Development of Databases on Iodine in Foods and Dietary

nutrients
Review
Development of Databases on Iodine in Foods and
Dietary Supplements
Abby G. Ershow 1, *, Sheila A. Skeaff 2 , Joyce M. Merkel 1 and Pamela R. Pehrsson 3
1
2
3
*
Office of Dietary Supplements, National Institutes of Health, Bethesda, MD 20892, USA; merkelj@od.nih.gov
Department of Human Nutrition, University of Otago, Dunedin 9010, New Zealand;
sheila.skeaff@otago.ac.nz
Nutrient Data Laboratory, US Department of Agriculture, Beltsville, MD 20705, USA;
pamela.pehrsson@ars.usda.gov
Correspondence: ershowa@od.nih.gov; Tel.: +1-301-435-2920
Received: 16 November 2017; Accepted: 10 January 2018; Published: 17 January 2018
Abstract: Iodine is an essential micronutrient required for normal growth and neurodevelopment;
thus, an adequate intake of iodine is particularly important for pregnant and lactating women, and
throughout childhood. Low levels of iodine in the soil and groundwater are common in many parts
of the world, often leading to diets that are low in iodine. Widespread salt iodization has eradicated
severe iodine deficiency, but mild-to-moderate deficiency is still prevalent even in many developed
countries. To understand patterns of iodine intake and to develop strategies for improving intake, it is
important to characterize all sources of dietary iodine, and national databases on the iodine content
of major dietary contributors (including foods, beverages, water, salts, and supplements) provide
a key information resource. This paper discusses the importance of well-constructed databases on
the iodine content of foods, beverages, and dietary supplements; the availability of iodine databases
worldwide; and factors related to variability in iodine content that should be considered when
developing such databases. We also describe current efforts in iodine database development in
the United States, the use of iodine composition data to develop food fortification policies in New
Zealand, and how iodine content databases might be used when considering the iodine intake and
status of individuals and populations.
Keywords: iodine; database; food; dietary supplements; food composition
1. Introduction
Low levels of iodine in the soil and groundwater are common in many parts of the world, often
leading to diets that are low in iodine. Severe iodine deficiency is now rare due to widespread salt
iodization, but mild-to-moderate deficiency is still prevalent even in many developed countries [1].
Knowledge about all sources of dietary iodine, including foods, beverages, water, salts, and
supplements, is important for understanding patterns of iodine intake and for planning interventions.
Robust food composition tables specific to individual countries are a key practical resource in providing
population-level and individual-level guidance for better iodine nutrition. This article will discuss the
importance of well-constructed databases on the iodine content of foods and dietary supplements, the
primary causes of variability in iodine content, the desirable characteristics of these databases, and
their current availability worldwide. We also describe recent progress in iodine database development
and use in the United States (US) and New Zealand, and consider database applications relevant to
the assessment of iodine intake of populations and individuals.
Nutrients 2018, 10, 100; doi:10.3390/nu10010100
www.mdpi.com/journal/nutrients
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2. Background
Iodine is essential for the synthesis of thyroid hormone and thus is required for normal physical,
neurological, and intellectual growth of infants and children, and for normal metabolism and
function in adults. On a body weight basis, infancy and early childhood are the times of highest
iodine requirements. Pregnant and lactating women also have increased requirements to meet their
heightened physiologic needs. It is critical that women who are likely to conceive, or are pregnant or
lactating, have iodine reserves sufficient for their own health and also sufficient to provide the fetus and
infant with the necessary iodine supply [2,3]. The most serious consequences of iodine deficiency are
well characterized and include hypothyroidism, neuro-cognitive impairment, and, in cases of severe
deficiency in pregnancy, cretinism in the infant. In contrast, the consequences of mild-to-moderate
iodine deficiency are less well understood and are an important priority for research and public
health practice. In particular, concerns center on the impact of mild-to-moderate iodine deficiency
in pregnancy, which has a high prevalence worldwide [1], on child development. Two observational
studies found an association between inadequate iodine status in pregnancy and poorer academic
performance in their children [4,5], although a recent randomized controlled trial reported no difference
in cognitive scores of children born to mildly iodine deficient mothers supplemented with iodine or
placebo in pregnancy [6].
Satisfactory iodine nutrition can be achieved in most circumstances through intake of adequately
iodized salt in sufficient quantities and/or intake of other iodine-rich foods that are commonly
consumed within a country [7]. In the early 1920s, iodized table salt was introduced in many countries,
a practice that since then has spread to include the majority of countries with about 86% of the world’s
population recently estimated as having access to iodized salt [8]. World Health Organization (WHO)
recommends Universal Salt Iodization, whereby all salt for human and animal consumption is iodized
including salt used in the food industry [9]. Some countries add iodized salt to only a few foods,
for example in New Zealand and Australia, where the mandatory use of iodized salt in commercial
bread production was implemented in 2009. However, in other countries iodized salt may be available
but not used in commercially prepared food [10,11], or the salt may be iodized but at a very low
level [10].
Iodine deficiency has re-emerged in countries such as Australia [12] and New Zealand [13]. A drop
in iodine intake may reflect recent changes in food consumption patterns in which home-prepared
foods, traditionally made with iodized salt, have been replaced with commercially prepared foods
made with non-iodized salt. For example, in the case of the US, this point is reinforced by surveys
documenting that retail sales of iodized salt have declined [11] and that less time is now spent preparing
foods at home [14]. This situation has raised concerns about potentially inadequate intakes of iodine
despite high intakes of salt from commercially prepared foods [15]. Likewise, individuals or ethnic
groups whose diets exclude or restrict iodine-rich food sources for health, religious, or other reasons
(such as vegan/vegetarian diet patterns, lactose intolerance, or low salt diets) may be at risk for
inadequate iodine intake [16]. Knowing the iodine content of available foods thus becomes a key
component in understanding which foods are the most important contributors to iodine intake for
populations as well as individuals.
