Acta agriculturae Slovenica, 87 - 2, september 2006
str. 245 - 253
Agrovoc descriptors: foods, supplements, quality, iodine, vitamins, minerals,
proximate composition, dietary guidelines, recommended dietary allowances
Agris category code: Q04, Q05, S30
COBISS code 1.01
Investigation of declared values of iodine in food
supplements on the Slovene market
Andrej OSTERC1, Vekoslava STIBILJ2
Received May 30, 2006; accepted August 10, 2006.
Delo je prispelo 30. maja 2006; sprejeto 10. avgusta 2006.
The essentiality of iodine for man is well recognized. The primary sources of iodine in a
normal, balanced diet are fish, shellfish, milk and iodinated salt. Alternative sources of iodine
meant to complement the normal diet are food supplements containing iodine. The complex
composition of food supplements, due to their content of various vitamins and minerals,
makes determination of iodine very difficult. In our work we employed radiochemical neutron
activation analysis to determine the content of iodine in different series of seven different food
supplements available on the Slovene market intended for adults, children and sportsmen.
The levels of iodine found ranged from 21 to 231 µg/g. Three food supplements were not in
agreement with the declaration made by the producer and two food supplements were not in
agreement with the recommendations made by U.S. Pharmacopoeia that dietary
supplements should contain not less then 90 % and not more then 200 % of the declared
amount of substance.
Keywords: food supplements, iodine, RNAA
PREVERJANJE DEKLARIRANIH VREDNOSTI JODA V PREHRANSKIH DOPOLNILIH NA
Pomembnost joda za ?loveka je dobro znana. Osnoven vir joda v normalni, uravnoteženi
prehrani predstavljajo ribe, lupinarji (školjke, polži, raki), mleko in jodirana sol. Alternativen vir
joda so prehranska dopolnila, ki vsebujejo jod. Njihova uporaba je upravi?ena predvsem v
primerih, kadar je težko zagotoviti uravnoteženo prehrano in so nepogrešljivi med
nose?nostjo ter v ?asu bolezni, poškodb, izrednega stresa ali telesnih obremenitev. Vsakršno
prekomerno ali nesorazmerno vnašanje v zdravo telo pa je zanj le obremenitev metabolizma
ali pa celo vodi v kopi?enje v dolo?enih organih, kjer povzro?ajo nekateri elementi celo
toksi?ne u?inke. Prehranska dopolnila imajo kompleksno sestavo, saj vsebujejo številne
vitamine in minerale, kar precej otežuje dolo?anje joda. Pri našem delu smo uporabili
radiokemijsko nevtronsko aktivacijsko analizo ter dolo?ili vsebnost joda v sedmih razli?nih
prehranskih dopolnilih, iz razli?nih proizvodnih serij, ki so dosegljiva na slovenskem tržiš?u in
1 Department of Environmental Sciences, Jožef Stefan Institute, Jamova 39 Ljubljana,
2 Corresponding author: Vekoslava Stibilj, Tel: +386 1 5885352, Fax: +386 1 5885346,
e-mail address: email@example.com
246 Acta agriculturae Slovenica, 87 - 2, september 2006
so namenjena odraslim, otrokom in športnikom. Vsebnost joda je bila v obmo?ju od 21 do 231
µg g-1. Pri treh prehranskih dopolnilih se dolo?ena vrednost joda ni ujemala z deklarirano
vrednostjo proizvajalca, pri dveh pa se dolo?ena vrednost ni ujemala z priporo?ilom U.S.
Pharmacopoeia, ki navaja, da prehranska dopolnila naj ne bi vsebovala manj kot 90 % in ne
ve? kot 200 % deklarirane vrednosti dolo?ene sestavine.
Klju?ne besede: prehranska dopolnila, jod, RNAA
Iodine is a mineral essential for normal functioning of the thyroid gland and for
mental and physical development. Lack of iodine results in insufficient production of
thyroid hormones, which are involved in the regulation of various enzymes and
metabolic processes. When physiological requirements for iodine are not satisfied
iodine deficiency effects, known as “iodine deficiency disorders” occur (DRI, 2000).
