Swapping dairy for plant-based milk is projected to push iodine deficiency in teen girls from 54% to 65%

Public health advice about dairy has spent the last decade mostly pointing one direction: plant-based alternatives are fine, sometimes better, for the planet and for people who can't or don't drink milk. A new modeling study puts a number on what that swap costs for one specific nutrient, and it is not a small number for the group most at risk.

The study

Researchers at the University of Surrey and Queen's University Belfast, publishing in The Lancet Planetary Health, combined two kinds of data. First, they surveyed 289 plant-based dairy alternatives on the UK market, milks, yogurts and cheeses, for their iodine, calcium, vitamin B12 and vitamin B2 content as sold. Second, they used dietary intake data from more than 3,500 people ranging from 18 months old to 100 years old to model what happens to population-wide nutrient adequacy under different levels of dairy substitution.

This is a modeling study, not a clinical trial and not a measurement of deficiency in people who have actually switched their diets. It estimates what current UK diets would look like, on paper, if a given share of dairy intake were replaced with plant-based products at today's fortification rates. That distinction matters for how much weight the numbers below should carry.

The finding

Only 12% of the 289 products surveyed contained any added iodine, compared with calcium (61%), vitamin B12 (46%) and vitamin B2, also called riboflavin, (19%), the other nutrients dairy reliably supplies. Iodine was the clear outlier, and the one the researchers flagged as the main concern.

Modeling a swap of 20% to 35% of dairy intake to plant-based alternatives at current fortification levels raised the projected share of 14-to-18-year-old girls with inadequate iodine intake from 54% to 65%. Women aged 19 to 49 went from an estimated 32% to 42%. Modeling a full replacement of dairy pushed the estimate for teenage girls to 80%, and for teenage boys of the same age to 72%.

Even with fortification added to every product, the model did not fully close the gap: the researchers estimated roughly 59% of adolescent girls would still fall short of adequate iodine intake under a fully fortified scenario. Their modeling suggested the combination that closed the most ground was mandatory fortification of plant-based dairy alternatives paired with iodine-fortified bread, a strategy the UK does not currently use for either product category.

What this design can and cannot tell you

Every number above comes from a model applied to existing dietary survey data, not from measuring thyroid hormone or urinary iodine in people who have made the swap. It assumes current eating patterns stay otherwise the same and current fortification rates hold, and it cannot capture an individual who deliberately seeks out an iodine-fortified plant milk or who eats other iodine sources, like fish or iodized salt, to compensate. A model is only as good as the substitution scenario fed into it, and a 20% to 35% dairy swap is a projection of a trend already underway in the UK and US, not a guarantee of how any one household's diet changes.

None of that makes the direction of the finding surprising. Iodine in dairy milk is a side effect of cattle feed and farm sanitizing practices, not a property of milk itself, so an alternative made from oats, almonds, soy or rice starts with structurally less of it unless a manufacturer adds it back. The study's contribution is putting a population-scale number on a gap that nutrition researchers have flagged qualitatively for years, in a group, adolescent girls and women of childbearing age, where the cost of getting it wrong falls hardest.

Iodine content of common foods

Iodine content varies enormously by source, and most nutrition labels don't list it at all unless a manufacturer has added it. Ordered by amount per typical serving, using the NIH Office of Dietary Supplements' food table:

• Enriched white bread made with an iodate dough conditioner, 2 slices: 296 mcg (197% of the daily value), when that specific conditioner is used
• Cod, baked, 3 oz (85 g): 146 mcg (97% DV)
• Greek yogurt, plain, nonfat, ¾ cup: 87 mcg (58% DV)
• Milk, reduced-fat, 1 cup: 84 mcg (56% DV)
• Iodized table salt, ¼ teaspoon: 78 mcg (52% DV)
• Fish sticks, cooked, 3 oz: 57 mcg (38% DV)
• Egg, hard-boiled, 1 large: 31 mcg (21% DV)
• Enriched pasta, boiled in water with iodized salt, 1 cup: 30 mcg (20% DV)
• Shrimp, cooked, 3 oz: 13 mcg (9% DV)
• Tuna, canned in water, drained, 3 oz: 7 mcg (5% DV)
• Seaweed (kelp, nori, kombu, wakame): the single most concentrated source, but also the least predictable; commercially available seaweed ranges from about 16 mcg to nearly 3,000 mcg per gram depending on species and processing, enough to swing from negligible to well past the daily upper limit in one serving
• Lima beans, boiled, ½ cup: close to 0 mcg, despite appearing on many "iodine-rich foods" lists online
• Plain bread made without an iodate dough conditioner, 2 slices: about 1 mcg, since most US and UK bakeries now use other dough conditioners