The need for improved data on the iodine content of foods and beverages has been noted by
several expert committees [17,18] and has recently been reviewed in detail [19]. Also, iodine derived
from dietary supplements must be included along with foods in order to accurately assess total
intake; therefore, data are needed on the iodine content of supplements [20]. Robust approaches to
developing databases will include choosing appropriate analytical methodology (including use of
standard reference materials [21,22]); designing and implementing sampling plans with good coverage
of major country-specific contributors (from foods, beverages, dietary supplements, and salt); and
publishing the results in database formats or tables that allow linkage with population surveys and
individual intake records (such as food frequency questionnaires and 24-h recalls).
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3. The Availability of Databases Including Iodine Content
Many countries have developed national databases that include information on the iodine content
of foods, beverages, and salts, and other food components. To identify food and nutrient composition
databases for individual countries and to determine if these databases included values for iodine, and
also to identify information on national iodization programs for salt or other foods, we conducted
an extensive Internet search using resources from FAO INFOODS [23], the recently released ILSI
interactive tool [24], and other sites including Google and Google Scholar. In most cases the keywords
used were “food composition”, “food composition database”, “iodine composition”, “iodine in foods”,
and the name of the country using countries listed by the US Department of State. (https://www.state.
gov/misc/list/). We did not specify any particular language in our search, although many databases
were available in English either in their original form or in translation.
Over 124 countries have known salt iodization programs, either mandatory or voluntary [25],
but for many of these countries it was not possible to determine if the country had a national food
composition database, and if so, to ascertain the presence or absence of iodine data. Table 1 presents
information on the availability of national food composition databases, by country, that have iodine
content data, and it also describes national iodization practices. Most databases provide for all kinds of
foods with a few exceptions, such as Turkey’s database, which contains iodine values only for table salt,
fish, and shellfish. However, as shown in Table 2, national iodine databases are not currently available
for many countries that do have national food composition databases containing information on other
nutrients. In some cases, limited iodine datasets have been published as part of scholarly or other
journal publications [26–31]. (NOTE: The compilation presented in Tables 1 and 2 is not exhaustive,
nor is it equivalent to a systematic literature review. The information is dynamic, meaning that at the
time of this publication, links to databases were verified to be active; however, links may change or
become inactive over time.)
Table 1. National food composition databases that include iodine.
Country
Year of Salt
Iodization
Armenia
2004 [32]
URL
Database Name
pdf.usaid.gov/pdf_docs/Pdach758.pdf
Armenian Food Composition Table 2010
AUSNUT 2011-13 Food Nutrient Database
Australia
1953/54 1 [33]
www.foodstandards.gov.au/science/
monitoringnutrients/ausnut/foodnutrient/
Austria
1963 [34]
www.oenwt.at/content/naehrwert-suche/
OENWT Österreichische Nährwerttabelle
www.fao.org/fileadmin/templates/food_
composition/documents/pdf/
FOODCOMPOSITONTABLESFORBAHRAIN.pdf
Food Composition Tables for the Kingdome
of Bahrain
Czech Food Composition Database
Bahrain
Not found 2
Czech Republic
1950 [34]
www.nutridatabaze.cz/en/
Denmark
1998 [34]
frida.fooddata.dk/AlpList.php
Danish Food Composition Databank
Finland
1949 [34]
fineli.fi/fineli/en/index
Fineli, National Food Composition
Database in Finland
France
1952 [34]
pro.anses.fr/TableCIQUAL/
CIQUAL French Food Composition Table
www.bda-ieo.it/wordpress/en/
Food Composition Database for
Epidemiological Studies in Italy (Banca
Dati di Composizione degli Alimenti per
Studi Epidemiologici in Italia—BDA
www.mext.go.jp/en/policy/science_technology/
policy/title01/detail01/sdetail01/sdetail01/
1385122.htm
Tables of Food Composition in
Japan-2015-(7th Revised Ed)
Italy
1972 [35]
Japan
No program
Malaysia
2000 [36]
myfcd.moh.gov.my/
Malaysian Food Composition Database
(MYFCD)
New Zealand 3
1924 [37]
www.foodcomposition.co.nz/concise-tables
Concise New Zealand Food Composition
Tables 12th Ed
The Netherlands
1942 [34]
nevo-online.rivm.nl/ProductenZoeken.aspx
Dutch Nutrient Material File (NEVO)
Norway
1920 [34]
www.matvaretabellen.no/
Norwegian Food Composition Table
Poland
1997 [34]
www.izz.waw.pl/index.php?lang=en
Poland Food Composition Tables Database
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Table 1. Cont.
Country
Year of Salt
Iodization
Slovakia
URL
Database Name
1953 [34]
www.pbd-online.sk/
Slovak Food Composition Database Online
1964 [34]
opkp.si/en_GB/cms/vstopna-stran
OPKP (Open Platform for Clinical
Nutrition)
1982 [35]
www.bedca.net/bdpub/index_en.php
Spanish Food Composition Database
Sweden
1936 [35]
www.livsmedelsverket.se/en/food-and-content/
naringsamnen/livsmedelsdatabasen
Livsmedelsdatabasen—Swedish Food
Composition Database
Switzerland
1922 [34]
www.naehrwertdaten.ch/
Swiss Food Composition Database
Tanzania
1990s [38]
www.hsph.harvard.edu/nutritionsource/foodtables/
Tanzania Food Composition Tables
Tunisia
1990s [39]
www.mpl.ird.fr/tahina/home/doc/sommaire_
table_composition.pdf
Table de Composistion des Aliments
Tunisiens
Turkey 5
1968 [34]
www.turkomp.gov.tr/?locale=en
Turkish Food Composition Database,
TürKomp 2
UK
No program
www.gov.uk/government/publications/
composition-of-foods-integrated-dataset-cofid
Composition of Foods Integrated Dataset
(CoFID)
Slovenia
4
Spain
1
Iodized salt was only added to bread in 1953/1954 but was discontinued in the 1980s; iodized salt was available
in Australia from this time. A 2009 iodization program applied to salt added to most bread [33,40]. 2 Search was
conducted and salt iodization details were not located, but this does not confirm the absence of a program. 3
Database does not have updated iodine values for all fortified breads. 4 Database lacks some milk/dairy and
beverage products. 5 Database iodine content is limited to table salt, fish, and shellfish.
