Plant-based foods have become a familiar part of the supermarket landscape, but a new study suggests that replacing dairy is not always a simple nutritional swap.

Researchers from the University of Surrey in the United Kingdom, working with colleagues at Queens University Belfast, Ireland, examined what could happen to people's micronutrient intake when milk, yogurt and cheese are replaced with commercially available plant-based alternatives.

Removing a food from the diet can remove nutrients along with it.

Their findings point to iodine as a particular concern, especially for adolescent girls and women of childbearing age.

The researchers used dietary information from the UK National Diet and Nutrition Survey covering more than 3,500 people ranging in age from 18 months to 100 years. Rather than asking participants to change what they ate, the investigators used a dietary modeling technique to simulate what would happen if dairy foods in their diets were replaced with plant-based products.

They focused specifically on four nutrients supplied by dairy: iodine, calcium, riboflavin and vitamin B12.

The researchers also considered the wide variation among plant-based products by modeling replacements using commercially available alternatives.

The study found that even partial substitution could reduce intake of all four nutrients. Iodine was especially sensitive.

When 20 to 35 percent of dairy consumption was replaced under current levels of product fortification, the proportion of adolescent females with an adequate iodine intake rose from 54 percent to 65 percent. Among women ages 19 to 49, it increased from 32 percent to 42 percent. With complete replacement, the model projected inadequate iodine intake in 80 percent of adolescent females and 72 percent of adolescent males.

Why does iodine matter? The mineral is required to make thyroid hormones, which help regulate metabolism and are important for normal growth and brain development. Iodine needs also increased during pregnancy, making adequate intake particularly important for women who may become pregnant.

“People are also unaware that dairy products are a major source of iodide [a form of iodine], particularly in the UK, and that plant-based alternatives, unless fortified, have a very low iodine, content," the study's lead author, Sarah Bath, a registered dietician and associate professor of public health nutrition at the University of Surrey, said in a press release.

A plant-based diet can be nutritionally adequate, but replacing dairy requires careful attention to where lost nutrients will come from. Going dairy-free does not have to mean going nutrition-poor.

The investigators also developed a model of a potential solution: mandatory fortification of plant-based dairy alternatives. This prevented the declines in micronutrient intake caused by dairy replacement, but it did not completely solve the iodine problem. Iodine inadequacy remained at 59 percent among adolescent females in the model.

Iodine-fortified bread made a larger difference in the model, it reduced iodine inadequacy among adolescent females to 37 percent. The researchers concluded that combining mandatory fortification of plant-based dairy alternatives with iodine-fortified bread could be the most effective strategy for reducing inadequacy.

Choosing oat, soy, almond or other plant-based alternatives will not automatically make someone nutrient-deficient. The study was based on modeling — the researchers were estimating what could happen to nutrient intake under different replacement scenarios; it was not a clinical trial. It does point to deficiencies likely to creep in when dairy is removed from a person's diet.

For people who choose plant-based alternatives to milk and other dairy products, the practical message is to read the nutrition label rather than assuming that every product is nutritionally equivalent to dairy. Look specifically for iodine as well as calcium, vitamin B12 and riboflavin. Different food products can differ substantially in what they contain.

Combining mandatory fortification of plant-based dairy alternatives with iodine-fortified bread could be the most effective strategy for reducing the most serious nutritional inadequacy seen in plant-based diets.

The findings also underscore a broader point. Removing a food from the diet can remove nutrients along with it. A plant-based diet can be nutritionally adequate, but replacing dairy with plant-based options requires that you pay careful attention to where lost nutrients will come from. Going dairy-free does not have to mean going nutrition-poor. The nutrients that were supplied by milk, yogurt and cheese have to come from somewhere. Checking labels and maintaining a varied diet can help make that transition a healthier one.

The study is published in The Lancet Planetary Health.