That last pair is the detail easy to miss: two loaves of bread that look identical on a shelf can differ by nearly 300 mcg of iodine depending on which dough conditioner the bakery uses, and almost nothing on the package tells a shopper which one they bought.

The recommended intake

The US and UK recommended dietary allowance (RDA) for iodine, from the NIH Office of Dietary Supplements:

• Infants 0–6 months: 110 mcg/day (Adequate Intake, a softer standard than an RDA)
• Infants 7–12 months: 130 mcg/day (AI)
• Children 1–8 years: 90 mcg/day
• Children 9–13 years: 120 mcg/day
• Teens 14–18 years: 150 mcg/day, the same as an adult
• Adults 19 and older: 150 mcg/day
• Pregnancy: 220 mcg/day
• Breastfeeding: 290 mcg/day

The upper limit (UL), the highest daily intake unlikely to cause harm, is 1,100 mcg/day for adults, and 900 mcg/day for teens 14 to 18, counting iodine from food, fortified products and supplements combined. These limits don't apply to iodine taken under a doctor's supervision for a medical reason.

Too little, and too much

Too little shows up first as a goiter, an enlarged thyroid gland straining to make enough hormone from too little raw material, often the first visible sign something is wrong. In adults, the NIH's health professional fact sheet notes that mild to moderate deficiency can cause goiter and, through the resulting hypothyroidism, impaired mental function and work productivity. The stakes rise sharply in pregnancy: the same fact sheet states that iodine deficiency of this magnitude can cause major neurodevelopmental deficits and growth retardation in the fetus, as well as miscarriage and stillbirth. That is the reason this particular modeling study singled out adolescent girls and women of childbearing age as the groups to watch.

Too much can, confusingly, cause some of the same problems as too little. The NIH's consumer fact sheet notes that high iodine intake can produce goiter by the same mechanism as deficiency, and the health professional version adds that excess intake can trigger thyroiditis and thyroid papillary cancer in susceptible people. Iodine supplements can also interact with antithyroid medications such as methimazole, pushing thyroid hormone production too low rather than too high. Harvard's Nutrition Source notes that goiter from excess iodine typically requires sustained intake well above 18,000 mcg a day, far beyond what anyone would get from food and fortified products alone, so this is mainly a concern for supplement users and people with existing thyroid disease rather than someone drinking fortified oat milk.

Tracking it in AIM

AIM does not track iodine as standard, so add it as a custom metric: give it a name, a unit (mcg), and a daily target of 150 mcg, 220 mcg if pregnant, or 290 mcg if breastfeeding, and it is estimated on every meal from then on, pinned to your cards or left behind "more." Any custom nutrient works the same way. If dairy makes up a meaningful share of your current diet and plant-based alternatives are replacing it, checking the label for "iodine" or "potassium iodide" in the ingredient list, most don't carry it yet, is the one habit this study's finding actually supports.

Estimates from a photograph carry real uncertainty for micronutrients, because content varies with soil, cultivar, storage and cooking in ways no image contains. Iodine is a harder case than most: it isn't on nutrition labels unless added, and this study's own 289-product survey is the kind of ingredient-list checking a photo cannot do at all. Treat the week as meaningful and any single day as approximate, and treat this study's projections the same way: a population-level model worth planning around, not a measurement of any individual's thyroid status.