Table 2. National food composition databases that do not include iodine.
Country
Year of Salt
Iodization
URL
Source Name
Belgium
1990 [34]
www.nubel.com/fr/table-de-composition-desaliments.html
Belgian Table of Food Composition
Brazil
No program
www.fcf.usp.br/tbca/
Brazilian Food Composition Table (TBCA)
Cameroon
Not found 1
www.academia.edu/5451699/A_review_of_
composition_studies_of_Cameroon_traditional_
dishes_Macronutrients_and_minerals
Journal Publication
Canada
1949 [41]
food-nutrition.canada.ca/cnf-fce/index-eng.jsp
Canadian Nutrient File (CNF)
Chile
1979 [42]
web.minsal.cl/composicion-de-alimentos/
Chilean Table of Chemical Composition of
Foods, Update 2010
China
1995 [43]
www.neasiafoods.org/dataCenter.do?level=yycfk&
language=us
Food and Nutrient Database,
Food Nutrition Library
Costa Rica
1970 [44]
www.inciensa.sa.cr/actualidad/Tabla%
20Composicion%20Alimentos.aspx
Tablas de Composición de Alimentos
Cuba
Not found 1
www.inha.sld.cu/
Tabla de Composición de Alimentos
Utilizados en Cuba
Gambia
Not found 1
ilsirf.org/wp-content/uploads/sites/5/2017/03/
Gambia2011FCT.pdf
Food Composition Table for Use in
The Gambia
Germany
1959 [34]
www.blsdb.de/
German Nutrient
Database—Bundeslebensmittelschlüssel
Greece
No program
www.hhf-greece.gr/tables/Home.aspx?l=en
Composition Tables of Foods and
Greek Dishes
Iceland
No program
old.matis.is/english/service/productdevelopment-and-entrepreneurship/nutrition/
isgem-the-icelandic-food-composition-database/
ISGEM (The Icelandic Food
Composition Database)
India
1983 [45]
ifct2017.com/wp-content/uploads/2017/05/ifctdoc.pdf
Indian Food Composition Tables
Ireland
No program
www.ucc.ie/archive/ifcdb/
Irish Food Composition Database
Israel
No program
www.health.gov.il/Subjects/FoodAndNutrition/
Nutrition/professionals/Pages/Tzameret.aspx
Tzameret
Korea
No program
koreanfood.rda.go.kr/eng/fctFoodSrchEng/
engMain
Korean Standard Food Composition Table
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Table 2. Cont.
Country
Year of Salt
Iodization
URL
Source Name
Lesotho
2000 [46]
ilsirf.org/wp-content/uploads/sites/5/2017/03/
Lesotho2006FCT.pdf
Lesotho Food Composition Tables
Mexico
Not found 1
www.innsz.mx/2017/Tablas/index.html#page/8
Tablas de Composicion de Alimentos y
Productos Alimenticios Mexicanos (Version
condensada 2015)
Mozambique
2000 [47]
ilsirf.org/wp-content/uploads/sites/5/2017/03/
Mozambique2011FCT.pdf
Food Composition Tables for Mozambique,
Version 2
Nepal
1973 [48]
www.fao.org/fileadmin/templates/food_
composition/documents/regional/Nepal_Food_
Composition_table_2012.pdf
Food Composition Table for Nepal 2012
Papua New
Guinea
1995 [49]
www.fao.org/docrep/007/y5432e/y5432e00.htm
The Pacific Islands Food Composition
Tables
Portugal
No program
portfir.insa.pt/foodcomp/search
Portuguese Food Composition Table
Serbia
1937 [50]
www.serbianfood.info/lozinka1.php
Serbian Food & Nutrition Database
Singapore
1988 [51]
focos.hpb.gov.sg/eservices/ENCF/
Energy and Nutrient Composition of Food
South African
1995 [52]
safoods-apps.mrc.ac.za/foodcomposition/
South African Food Database System
(SAFOODS)
Sweden
1936 [34]
www.livsmedelsverket.se/en/food-and-content/
naringsamnen/livsmedelsdatabasen
Livsmedelsdatabasen—Swedish Food
Composition Database
Thailand
1994 [53]
www.inmu.mahidol.ac.th/aseanfoods/download/
books/dl1.php?file=A1
ASEAN Food Composition Tables
Togo
Not found 1
ilsirf.org/wp-content/uploads/sites/5/2017/03/
TogoTable_de_Composition_des_Aliments.pdf
Table de Composition des Aliments
du Togo
Uganda
1990s [54]
www.harvestplus.org/category/resource-type/
technical-monographs
A Food Composition Table for Central and
Eastern Uganda
Vietnam
1999 [55]
www.fao.org/fileadmin/templates/food_
composition/documents/pdf/VTN_FCT_2007.pdf
Bảng Thành Phần Thực Phẩm Việt Nam
Vietnamese Food Composition Table
United States
1924 [56]
ndb.nal.usda.gov/ndb/
USDA Food Composition Database
1
Search was conducted and salt iodization details were not located, but this does not confirm the absence of
a program.
