Ditching dairy for plant-based milks may leave you short on iodine, study finds

Swapping milk, yogurt and cheese for plant-based stand-ins may quietly strip key nutrients out of your diet — and iodine takes the hardest hit, according to a new study from the University of Surrey. The research, run with colleagues at Queen's University Belfast, is the first of its kind in the United Kingdom and lands in The Lancet Planetary Health.

The team pulled data from the National Diet and Nutrition Survey, covering more than 3,500 people from 18 months to 100 years old, then built a modeling technique to simulate what happens when dairy gets swapped out for the plant-based alternatives already sitting on supermarket shelves. They tracked four nutrients dairy is known for: iodine, calcium, riboflavin and vitamin B12.

Even partial swaps moved the needle fast. When 20% to 35% of dairy was replaced with unfortified alternatives at current market fortification levels, iodine inadequacy among adolescent females climbed to 65%, up from 54%. For women aged 19 to 49, it rose to 42%, up from 32%. Go all the way and ditch dairy entirely, and inadequacy reaches 80% in adolescent females and 72% in adolescent males.

Dr. Sarah Bath, associate professor at Surrey and lead author, said the U.K. is especially exposed because it has no iodine fortification policy like the universal salt iodization many other countries use. She noted iodine is needed to make thyroid hormones that regulate metabolism and support growth and brain development — and that it matters before and during pregnancy. Dairy is a major iodine source in the U.K.; unfortified plant alternatives barely register.

The researchers also tested fixes. Forcing mandatory fortification of plant-based dairy alternatives stopped the overall nutrient slide, though iodine still came up short for 59% of adolescent females. Iodine-fortified bread did better, cutting that inadequacy to 37%. The strongest result came from combining both: fortified plant alternatives plus iodine-fortified bread.

Co-author Dr. Katie Nicol, a research fellow at Surrey, said nutrition policy has to move alongside environmental goals — sustainable diets shouldn't come at the cost of public health. Fortification is one route, she added, but it takes government policy to make it happen. Dr. Anne Nugent of Queen's University Belfast explained the method: the team flagged every dairy-containing food and recipe in the national survey, substituted equal weights of plant-based alternatives, gave each product an equal shot at selection, then tested whether fortification could close the gaps without overshooting intake.