The recommendations for minimum daily iodine intake differ among different
institutions. The U.S. Food and Nutrition Board recommends for adults a daily iodine
intake of 150 µg/day (DRI for Vitamin C…, 2000). In contrast, the German Nutrition
Society recommends for adults a daily iodine intake of 200 µg/day (Reference Values
for Nutrient Intake, 2002). The main sources of iodine in an average diet are marine
fish, shellfish, milk, dairy products and salt fortified with iodine (Haldimann et. al.,
2005). With a well balanced diet the nutritional requirements for iodine can be met,
but some groups of people like vegetarians, sufferers from milk allergy, those with
lactose intolerance, fish allergy or persons on a low salt diet can develop iodine
deficiency. For these people food (dietary) supplements containing iodine are an
important source of iodine in the daily diet (DRI, 2000; European Commission,
The term dietary supplement is used in the United States for a wider range of
products. At the moment the term food supplement is used in the European Union for
foodstuffs containing vitamins and minerals. The definitions are as follows:
a) Food supplements in the European Union are defined in Directive 2002/46/EC,
which become law in August 2005, as foodstuffs meant to supplement the normal
diet and which are concentrated sources of nutrients or other substances with a
nutritional or physiological effect, alone or in combination (nutrients means
vitamins and minerals) (Directive 2002/46/EC, 2002).
b) Dietary supplements in the United States are defined in the Dietary Supplement
Health and Education Act (DSHEA) of 1994 as a product intended to supplement
the diet and that contains a “dietary ingredient” (vitamins, minerals, herbs or
other botanicals, amino acids, and substances such as enzymes, organ tissues,
glands and metabolites) (DSHEA, 1994).
Dietary and food supplements may be found in forms such as tablets, capsules,
pastilles, pills, soft gels, gelcaps, liquids or powders. They are sold in the United
States and in the European Union with little or no regulation. In the European Union
only herbal products in Germany are regulated by the same standards as drugs
(Directive 2002/46/EC, 2002; DSHEA, 1994).
The Directive 2002/46/EC of the European Parliament and of the Council introduces
rules for the labelling of food supplements and introduces specific rules for vitamins
OSTERC, A., STIBILJ, V.: Investigation of declared values of iodine in food ... 247
and minerals; medicinal products as defined by Directive are excluded (Directive
2001/83/EC, 2001). It prescribes which vitamins and minerals (Annex 1) and in which
chemical form (Annex 2) may be used for the production of food supplements
(positive lists). Trade in products containing vitamins and minerals not listed was
prohibited from the 1st of August 2005. For example, iodine may be present in the
form of sodium iodide/iodate or potassium iodide/iodate. (Directive 2002/46/EC,
Dietary and food supplements are not under strict regulations like drugs, which must
be proven safe and effective before marketing and before they reach the costumer.
The institutes responsible for food safety focus their resources first on products that
may cause injury or illness, meaning that they do not monitor all dietary supplements.
The information about a specific dietary supplement is provided on its label or
declaration and the costumer has to trust the producer to whom control and quality
assurance in the production of dietary supplements is left (DRI, 2000).
Food supplements contain a variety of different vitamins and minerals in different
chemical forms, which can interact with each other and make the determination of an
individual element (mineral) very difficult. For example, the presence of ascorbic acid
(vitamin C), a reducing agent, can change the chemical form of the element selenium
From the point of view of human nutrition and metabolism the fact that some minerals
decrease the absorption of others is important, because they compete for the same
binding sites, and transport pathways in the cell. Iodine absorption is reduced by iron,
calcium and magnesium. Iron may impair zinc absorption, which is needed especially
by lactating women for milk production (Carolyn et. al., 2002; Ubom, 1991).
The aim of our work was to determine the content of iodine in some commercially
available food supplements on the Slovene market and to find out whether consumers
can trust the declared values on the label. To our knowledge there are no data
concerning iodine in food supplements available in literature. Radiochemical neutron
activation analysis (RNAA) was employed as a sensitive, accurate and reliable
MATERIALS AND METHODS
From each food supplement ten to twenty tablets/capsules were taken and ground in an
agate mortar. Two to four aliquots of each homogenised sample of food supplement were
taken to determine the content of iodine.
The samples investigated, bought on the Slovene market in period of two years (2003-2005),
were as follows: ABC plus® (3 lots), Centrum from A to Zinc® (3 lots), Centrum Junior® (3
lots), Daily one caps®, Sport fuel with Iron®, Unicap M® (3 lots) and Unicap T® (3 lots). These
so called multivitamin-mineral supplements contain different vitamins and minerals (Table 1)
and are intended to provide balanced nutritional support for children, adults and sportsmen.