4. Sources of Variability in Food Iodine Content
The amount of iodine in foods can be highly variable, and the nature and degree of this
variability can have implications for the complexity and cost of developing databases of iodine
content. For example, high variability may affect sampling plans such that more samples may need
to be collected over a wider range of geographic areas and a larger number of sales or distribution
venues [57]. Also, different chemical assay approaches (i.e., methods and reference materials) may
be needed, depending on the anticipated range of iodine concentrations. (Note: see Section 5, below,
for further discussion of these methodological issues). Highly variable and non-normal (skewed)
distributions of iodine content may be best served by presentations of descriptive statistics that include
multiple indicators of central tendency and range [58]. Some of the factors affecting between-country
and within-country variability in iodine content of foods are described below.
4.1. Water
Drinking water is particularly variable in its iodine content between and within countries,
especially if they are geographically diverse. Levels of iodine in drinking water supplies are reflective
of factors such as iodine in the soil and water table, proximity to sea water, and agricultural runoff [59].
Therefore, assessment of iodine intake from drinking water may require data at the regional or local
level. For example, the iodine content of water in some regions of China is sufficiently high to lead to
excessive intake and potential thyroid hypertrophy in school children [60]. Conversely, the desalinated
water used in many parts of Israel has been noted to have very low, essentially zero, iodine levels [61].
In many countries, there is very little information available on the iodine content of drinking water
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supplies, nor on the iodine content of bottled waters. Food composition databases should include a
range of values or a default average value for the iodine content of drinking water.
4.2. Salt
Salt is the preferred vehicle for iodine fortification, and salt iodization has been implemented in
124 countries [62]. Salt can be produced from underground rock salt deposits, natural brine, or by
evaporated seawater, with the latter containing <1 mg iodine (I)/kg of salt. This relatively low level
of iodine in un-fortified sea salt may not always be appreciated by consumers. Most food-grade salt
requires the addition of iodine, usually as potassium iodate or potassium iodide. WHO suggests that
the amount of iodine added to salt should be based on the estimated salt consumed by the population;
this ranges from 14 mg I/kg when estimated salt intake is high (i.e., 14 g/day) to 65 mg I/kg when
salt intake is low (i.e., 3 g/day) [63]. In many countries, the iodine content of iodized salt is legislated
and specified by a food standards code. However, the actual iodine content of iodized salt may differ
from the reported content, particularly when iodized salt is kept in open containers and exposed to
high humidity; the iodine content of salt can vary by 8% to 49% under such conditions [64]. Database
developers must consider whether the country practices Universal Salt Iodization, in which case all
salt for human use must be iodized, and therefore iodized salt will be used for commercial food
products [9]. Alternatively, databases might include paired iodine values for some food products that
have been prepared with iodized or non-iodized salt.
4.3. Agricultural Practices—Soils and Crops
Iodine occurs naturally in the earth’s crust and is present everywhere in the environment. Soils,
shales, and coal rich in organic matter are generally higher in iodine than hard rock [65,66]. Also
the iodine content of the soil can be influenced by the proximity of the growing area to ocean water
(through which atmospheric iodine is incorporated into rainfall, and thereby raises the iodine content
of the soil), the iodine content of ground waters and irrigation waters, and the use of iodine-containing
fertilizers [67]. The iodine content of plant crops is affected by the content of iodine in the soil (i.e.,
plants grown on high iodine soils will contain more iodine than those grown on low iodine soils), but
in general, plant-based foods such as vegetables and fruits are relatively poor sources of iodine [59].
The exceptions are seaweeds, which have a great capacity to concentrate iodine [68–70].
4.4. Agricultural Practices—Animal Husbandry
Dairy products and eggs may contain significant but variable amounts of iodine, influenced, to
some degree, by the iodine content of supplements in animal feeds and salt licks. These supplements
are often provided as part of animal husbandry practice to ensure good health and reproductive
outcomes in dairy and beef cattle, sheep, goats, and poultry. Dairy products also have contained
adventitious iodine from iodophors, iodine-containing disinfectants used at various points in the
production of milk. Iodophors can be used to clean udders, but if the cleansing has not been performed
properly some of the iodine from the teat dips can be absorbed and transferred to milk and meat.
A recent US Department of Agriculture (USDA) report found that 55% of dairy operations were using
iodophor teat dips [71], suggesting that the practice is still relatively common in the US. Iodophors
were also used in the cleaning of industrial equipment for processing milk in dairies; a decline in
this practice in New Zealand by the mid-1980s is suggested to be responsible for a drop in the iodine
content of milk and dairy products [13].
4.5. Food Processing
Commercial baked goods are another source of iodine when iodates are used in the commercial
baking industry as dough conditioners. Iodates were introduced in the US 40 years ago and in
Tasmania, Australia, over 50 years ago, but now other dough conditioners are being used. Also, some
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commercial baked goods contain erythrosine (Red No. 3/E127), a common food coloring that contains
iodine; however, the iodine from erythrosine is only partially bioavailable [72].
5. Developing an Iodine Database for US Foods—Recent Progress
In 2014, the National Institutes of Health’s Office of Dietary Supplements (ODS), Bethesda MD,
convened several working groups to consider clinical and population research relevant to human
iodine nutrition, particularly in the US [73]. Key areas of applied and clinical research that were
reviewed and identified as needing greater effort included: assessing the iodine concentration of US
foods and drinking water supplies; evaluating iodine intakes of various US population subgroups; and
having a sufficient scientific knowledge base to determine iodine requirements at different lifecycle
stages [73]. A database on the iodine content of US foods was considered to be a critical tool for
conducting research on all of the identified gap areas. This priority has been operationalized through
an interagency agreement (NIH IAA-AOD-17002) between ODS and the USDA to develop the USDA
Food Iodine Database. Other federal partners involved in this project include the US Food and Drug
Administration and the National Institute of Standards and Technology (NIST).
