Iodine Content of the Best-Selling United States Adult and Prenatal Multivitamin Preparations

Introduction

Iodine is essential for the production of thyroid hormone and normal fetal development. Severe iodine deficiency is associated with cretinism, impaired neurodevelopment, and growth retardation in fetuses and infants (1,2). Even mild maternal iodine deficiency has been associated with neurocognitive deficits in offspring (3 –5). Multiple studies have demonstrated higher iodine requirements during pregnancy and lactation because sufficient iodine is needed for increased thyroid hormone synthesis during gestation and for secretion into breast milk (2,6,7).

In the United States, sources of iodine are not labeled on most food packaging and have been challenging to identify. Dairy products have been identified as a major food source of iodine, but dairy intakes are highly variable across the U.S. population (8). Seafood is another important iodine source, but both content and intake are highly variable (6,9,10). While universal salt iodization has been a successful intervention to prevent iodine deficiency in many regions of the world, iodization of salt has never been mandated in the United States. Salt in U.S. processed foods typically is not iodized, and currently only 53% of salt sold for household use in the United States contains iodine (11). Despite iodine sufficiency in the general U.S. population (12), the high variability of iodine content in food sources and differences in dietary patterns may leave some populations at risk for iodine deficiency. In particular, mild iodine deficiency has recently reemerged among pregnant U.S. women (10). Consequently, the American Thyroid Association, Endocrine Society, and American Academy of Pediatrics have all recommended that U.S. women who are planning pregnancy, pregnant, or lactating should ingest a daily supplement containing 150 μg of iodine (13 –15).

The importance of multivitamin supplements as a source of iodine for pregnant and nonpregnant U.S. adults is not well understood. From 2011 to 2012, 52% of U.S. adults and 46% of women of childbearing age reported use of multivitamin supplements within the past 30 days, of whom 20% and 14%, respectively, reported taking a supplement containing iodine (16). A systematic review of studies on multivitamin supplementation in the United States and Canada reported that supplement use ranged from 7% to 85% (17), exemplifying the difficulty of accurately determining the extent of multivitamin use. The proportion of nonpregnant U.S. adults taking iodine-containing multivitamins has not been reported. Analysis of the National Health and Nutrition Examination Survey (NHANES) data set from 1999 to 2006 showed that 77% of pregnant women reported supplement use (18). A more recent analysis of the NHANES data set from 2011 to 2014 showed that although 72.2% of 122 pregnant women reported using a supplement, only 17.8% used iodine-containing supplements (19). In addition, it was recently reported that only 60% of the different types of prenatal multivitamin (PMV) supplements marketed in the United States list iodine among their ingredients (20). It has also previously been shown that iodine content is the most variable and least accurately labeled nutrient contained in U.S. adult multivitamins (AMV) (9).

It was not previously possible to identify the market share for iodine-containing prenatal vitamin preparations. In order to assess whether multivitamins are a significant source of iodine in nonpregnant and pregnant adults, the proportion of the most frequently purchased U.S. AMV) and PMV preparations that contain iodine were quantified.

Methods

Information Resources, Inc. (Chicago, IL), a market research firm, provided information about the 99 U.S. AMV and 60 U.S. PMV with the largest market share from July 2016 to July 2017. Supplements purchased at food, drug, and mass merchandise stores, club stores, value chains, and military stores were included. However, sales by direct selling and specialty stores and over the Internet were not ascertained. Product names, dollar sales, unit and volume sales, and recommended daily intakes were provided. Product labels were used to determine the listed iodine content and iodine source for each of the multivitamins. Ten AMV represented private-label brands for which iodine content could not be determined and were therefore excluded. One PMV was not analyzed because its ingredient label was unavailable and it was not possible to determine the iodine content, leaving a final sample of the top 89 AMV and 59 PMV.

