As plant-based milk becomes a staple in UK diets, experts warn that without proper iodine fortification, consumers, especially young women, may face hidden health risks.
Study: Plant-based milk alternatives: can they replace the iodine from UK cow’s milk? Image credit: Garna Zarina/Shutterstock.com
As traditional food systems, often containing animal-source foods, are replaced by plant-based alternatives, questions arise about their nutritional value. A recent paper published in Proceedings of the Nutrition Society examined the iodine content of plant-based milk substitutes compared to cow’s milk.
Introduction
Cow’s milk is the major milk consumed in the UK, but its consumption has decreased over the last four decades, coinciding with increased use of plant-based milks like soy, grain, or nut milk. These choices, often driven by concerns about climate change, sustainability, and human health, have not been adequately assessed for their nutritional comparability.
Plant-based milk alternatives are chosen primarily by young women and can easily be substituted for cow’s milk. This consumption pattern raises public health concerns about potential iodine and other micronutrient deficiencies in this demographic, which may have implications for future generations.
The importance of iodine
Trace amounts of iodine are essential to synthesize the thyroid hormones thyroxine and triiodothyronine (T4 and T3, respectively). These hormones are key metabolism regulators necessary for developing and maturing the central nervous, muscular, skeletal, and respiratory systems.
Iodine deficiency disorders present as hypothyroidism, stillbirth, reduced mental function, and congenital anomalies. In some cases, excessive iodine supplementation can also cause hyperthyroidism, although this is less common.
Iodine deficiency in pregnancy is the primary cause of preventable infant brain damage. Congenital iodine deficiency disorder causes growth retardation and intellectual disability, as well as miscarriages, stillbirths, and infant deaths. Even subtle iodine deficiency in pregnancy affects the offspring’s cognitive and academic performance, increasing the risk of neuropsychological problems.
Iodine recommendations
The World Health Organization (WHO) recommends iodine intake of 90-120 μg of iodine per day up to 12 years, and 150 μg per day after that. For pregnancy and lactation, the recommended intake is 250 μg per day.
UK recommendations advise maintaining adequate intake throughout reproductive life, assuming that women can thus ensure sufficient iodine stores. Unlike WHO, the UK does not recommend an increased iodine intake during pregnancy, but this approach may not be sufficient if iodine intake has been low for an extended period. This may not be true in the UK and other countries where iodized salt is not the norm, and the WHO recommends iodine supplements for all women of reproductive age in such places.
Iodine status
Despite iodine sufficiency in children, dairy consumption falls with age. In 2011, a UK survey showed mild iodine deficiency among girls aged 14-15. Girls aged 11-18 were at 65% of the recommended intake, suggesting a high risk of abnormal neurodevelopment in their offspring.
The latest (2018/19) National Diet and Nutrition Survey (NDNS) report shows inadequate intake among adults. In 10% of all individuals and 12% of women, intake fell below that required to prevent goiter.
NDNS spot sample collection in years 9-11 also demonstrated inadequate iodine intake, with iodine deficiency in 21% of reproductive-age women. This fits the definition of population-level iodine deficiency in this group. Reasons may include their preference for plant-based diets or excluding seafood and dairy products, though nearly 80% of people who use milk alternatives also use cow’s milk. Therefore, the risk of deficiency depends on whether plant-based milks are used in addition to or as a total replacement for cow’s milk and the overall iodine content of the diet. Vegans who do not supplement with iodine or seaweed are another high-risk group.
Iodine sources
Salt iodization is not legally mandatory in the UK, and iodized salt is difficult to obtain. Iodine sources are mainly seafood, eggs, and dairy foods. Adequate intake would mean consuming one portion each of white fish and oily fish or seafood per week, with three portions of dairy per day. Thus, excluding dairy from the diet is a high-risk factor for iodine deficiency in the UK.
Fortified cattle feed and iodophor use during milking increase cow's milk's naturally low iodine content to 427 μg/l, or 85 μg per 200 mL glass of milk. This is 34% of the recommended intake for pregnant women, and 57% for adults. Conversely, a third of UK adults, mostly young adults, approximately 45% of people aged 16-34, and people with children younger than 18, use plant-based milks. These provide only about 2% of the iodine in cow’s milk when unfortified.
While iodine-fortified milk alternatives increased to 20% by 2020, most remain unfortified. When plant-based milk alternatives are fortified, their iodine content usually ranges from about 43% to 150% of cow’s milk, with most at around 50% of cow’s milk’s iodine content. Some fortified products are nearly equal to cow’s milk, but this is uncommon. The exclusive use of plant-based milk in the UK may cause iodine deficiency. However, the population risk varies depending on the presence of other iodine-rich foods or fortified milks and the stage of life.
Many dietary guidelines, such as the British Dietetic Association’s (BDA) One Blue Dot policy and the UK Eatwell Guide, ignore this risk, treating plant-based milks as interchangeable with dairy. These recommendations often encourage calcium fortification but do not specifically advise iodine fortification, which may lead consumers to believe these products are nutritionally equivalent to cow’s milk. Even the EAT-Lancet Commission’s reference diet passes over iodine as an essential micronutrient; thus, iodine content ranges from 6% to 196% of the adult recommended intake depending on the actual foods chosen in each category.
The issue and the way ahead
Modeling reveals that replacing dairy with plant-based milk consumption is associated with an increased risk of iodine insufficiency in children and women of reproductive age, unless fortified to about 27.4 µg/100 g. Further, based on current consumer behavior, iodine intake is projected to decrease by 18-44%.
Consuming only fortified plant milk alternatives at the highest level could cause excessive iodine intake for young children, though the consequences are little understood at present.
If plant-based milk is treated as equivalent to cow’s milk, a well-designed standardized fortification protocol is necessary to increase iodine intake effectively. Iodine bioavailability and proper mixing with the solution require further study. Consumers must be educated to accept fortified products.
To avoid overdosage, plant milk consumers can choose fortified milks, adequate amounts of other iodine-rich foods, or iodine supplementation with potassium salts (150 µg/d or less) rather than seaweed.
Conclusions
Sustainability alone cannot define the optimal diet. “The framing of plant-based milk as ‘alternatives’ to dairy milk is based on their intended use, without considering micronutrients, bioavailability, or the health effects of replacing cow’s milk with plant-based milk alternatives.” This can lead to iodine deficiency in the present and future generations.
Public health experts should guide consumers to use fortified foods or supplements. It is also important to recognize that the impact of replacing cow's milk with plant-based alternatives will depend on whether replacement is total or partial, the iodine content of other foods in the diet, and the life stage of the consumer. Research on plant milk is necessary to assess the bioavailability of added iodine and understand consumer perceptions and the population-level effects of fortification.
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