In this section, as summarized in Table 3, we will describe the main features of the project plan
for the nascent USDA Food Iodine Database. A similar scientific and technical approach has been
used to develop other databases, including those for choline and flavonoids [74,75]. We note that the
scientific and technical approach used by the USDA is, of course, most suitable for the US; however,
the concepts and operational components driving the design and execution of the project plan can
serve as a prototype for development of iodine databases in other countries.
Table 3. Project Components for Developing the USDA Food Iodine Database.
Design phase (Completed)
•
Define research needs
•
Review existing data
•
Assess iodine distribution in the food supply
•
Develop sampling design and calculate sample sizes
Preliminary study phase (Completed)
•
Conduct stability studies
•
Develop sample handling protocols
•
Identify appropriate analytical methods and quality control materials
Research implementation phase (Ongoing)
•
Conduct analyses and data quality control reviews
•
Conduct ancillary studies
•
Disseminate data and documentation
5.1. The Design Phase
For any research initiative designed to provide foundational data to explore the connection
between food and health, the researcher must address what and why it needs to be done, and how to
achieve the answers. Put another way, when developing a new database or dataset, it is important
to define the research needs and potential impact at the outset of the project. In the case of iodine,
sufficient iodine composition data on food and dietary supplements are needed to estimate intakes
and to assess the consequences of deficiency or excess.
Preliminary planning activities include: identifying the “population of interest” of foods and
supplements; designing the sampling plan; developing a defensible analytical process (methods
and quality control); planning for statistical analysis of the results; and considering a means of data
dissemination. In the case of iodine, although the USDA has developed special databases for other
nutrient-focused datasets, it was important to identify unique characteristics of iodine-contributing
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foods and dietary supplements. Other specific challenges related to database development included
the need for methods for effective sampling and analysis of foods [19], and for statistical approaches
that can accommodate variability in the iodine content of foods [58].
Since resources for composition analyses are rarely sufficient to analyze all potential contributors
to iodine intake, it is essential to identify existing useable data of adequate scientific quality. The USDA
has long sought to take advantage of published and other available data when data quality criteria
have been met to satisfaction [76]. Also, for this same reason of efficiency, when possible, the USDA
seeks to harmonize its data with other data generated by complementary activities, most notably,
the US Food and Drug Administration (FDA) Total Diet Study (TDS). The FDA TDS is an ongoing
program that obtains about 290 foods of various matrices, four times a year, each in a different US
location, sampled in order to monitor an array of nutrients and other constituents of the US food
supply [77]. For iodine analysis of TDS foods, the FDA has recently synchronized their laboratory assay
method, inductively coupled-mass spectrometry (ICP-MS), to that used by the USDA; in addition,
the possibility of coordinating selected efforts is under discussion. This approach allows inter-agency
exchange of data on iodine. Also, enhanced quality control is achieved through close collaboration
to utilize NIST standard reference materials and methods. Thus, existing data from the TDS will
be used to enhance the national iodine dataset and also to set priorities for planned new or repeat
analyses [19]. When planning the sampling of specific foods, it is critical to identify and acquire other
relevant data that clarifies the distribution of iodine in the food supply (e.g., industry uses, agricultural
production, sales, and other sources). Another step involves preparing country-specific proportional
weighting factors for different versions of the food; an example would the use of industry data on
relative amounts sold or used of different types of iodized and non-iodized salt (e.g., conventional, sea,
Kosher, etc.).
In developing the sampling plan, it is necessary to determine an appropriate sample size.
The acquired food samples must be nationally representative, and the number of samples must be
sufficiently large to develop statistically defensible variability estimates. The approach to this plan must
be suitable for the country whose food supply is being analyzed; countries with a highly structured and
nationally distributed food supply will need an approach different from that in countries where foods
are acquired (grown, hunted, or foraged) and consumed within multiple smaller localities. An example
of necessary adaptations is that of acquiring food samples from American Indian reservations and
Alaska Native villages [78]. In the US, the sampling plan includes identification and procurement of
US-representative food samples; many of these samples are selected for analysis under the NIH-USDA
National Food and Nutrient Analysis Program (NFNAP) [79]. The USDA typically acquires its
food samples in a minimum of 12 geographically diverse locations selected according to current
population density data, retail sales data, and other national-level information. NFNAP foods under
evaluation for iodine analysis include finfish and other seafood products; seaweeds/seaweed extracts;
iodine-containing commercial ingredients and additives; highly-consumed dairy and egg products,
commercially processed mixed dishes; retail salts; and other foods containing significant amounts
of iodine.
5.2. Preliminary Study Phase
It is necessary to confirm the stability of the analytes in both new and archived analytical samples,
particularly when samples are shared from other studies or have been stored for some time. This is a
particularly important step when analyzing for iodine in salt and other food substrates. To investigate
the possible loss of iodine during sample storage, the USDA reanalyzed a set of samples that had
been initially analyzed 5 years previously and was able to confirm that the iodine content of archived
NFNAP foods stored at −60 ◦ C had not changed [78,80]. The USDA also plans to evaluate salt industry
data regarding iodine volatilization under varying storage, humidity, and temperature conditions.
Sample handling protocols must be developed prior to sample collection to ensure sample
integrity during shipping and storage. Pilot testing of protocols including chain-of-custody plans may
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be warranted to ensure that sample integrity can be maintained from the point of purchase all the way
through to analysis. The USDA has developed sample handling protocols for NFNAP that can serve
as examples for others who are undertaking collection and eventual assay of foods [76]. An important
precaution when utilizing samples of varying provenance (such as from a multicenter study) is to
ensure that there has not been inadvertent contamination during the collection or storage process;
sources of contamination may include leaching from storage containers that can release iodine into the
sample over time and use of iodine-containing preservatives or disinfectants.