248 Acta agriculturae Slovenica, 87 - 2, september 2006
Table 1: Labelled content of each tablet/capsule of food supplement
Vitamin A (µg)
Vitamin C (mg)
Vitamin D (µg)
Vitamin E (mg)
Vitamin K (µg)
Vitamin B6 (mg)
Folic Acid (µg)
Vitamin B12 (µg)
Pantothenic Acid (mg)
B, Ni, Si, Sn, V, Lutein and Lycopene
Para-aminobenzoic Acid, Choline Bitartrate and Inositol
Para-aminobenzoic Acid, Choline Bitartrate, Inositol, Co-Enzyme Q10, L-Glutathione, N-
Acetyl-Cysteine, Alpha Lipoic Acid and L-Carnitine
Iodine determination method
In brief, samples were heat sealed in a plastic tube and irradiated simultaneously with an
appropriate aliquot of standard iodine solution in a rabbit for approximately 20-30 seconds in
the pneumatic system of our TRIGA Mark II Reactor at a neutron fluence of 4 x 1012 n·cm-2 s-
1. The irradiated samples were then combusted with iodine carrier in an oxygen atmosphere
(4L Schöniger flask) containing a reducing acid solution (0.05 M H2SO4 and 10 % Na2SO3),
followed by extraction and purification of iodine by a selective extraction-stripping-reextraction
cycle via NaNO2/CHCl3 and Na2SO3. The purified organic solution of iodine in CHCl3 was
transferred to a 10 mL vial for gamma-ray measurement of the induced radionuclide
128I(127I(n,?)128I; t½ = 25 min, E? = 443 keV).
The chemical yield for each sample aliquot was determined spectrophotometrically at 517 nm
with an MA 9525 – Spekol 210 spectrophotometer from the recovery of the added carier. The
method for iodine determination by RNAA is described in more detail elsewhere (Dermelj et.
OSTERC, A., STIBILJ, V.: Investigation of declared values of iodine in food ... 249
RESULTS AND DISCUSSION
The critical step in iodine determination is sample decomposition, because iodine is
highly volatile and the reagents used should have a low blank and not cause
interference with the measurement. Procedures such as dry (alkaline) ashing, wet
ashing using oxidizing acids or acid mixtures and combustion with oxygen in closed
systems are used. For measurement of iodine at µg/g levels ion selective electrodes,
X-ray fluorescence, ion chromatography, cathodic stripping voltammetry, ICP-MS
and radiochemical neutron activation analysis are suitable, but for its determination in
foods only the high selectivity ICP-MS and RNAA have low enough detection limits
(Knapp et al, 1998).
The main advantage of the RNAA method used is that the sample is not subjected to
any chemical treatment before irradiation, so the possibility of sample contamination
is minimal, i.e. virtually blank free. The method is not time consuming but costly and
requires special equipment. During irradiation the iodine present in the sample (and
standard) is activated via the (n, ?) nuclear reaction induced by thermal neutrons to
128I, and then extracted from the combusted matrix during the radiochemical
purification with chloroform via the redox reaction with NaNO2 in H2SO4 medium.
Stripping of elemental iodine from the organic phase with Na2SO3 followed to
separate iodine from any traces of bromine and chlorine. The purified iodine was then
re-extracted into chloroform. The separation step is performed after irradiation,
meaning that there is no influence from the reagents used for the isolation of iodine on
the activity of 128I separated. The detection limit of the method is about 1 ng/g. There
is no reference material with a similar matrix to food supplements on the market. So
we tested the method with the reference material NIST 1549 Whole milk powder.
Good agreement with the certified value was obtained (3.37 ± 0.12 µg/g, certified
value 3.38 ± 0.02 µg/g).
The levels of iodine in seven investigated food supplements ranged from 21 to 231 µg
per g and they were in most cases, with the exception of three (A, F and G), in
agreement with the declaration made by the producer (Table 2). The levels of iodine
were constantly lower for food supplement A found in period of two years. Two food
supplements (A and G) were not in agreement with the recommendations made by
U.S. Pharmacopoeia, that dietary supplements should contain not less then 90 % and
not more then 200 % of the declared amounts of iodine (U.S. Pharmacopoeia, 2004).
