Iodine Content in Milk Alternatives

Introduction

Iodine is necessary for the production of the iodine-containing hormones thyroxine (T4) and triiodothyronine (T3). The Institute of Medicine's recommendation of iodine intake is 150 μg/day in adults, 220 μg/day in pregnant women, and 290 μg/day in breastfeeding women (1). Iodine deficiency may result in goiter, hypothyroidism, and decreased fertility. Adequate iodine intake is especially important for pregnant or nursing women, as iodine is crucial for neural development in the developing fetus and newborn infant. Maternal iodine deficiency is associated with cretinism, increased neonatal mortality, and a decreased IQ of 12 points on average (2,3). Iodine deficiency remains the leading cause of preventable mental retardation worldwide in 30 million children born annually (4).

Iodine deficiency is a global health issue. The World Health Organization (WHO) estimates >2.2 billion people may be iodine deficient, with up to 50 million suffering from symptoms of iodine deficiency (4). Urinary iodine levels are used to evaluate iodine intake. Those who are defined as iodine deficient have median urinary iodine concentrations of ≤100 μg/L in adults and ≤150 μg/L during pregnancy (4). The National Health and Nutrition Examination Surveys (NHANES) reports that the median urinary iodine concentration in U.S. adults decreased by >50% from 1971 to 1994 (5). At the same time, women of childbearing age had approximately a fourfold increase in moderate to severe iodine deficiency. Despite the stabilization of urinary iodine concentrations shown by subsequent studies (6 –9), the most recent NHANES survey (2005–2008) demonstrated that 56.9% of pregnant women may have low iodine intake (7), which remains a problem in the United States.

Dairy products are a primary source of U.S. dietary iodine (10). The average iodine content of milk is 110 μg/250 mL serving size, which is approximately one cup of milk. It is recommended to drink four cups of milk daily to meet dietary calcium requirements, which would provide 440 μg of iodine daily from cow's milk (10). This intake is 2.9 times the minimum recommended iodine intake for adults (10). Individuals with restricted dairy-product consumption are at risk for inadequate dietary iodine intake. In a cross-sectional survey of 700 teenage girls from the United Kingdom, those who drank no milk had daily iodine intake below recommendations and the lowest urinary iodine levels (11). Two major populations in the United States at risk for iodine deficiency are vegans, who avoid all animal products, and those who are lactose intolerant, who cannot consume dairy products due to their high lactose content. In 2013, there were approximately one million vegans in the United States, and 65% of the population worldwide is lactose intolerant. Veganism is on the rise in the United States, with Google trends reporting a threefold increase in vegan interest from 2005 to 2016 (12). Previous studies have shown that the mean urinary iodine concentration of vegans in the Boston area of the United States is 78.6 μg/L, which strongly suggests that vegans are mildly iodine insufficient (13). There are few data on the iodine status of those who are lactose intolerant. Those who do not drink cow's milk use other sources to achieve adequate calcium and protein intake, but often they are unaware of iodine insufficiency.

Milk alternatives such as soymilk, almond milk, and coconut milk are used to replace cow's milk in the diet of vegans, lactose-intolerant individuals, and others who do not consume milk. These milk alternatives contain no dairy or lactose and are made from plant-based products. A common type of milk alternative is made from soy. The consumption of soy, which contains isoflavones, genenstein, and daidzein, may be associated with iodine deficiency and hypothyroidism (13). The objective of the current study was to measure the iodine content in 30 different brands of milk alternatives from 16 different companies to determine whether milk alternatives offer comparable iodine levels to that of cow's milk.

Materials and Methods

Thirty different brands of milk alternatives were obtained from local supermarkets. Total iodine concentrations were measured using the chloric acid digestion with spectrophotometric detection of the Sandell–Kolthoff reaction method. Iodine concentrations from all samples were measured three times. In all of the samples, the three measurements were within 10% of one another. The values are represented as the mean ± SD. Whole cow's milk from Garelick Farms and 100% lactose-free, fat-free cow's milk from Lactaid were measured to serve as our positive controls. Recovery rates of iodine in accuracy assessments were between 88% and 106%. The detection limit was 1.2 μg/L. The interassay coefficient of variation was 3.4%.

Results

A total of 30 brands of milk alternatives were analyzed from 16 different companies. The types of milk alternatives were derived from a variety of nuts and plants such as soy, walnut, coconut, cashew, rice, and others (Table 1). The companies producing these milk alternatives are located throughout the United States.

The 30 brands of milk alternatives contained an average of 3.1 ± 2.5 μg/250 mL (∼8 oz. or one cup) of iodine. Iodine content ranged from 0.5 μg/250 mL to 10.9 μg/250 mL (Table 1). Whole cow's milk from Garelick Farms and 100% lactose-free, fat-free cow's milk from Lactaid had iodine levels of 96.8 μg/250 mL and 101.1 μg/250 mL, respectively, similar to previously measured cow's milk iodine levels of 116.0 ± 22.1 μg/250 mL (10). Since four cups of milk daily are recommended to meet the daily calcium intake, the iodine content in four cups of milk alternatives was calculated. The 30 brands of milk alternatives contained iodine levels of 12.3 ± 10.1 μg/L (∼24 oz. or four cups).

Discussion

The iodine content in 30 different brands of milk alternatives is 3.1 ± 2.5 μg/250 mL, which is significantly lower than previously measured cow's milk iodine levels of 116.0 ± 22.1 μg/250 mL (10). The recommended intake for adequate calcium nutrition of four cups daily would provide 12.3 ± 10.1 μg for milk alternatives, which is 12 times less than the daily recommended intake for adults. Iodine supplementation of cattle feed and local application of iodophor to teats and collecting vessels contribute to the iodine content of cow's milk (10). Thus, iodine levels in milk alternatives are far lower than the iodine content in cow's milk.

These results help identify one cause of low iodine consumption in vegans reported in previous studies (13) and suggest further iodine screening of those who have restricted dairy-product consumption and who may consume high amounts of milk alternatives. For those who are lactose intolerant, lactose-free milk may be a better option than plant-based milk alternatives in providing sufficient iodine intake.

Although this study reports results in the United States, it is likely that milk alternatives produced worldwide would be iodine deficient as well. Further research should be conducted to understand better the iodine status of milk alternatives and its effects on people who cannot drink cow's milk. Including iodine content on food labels would give consumers more information about their daily iodine intake. For better thyroid health of the U.S. population who do not drink cow's milk, it may be beneficial to add iodine to milk alternatives to achieve adequate levels of iodine from their diet. It is recommended that individuals who do not drink cow's milk take a daily multivitamin containing 150 μg of iodine, as has been recently recommended for women planning pregnancy, who are pregnant, or who are breastfeeding (14,15). Increased awareness of the importance of adequate iodine nutrition and proper action to ensure iodine sufficiency in U.S. populations is strongly recommended.