For the chemical analysis of foods, it is important to identify appropriate analytical methods and
to select a capable laboratory through precertification testing with blinded samples [75]. In terms
of analytical methodology, the ICP-MS method [76] is suitable for the analysis of iodine in a variety
of foods, as well as dietary supplements, and is known to have good precision and accuracy. Using
the ICP-MS method, the USDA contract laboratory has shown that it can generate accurate iodine
values that compare well with the iodine values of several NIST-certified reference materials (CRM),
notably NIST 1849a Infant/Adult Nutritional Formula and 1548a Typical Diet [21,78,80]. For quality
control during analysis of samples, the USDA is using these CRM reference materials, as well as
in-house control materials cross-validated to the NIST materials. Other NIST CRM materials with
available iodine values (such as iodized salt, egg powder, whole milk powder, and kelp) will be used
as warranted as the analytical plan progresses.
5.3. Research Implementation Phase
The USDA’s experience with other database development projects has revealed that the greatest
effort and cost are incurred when acquiring food products according to the statistical design, conducting
direct assays of the prepared samples and quality control materials, and assembling the data in an
orderly format. This phase of the USDA Food Iodine Database project is now in progress. To date, over
135 unique foods have been sampled; some of these are newly acquired and others are existing frozen
samples that were archived as part of previously completed projects. About 350 prepared samples
representing these foods have been analyzed to date, along with quality control materials comprising
about 10% of total assays. The predominant food groups analyzed so far are multi-ingredient
commercial foods (e.g., restaurant hamburgers and macaroni and cheese, retail frozen pizza, milks and
yogurt) and several types of fish (shellfish, crustaceans, mollusks, and finfish).
Following data quality review, data dissemination in the form of accessible databases and
professional and peer-reviewed publications will allow transparency of the data and related
information. Additional ancillary studies can enhance the value and usefulness of database resources;
for example, the USDA is considering a sub-study to utilize direct chemical assay followed by statistical
modeling to evaluate the iodine content of home-prepared recipes in comparison with commercial
food equivalents. In addition, the USDA will use ancillary studies to explore other factors that that
may affect total dietary iodine such as geographic origin of foodstuffs (as reflecting iodine content of
soils), the iodine content of water supplies used for drinking and reconstituting foods, and the iodine
content of dietary supplements.
6. Iodine Content of Supplements
Nutrient-containing supplements may contribute substantially to total nutrient intake [27,81,82],
and therefore should be included along with foods when estimating total dietary intake of
iodine. Supplements are used worldwide but are regulated very differently among countries [83],
sometimes as foods (as in the US and Europe) [84,85], and sometimes as a form of herbal
medicines (as in Germany) [86,87], and product registration or licensing may be required under
certain circumstances [88]. Databases describing the content, ingredients, and other information
on supplements may be assembled by government entities, manufacturers, or researchers.
The information content, definitions, terminology, data formats, and sources of data can vary
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considerably, and this lack of uniformity and consistency has been noted as a limitation in making
between-country comparisons for research and other purposes [85].
Identifying and then accessing dietary supplement databases from different countries can be
challenging as there are few readily available and comprehensive compilations [85]. Nevertheless,
the task may be approached on a country-by-country basis. For example, the Netherlands provides
a database on the composition of supplements that is linked to the national food composition
tables [89]; this database has been used as part of an iodine intake assessment study in children
and adults [90]. Similarly, dietary supplement databases have been developed and then used for iodine
intake assessment in pregnant and lactating women in Norway [26–28] and in pregnant women [29]
and adults [30] in Denmark. The Australian dietary supplement database (AUSNUT 2011–2013)
contains 35 nutrient values, including iodine, for 2163 dietary supplements consumed during several
national nutrition and physical activity surveys conducted from 2011–2013 [91]. The method for
constructing a dietary supplement database for adult participants in one of the United Kingdom
sub-projects of a multi-country European cohort study has been described in detail [92].
In the US, the Dietary Supplement Label Database (DSLD), sponsored by ODS in collaboration
with other US federal agencies, is presently the most comprehensive listing of label information
(www.dsld.nlm.nih.gov/dsld-mobile/) [93,94]. The DSLD evolved from earlier questionnaire-based
efforts to understand the magnitude of the contribution of supplements to population nutrient intake
in the US, starting with the 1999 cycle of the National Health and Nutrition Examination Survey
(NHANES) [93,95]. Other currently available databases with label information for US products
include DailyMed (dailymed.nlm.nih.gov/dailymed/) and the industry-sponsored Supplement Online
Wellness Library (OWL) database (www.supplementowl.org).
Iodine content (per serving) and source (ingredients) are provided for US supplement products
as listed on the product label and the Supplement Facts Panel [96]. For example, the label may
indicate that the iodine in the supplement product may come from diverse sources such as iodine salts
(e.g., potassium iodide) or botanical ingredients (e.g., kelp). When using label information, whether
by direct inspection of the product container or through the label databases mentioned above, an
important caveat is that results from direct analysis of individual supplement products usually are not
publicly available.
To estimate typical nutrient content values—including for iodine—for certain grouped classes of
sampled supplement products (e.g., adult multivitamin/multiminerals (MVMs), pediatric MVMs, and
non-prescription prenatal MVMs), direct chemical analysis has been conducted on sampled products
and standard reference materials [97]; the analysis results have been made available through the Dietary
Supplements Ingredient Database (DSID) [20,98]. The DSID assay results suggest that the actual iodine
content for adult MVMs, pediatric MVMs, and non-prescription prenatal MVMs may typically exceed
the labeled amount by 20–26%. Furthermore, comparisons of labeled iodine contents of prescription
and non-prescription prenatal supplements sold in the US have found that, per tablet, non-prescription
prenatals contain approximately 10% more iodine than prescription prenatals [99,100].