Variability between each batch of production was observed. A possible explanation
for this difference could be, as already mentioned in the Introduction of the present
work, the huge variety of different vitamins and minerals present in food supplements.
Vitamins and minerals can interact with each other and thereby change their
chemico–-physical properties, causing loss of some vitamins, and/or minerals by
making them volatile (Table 2).
250 Acta agriculturae Slovenica, 87 - 2, september 2006
Table 2: Comparison of levels of iodine determined in food supplements and those
declared by the label
according to U.S.
determined and Pharmacopoeia
62.90 ± 3.50
77.74 ± 4.03
97 – 216
55.43 ± 2.34
104.81 ± 7.82
102.96 ± 5.16
94 – 208
112.50 ± 6.16
28.20 ± 1.39
37.00 ± 1.84
27 – 60
35.77 ± 2.40
212.00 ± 11.00
207.27 ± 11.22
208 – 463
227.64 ± 15.12
126.71 ± 6.51
121 – 268
123.14 ± 6.37
129.49 ± 7.53
230.77 ± 12.02
140 – 310
21.13 ± 0.74
69.89 ± 2.60
14.65 ± 0.54
74 – 165
56.96 ± 2.10
36.17 ± 1.20
Results are given with measurement uncertainty (k=2.0)
a) Numbers represent different lots, letters represent different food supplements
Sample G caused problems. The scattering of results was very high. We made five
determinations and the results are listed in Table 2. As shown, they range from 21 to
70 µg per g, and the iodine content is evidently non-homogeneous.
Amount of iodine consumed based on the determined and declared values is presented
in Table 3. One can see that, with the exception of food supplement A and F, there is
good agreement between the actual amount of iodine consumed and the declared
OSTERC, A., STIBILJ, V.: Investigation of declared values of iodine in food ... 251
values as labelled by the producer. This result is much better than in the case of study
of the iodine content of infant formulae (Osterc et al., 2005) where infant starting,
follow-on and special formulae were investigated. From the results obtained it was
clear that only three from eighteen investigated infant formulas were in good
agreement with the declared values.
The same food supplements were analysed by Stibilj et al. (2005) to investigate the
declared value of selenium. They found that two of the nine food supplements do not
comply with the U.S. Pharmacopoeia and also revealed variability between different
lots of the same brand. Their findings for selenium are comparable to ours for iodine
in the same food supplements.
Table 3: Comparison of iodine intakes from food supplements based on declared
values and experimentally determined values
iodine consumed iodine consumed
87.48 ± 4.87
108.11 ± 5.60
77.10 ± 3.25
150.83 ± 11.25
148.17 ± 7.43
161.89 ± 8.86
61.49 ± 3.03
80.69 ± 4.01
78.00 ± 5.23
134.21 ± 7.26
147.40 ± 9.79
141.83 ± 7.29
137.83 ± 7.13
144.94 ± 8.43
223.13 ± 11.62
Results are given with measurement uncertainty (k=2.0)
b) Numbers represent different lots, letters represent different food supplements
Rasmussen et al. (2002) investigated the effect of geography, supplements and food
choice on iodine intake in a Danish population. They estimated the iodine intake by
measuring the iodine excretion in urine samples. The iodine excretion was
252 Acta agriculturae Slovenica, 87 - 2, september 2006
significantly higher in participants who took a daily dietary supplement with 150 µg
of iodine in form of a multivitamin-mineral supplement and slightly higher in
participants who consumed at least 200g of fish/week and 0.5 L of milk/day, but who
did not get enough iodine. Rasmussen et al. concluded that the iodine deficiency
problem could not be overcome by dietary changes .
Haldimann et al. (2005) investigated the iodine content of several kinds of foods
representing different product groups available on the Swiss market. The samples
included foods such as cereals, meat, dairy products, fruit and vegetables. They
estimated per capita iodine intakes from the investigated foods using statistics on food
consumption and the corresponding mean iodine concentrations. They found bread
and milk to be significant daily sources of iodine in the Swiss diet, contributing 58
and 29 µg/day, respectively. The sum of the contributions of all basic food groups to
the total dietary iodine intake for the average consumer was estimated to be
approximately 140 µg/day, which is comparable to the food supplements investigated
in this study.
The intake of iodine per day declared by the producer was not always in agreement
with the actual contents of the supplements found experimentally. It would make
sense to introduce controls in the field of food supplements so as to assure the safety
and quality of these products.
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