Read: The epic battle between breast milk and infant-formula companies

In a study published today in the journal Public Health Nutrition, Yoon Choi, Alexis Ludwig, and Jennifer Harris, all from the University of Connecticut, found that as public-health organizations have promoted breastfeeding over infant formula, formula manufacturers have invested more of their advertising dollars in toddler milk. The study authors used data from Nielsen to analyze toddler-milk and infant-formula sales and advertising from 2006 to 2015. They found that sales of toddler milk more than doubled in the study period, from $39 million in 2006 to $92 million in 2015. In the same time period, TV, print, and online advertising spending for toddler milk increased from less than $5 million annually to more than $20 million annually, while advertising spending for infant formula declined.

Sales were especially high for the brands that had contracts to provide infant formula to participants in WIC—the food program for low-income women, infants, and children. The reason is there might be “some sort of loyalty to the brand because you’re using the infant formula for your baby,” says Fran Fleming-Milici, the director of marketing initiatives at the University of Connecticut’s Rudd Center for Food Policy and Obesity, who was not an author of the study.

To the study authors, and to other child-health advocates, the rise of toddler milk suggests that there should be more restrictions on how aggressively the product can be marketed to mothers. “I don’t think it’s a coincidence that infant-formula sales and advertising decreased at the same time that toddler-milk sales increased,” Choi told me.

Buying toddler milk instead of regular milk could have real consequences for a child’s development. Choi and her co-authors write that toddler milk might train children’s palates to prefer more sugary beverages, which could be harmful to their long-term health. The packaging of toddler milk is also similar to that of infant formula, the authors say, which might lead mothers to accidentally serve the product to infants. Toddler milk does not contain the same nutrients as infant formula, so in large quantities it could leave infants malnourished.

In an email, Lynn Kenney, the head of North American communications for Enfagrow’s parent company, RB, disputed this characterization of toddler milks. “DHA is an important omega-3 fatty acid and a building block of the brain,” she said. “Because toddlers are still developing and rapidly growing, it’s important that DHA remain a key part of toddlers’ diets.” She also said that toddler milks are good for children who have “nutrient deficiencies or dietary restrictions,” and that the packaging of the company’s toddler milk is very different from that of its infant formula.