Given the concerns about the adequacy of iodine intake by pregnant women, data from the
US National Health and Nutrition Examination Survey (NHANES) program has provided a means
of understanding the contribution of dietary supplements to iodine intake. Use of supplements
(multi-vitamins or MVMs) was found to be widespread (~75%) among US pregnant women surveyed
in 1999–2006; however, use of iodine-containing supplements was relatively low (~22%) [95]. Since
that time, various professional organizations including the American Academy of Pediatrics [101], the
Endocrine Society [102], the Teratology Association [103], and the American Thyroid Association [104],
as well as the Australian National Health and Medical Research Council [105] and the New Zealand
Ministry of Health [106], have recommended that pregnant and lactating women take a daily prenatal
MVM supplement that contains 150 µg of iodine. Data on time trends in usage of iodine-containing
supplements by pregnant and lactating women will help in understanding whether usage is changing
in response to professional recommendations. A recent study undertaken in New Zealand reported
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that 52% of pregnant and lactating women followed the recommendation for a 150 µg of iodine per
day [107].
7. New Zealand: A Case Study Illustrating the Need for a Database with Iodine Content
New Zealand has low levels of iodine in the soil, predisposing the population to iodine deficiency.
New Zealand was the first country after Switzerland to introduce iodized salt to address widespread
iodine deficiency, albeit at an initial low concentration of 5 mg I/kg; this was increased to 50 mg I/kg in
1939 [37]. Domestic household use of iodized salt, both at the table and in home cooking, successfully
eradicated iodine deficiency in New Zealand by the early 1950s. Additional iodine was derived from
dairy products when iodophors were used by the dairy industry from the 1960s to the 1980s. However,
changes in food habits, including a reduction in the consumption of iodized salt in the home and a
drop in the iodine content of dairy products when detergent-based sanitizers replaced iodophors, are
factors believed to have contributed to the re-emergence of mild iodine deficiency in the 1990s [13].
Given the widespread nature of the deficiency, which was reported in all population groups, the most
effective strategy identified by the government was mandatory fortification [41].
Dietary modeling was used to identify the best dietary approach to increase iodine intake without
exceeding the upper limit of intake, with a particular focus on reducing the prevalence of iodine
deficiency in pregnant women and children. In order to undertake this process, it was imperative
that the iodine content of foods be included in the New Zealand Food Composition Database [108].
Because the most commonly eaten staple foods in the New Zealand diet are low in iodine, and the
foods highest in iodine content are consumed in small quantities, a preliminary proposal was put
forward to mandate the replacement of non-iodized salt with iodized salt in breads, breakfast cereals,
and sweet biscuits (i.e., cookies). Because New Zealand imports and exports biscuits, a requirement
to fortify biscuits with iodine would require separate production lines for both overseas and New
Zealand biscuit producers. Furthermore, cereal manufacturers suggested that the application of a
brine spray to breakfast cereals would produce inconsistent amounts of iodine in breakfast cereals.
Thus, concerns about trade regulations for biscuits and technological concerns for breakfast cereal
meant that bread, a staple food consumed by 87% of the New Zealand population, was chosen as the
sole food for fortification; it was acknowledged that bread would not provide pregnant women with
enough iodine to meet their higher requirements. A change to the Food Standards Code came into
effect in September 2009 mandating the use of iodized salt in yeast-leavened bread; organic bread
was exempt from this requirement [33]. The New Zealand Food Composition Database has included
a revised iodine content for some breads, although more information on the iodine content of New
Zealand breads can be found in a separate government report [109].
8. Discussion
Iodine has emerged as a nutrient of concern in many developed countries, in part because retail
sales (and home use) of non-iodized salts may be increasing [11,110–112] and iodized salt is not
always used in the commercially prepared foods that make up an ever-increasing component of the
food supply yet whose consumption often leads to excessive sodium intake [11,110–112]. Also, some
countries have not yet implemented mandatory salt iodization programs [51,112,113]. Pregnant women
and young children are at highest risk of inadequate intake but other groups within populations may
also be at risk. At present there is no simple or reliable way to assess the iodine status of an individual.
Although other causes cannot be ruled out, altered thyroid function can suggest the presence of iodine
deficiency. In the future, validated biomarkers of individual status (such as thyroglobulin levels) may
become available [114]. In the meantime, despite the difficulty of directly assessing iodine status of
individuals, practical diet-based approaches may prove useful.
Individuals should be asked about the voluntary and involuntary factors that can affect iodine
intake and status. This information can provide a basis for assessing risk and providing counseling.
For example, interview methodology can be used to gather information on use of iodine-containing
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dietary supplements, and on intake of food sources of iodine such as dairy products and seafood,
including fresh and saltwater fish and seaweeds. A growing concern in many countries is the
adoption of dietary patterns that specifically exclude major sources of iodine, including low-salt [115],
vegan [116–118], and Paleo diets [119]. Questionnaires must be attuned to phrasing; for example, it is
important to enquire about use of dairy vs non-dairy milks, as the iodine content of milk alternatives
often is very low [120]. In some situations, it may even be desirable to develop individualized advice
regarding type of salt; for example, researchers in China are attempting to develop an online screening
tool that informs consumers if they should consume iodized or non-iodized salt [121].
An additional topic that needs appropriate interview methodology includes the type of salt used
in the home for cooking and at the table. For example, interviewers should ask how often new iodized
salt is purchased, as the iodine content may decline over long periods of storage. Interviewers should
also ascertain use of sea salt, which, if not fortified, may have a surprisingly low iodine content.