Results

The sample of 89 AMV represented $924,436,225 in annual retail sales. Sixty-six (74.2%) of these brands contained iodine according to their labels. Of the AMV that contained iodine, 50 (75.8%) brands contained 150 μg of iodine per daily serving (PDS). The median (range) iodine content was 150 μg (38–150 μg) PDS. Over the year analyzed, 8,924,371,955 doses of AMV were sold. Of these doses, 7,564,022,778 (84.8%) doses contained iodine, and 6,959,113,896 (78%) doses contained ≥150 μg of iodine. All of the AMV listed potassium iodide as their source of iodine.

The PMV sample represented $128,388,155 in annual retail sales and 466,927,559 doses sold. Thirty-four (57.6%) brands of the 59 PMV contained iodine, with a median (range) iodine content of 150 μg (25–290 μg) PDS. Thirty-one (91.2%) iodine-containing PMV brands had an iodine content of ≥150 μg PDS, of which only one contained >250 μg PDS. Of the doses of PMV sold, 358,764,603 (76.8%) contained iodine and 354,225,813 (75.9%) contained ≥150 μg iodine PDS. Twenty-five (73.5%) of the iodine-containing PMV brands used potassium iodide, eight (23.5%) used kelp, and one (2.9%) used inactivated Saccharomyces cerevisiae as the source of iodine.

Discussion

This study provides novel data on the scope of multivitamin use and more accurate estimates of multivitamin iodine content and consumption by adults in the United States. Data regarding sales of iodine-containing multivitamins have not been available previously. The results demonstrate that multivitamins are used by a substantial proportion of the population and can be an important source of iodine for pregnant women. The findings that AMV represent an approximately $1 billion business were consistent with previous studies that estimated high use of multivitamins in this country (17,21,22). Despite the wide range of iodine content found in the top 99 AMV, the majority contained 100% of the recommended daily allowance (RDA) for nonpregnant, non-lactating adults. U.S. census estimates suggest that the U.S. adult population in July 2017 included 246,324,986 people (23). After excluding 3,853,472 births in 2017 as an approximation of the number of pregnant women (24), it is estimated that the nonpregnant U.S. adult population in 2017 was about 242,471,514. Based on these data, approximately 9% of the iodine required by nonpregnant adults over the course a year can be accounted for by AMV sold over that period. There are no recommendations for iodine supplementation for U.S. adults who are not pregnant, lactating, or planning a pregnancy. However, it may be reasonable for U.S. adults who do not consume dairy or use iodized salt to consider daily use of iodine-containing supplements.

A smaller proportion of PMV doses sold contained any iodine compared to the AMV doses sold, despite the fact that supplementation is specifically recommended for the population consuming prenatal supplements. The Council for Responsible Nutrition has previously recommended that all of their manufacturers include ≥150 μg of iodine PDS in their PMV (25), but this has not to date been universally adopted. The majority of iodine-containing PMV contained 100% of the iodine RDA. The single PMV formulation, representing 0.004% of the top-selling PMV doses sold, containing >250 μg iodine PDS could potentially pose a risk for excess iodine exposure, depending on concomitant dietary iodine intake (26).

Some PMV used kelp and inactivated yeast as sources of iodine, although these were not employed in any of the AMV products. These sources have been previously shown to be variable in their iodine content (9,27). While it is unclear why PMV were less likely to use potassium iodide as an iodine source than AMV, it is possible that kelp or inactivated yeast may be seen as more appropriate for those PMV that were marketed as “natural” or “organic.”

This study is the first assessment of the iodine content in AMV and PMV with the largest market share in the United States. This study has several limitations. The study relied on manufacturers' listed iodine content, which may not match labeled iodine content (9). Although the majority of multivitamin products purchased over the year studied were included, it was not possible to account for supplements that were sold over the Internet and in specialty stores. It was possible to determine how many doses were purchased, but it was unclear how many doses were actually consumed.

In conclusion, multivitamin products appear to be a significant source of iodine nutrition for many U.S. adults. However, despite recommendations for use of daily iodine supplements by U.S. women who are planning pregnancy, pregnant, or breastfeeding (13 –15), 24% of the top-selling prenatal multivitamins do not contain iodine.