Even swapping a small amount of dairy for plant-based alternatives could leave people short of essential nutrients, with teenage girls and women particularly at risk, scientists have warned.
Researchers found that replacing traditional milk, cheese and yoghurt with vegan alternatives could significantly reduce the intake of iodine, calcium, vitamin B12 and riboflavin, which are important for maintaining good health.
The study, published in The Lancet Planetary Health, raises particular concerns about iodine, a mineral needed for healthy thyroid function, growth and brain development.
Scientists from the University of Surrey and Queen's University Belfast examined dietary data from more than 3,500 people aged between 18 months and 100 years.
They found that replacing just 20 to 35 per cent of dairy products with plant-based alternatives could increase the proportion of teenage girls aged 14 to 18 with inadequate iodine intake from 54 per cent to 65 per cent.
Among women aged 19 to 49, the figure could rise from 32 per cent to 42 per cent.
The researchers warned that the nutritional risks could be even greater for those who completely remove dairy products from their diets.
Their modelling suggested that inadequate iodine intake could affect 80 per cent of teenage girls and 72 per cent of teenage boys if dairy products were fully replaced.
Iodine is particularly important during pregnancy, when the body needs more of the mineral to support the baby's brain development.
Dr Sarah Bath, associate professor in public health nutrition at the University of Surrey, warned that "plant-based alternatives, unless fortified, have a very low iodine content."
The researchers also examined 289 plant-based milk, cheese and yoghurt products available in the UK and found that only 12 per cent contained added iodine.
Although some alternatives were fortified with calcium and vitamin B12, iodine was much less commonly added, raising concerns about whether consumers were receiving comparable nutritional benefits.
The findings have prompted researchers to call for mandatory fortification of plant-based dairy alternatives with essential vitamins and minerals, particularly iodine.
They also suggested that adding iodine to bread could help address nutritional shortfalls among vulnerable groups.
Dr Katie Nicol, a researcher at the University of Surrey, stressed the need for healthier dietary policies as more people turn towards plant-based eating.
She said "government policy is needed to make this happen."
However, experts cautioned that the findings were based on computer modelling rather than confirmed cases of nutrient deficiency.
Professor Ian Givens, emeritus professor of food chain nutrition at the University of Reading, said the results showed "a risk of inadequacy rather than confirming deficiency".
The researchers emphasised that plant-based alternatives can still form part of a balanced diet, but their nutritional value varies considerably depending on the product.
Consumers choosing dairy-free milk, cheese or yoghurt are therefore encouraged to check nutritional labels and look for products fortified with iodine and other essential nutrients.