Determining the amount of iodine coming from iodized cooking and table salt can be difficult to
quantify for a number of reasons. Firstly, the interpretation of a sprinkle or pinch of salt will vary
from person to person. Secondly, if food intake is being weighed, the weighing scales may not be
sensitive enough to measure salt added at the table and weight recorded as null (i.e., 0) grams. Thirdly,
when iodized salt is used for cooking vegetables or pasta, the amount of iodine in water that is
discarded and the amount that becomes incorporated into the cooked food is unknown. Thus, iodine
intakes determined using diet records or 24-h recalls are likely to underestimate actual iodine intake,
particularly in individuals who regularly and generously add iodized salt to their food. A simpler
approach is to add a set amount of iodine to anyone who reports use of iodized discretionary salt; in
New Zealand, an additional 48 µg of iodine, representing the consumption of 1 g of salt (48 mg I/kg)
per day, is included in the total daily iodine intake to account for discretionary use of iodized salt [109].
Data on the iodine content of foods can also be useful in research settings. Although seldom
considered, an estimate of the iodine content of the diet determined using a validated iodine-specific
food frequency questionnaire should be included as a variable in studies investigating thyroid function
on disease and child development. The iodine content of the diet could also be used to stratify
participants in randomized controlled trials of iodine supplementation.
Another use of iodine databases is in the treatment of thyroid disease. Patients undergoing
radioablation treatment for thyroid cancer are prescribed a several-week course of a low iodine diet,
in order to enhance uptake of radioactive iodine [122,123]. The degree of success in reducing iodine
intake usually is estimated using urinary iodine excretion [124]. Some patients find it difficult to
adhere to these diets. Also, there is debate about the necessary degree of dietary restriction, as well
as about the optimal time frame for dietary modification, both of which may depend, in part, on the
background iodine content of the patient’s usual diet [125,126]. In some circumstances, iodine data
from other countries has been used to develop dietary prescriptions for patients living in a different
country [127]. Improved national-level databases will be useful in clinical practice and in research on
low iodine diets.
Goitrogens are a chemically diverse group of compounds that have the capacity to interfere
with uptake or utilization of iodine by the thyroid gland, and thus pose an additional source of
dietary complexity important for understanding issues of adequacy of iodine intake [18]. The impact
on thyroid status of eating these foods may depend on the quantity eaten and the background
iodine content of the diet. High goitrogen intake may render marginal iodine intakes inadequate
for physiologic demands and under some circumstances may actually contribute to goiter endemics
and related disorders [128]. Dietary goitrogens often are inherent botanical constituents of foods;
examples include cassava (cyanogenic glucosides), cruciferous vegetables (glucosinolates), and soy
products (flavonoids). Dietary assessments should include information on intake of goitrogen-source
foods, which can be very specific to geographic region, including cultural practices and economic
issues related to the cost of foods. Special purpose databases or data tables with information on some
of the goitrogenic constituents of foods (e.g., the USDA Flavonoid Database [74,129]) can provide
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useful corollaries for iodine composition tables and would have potential application for research and
for counseling individuals at risk for thyroid disease. Environmental goitrogens also may present
dietary exposures of concern in some situations and include perchlorates, nitrates, and disulfides.
These compounds may derive from industrial contamination but may also occur naturally in soils and
can subsequently leach into the water supply and thereby into foods [130]. Tobacco smoke presents
another important goitrogenic exposure due to its thiocyanate content. As individuals are unlikely
to be aware of ingestion of most environmental goitrogens, it may be useful to consider estimating
exposure through the use of biomarkers [131,132].
In the US, the USDA Food Iodine Database project described in this paper complements the
ODS-supported DSID and DSLD resources that provide data on the iodine content of supplements,
necessary to determine total iodine intake. Once the USDA Food Iodine Database is complete, it will
be possible to generate more complete estimates of iodine intake and iodine sources through linkage
to dietary intake tools and to the data generated by surveys such as the NHANES, which heretofore
has based its estimates of iodine intake on urinary iodine levels [133]. Also, the eventual availability of
iodine values for a large number of foods with descriptive statistics that include multiple measures
of variability (e.g., percentile cutoffs, coefficients of variation, and standard deviation) and central
tendency (e.g., means and medians) will allow modeling of intakes that account for the spread of
iodine levels in many types of foods, thus helping to develop appropriate methods for estimating
intake of individuals and populations [58].
Another US concern, which also may be the case in other countries, is that information is available
about the iodine content of infant formula but not breast milk. Databases on the typical iodine content
of human milk would be desirable. Estimating the likely iodine intake of breast-fed infants is an
additional key aspect of understanding adequacy of population level iodine status.
9. Conclusions
Information about the iodine content of national food supplies is essential for understanding the
state of human iodine nutrition around the world, with important applications in nutrition research,
dietary counseling, treatment of thyroid disease, and public health practice. Collection and evaluation
of dietary data are essential components of this work because there is no simple way at present to
estimate iodine status of individuals. When possible, iodine composition databases for foods should
be complemented by databases for dietary supplements in countries where supplements make a
significant contribution to the total intake of iodine. Regularly updated composition data that reflect
current food supplies are needed to support nutrition surveys, which generate critical data resources
for understanding national-level concerns and identifying sub-populations at risk due to typical dietary
patterns or increased physiological need. In future surveys, characterizing the relationship between
iodine intake and thyroid function across populations and within population subgroups will require
information on total intake, from all sources.
Acknowledgments: We would like to acknowledge the assistance of Amanda Moran, University of Maryland.
This paper was supported by the NIH Office of Dietary Supplements.
Author Contributions: A.G.E., S.A.S. and P.R.P. developed the concept for the manuscript. A.G.E., S.A.S., J.M.M.
and P.R.P. wrote the manuscript.
Conflicts of Interest: The authors declare no conflicts of interest.
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