Persuading kids to eat more vegetables may be surprisingly simple yet somewhat counterintuitive, according to new research from The Pennsylvania State University: Doubling the portion sizes of broccoli and corn led preschool children to eat 68% more vegetables, but adding butter and salt did not change the amount consumed.

The study, published April 6 in Appetite, used a novel approach to examine both portion size and enhanced flavor. The Penn State researchers served a custom lunch to 67 children, ages 3 to 5, at two child care centers in State College, Pennsylvania, and employed kid-friendly methods to get results. "We frame the tasting assessment as a tasting game with cartoon faces," Ph.D. candidate Hanim E. Diktas, who was the lead author of the paper, told The Academic Times. "The kids love it when we come, they call it 'Restaurant Day' and get really excited," added Barbara J. Rolls, a professor of nutritional sciences at Penn State and a co-author of the study.

In the United States, 73.6% of people above the age of 20 are either overweight or obese. Previous research has shown that children who eat fewer vegetables are more likely to be overweight and stay overweight later in life. Both of these conditions can lead to medical issues such as heart disease, cancer and strokes. The World Health Organization has estimated that low fruit and vegetable consumption was responsible for about 3.9 million deaths in 2017. Yet most American kids don't eat the daily serving of vegetables recommended by the U.S. Department of Agriculture. Getting kids to eat vegetables remains a "big challenge," Diktas and Rolls said.

"When you give bigger portions of virtually every type of food except vegetables, it consistently leads to increased intake," Rolls noted. In a previous study, she found that broccoli consumption had the smallest change when children were served larger amounts of bread, pasta, grapes and vegetables in the same meal. Other research has shown that consumption is highly dependent on how vegetables are served. Perhaps unsurprisingly, french fries and ketchup were more popular than baked potatoes and tomatoes.

Choosing a vegetable that is well-liked and familiar to children for a study can be a delicate balance. Diktas said that they "tried everything" to find the right ones, even cutting carrots into different shapes to make them more visually attractive (though carrots did not make the final cut). Broccoli and corn were the most liked by the children, and both were served at least once a week at the child care center already.

The researchers used a two-by-two crossover design that compared four different meals: two with varying amounts of vegetables, and two with different seasonings. This method allowed them to present the choices in a randomized way, making it well-suited for young children who are affected by small changes in a study. 

The children were served either 60 grams or 120 grams of vegetables, offered either plain or with specific amounts of light butter and salt. These common condiments have been found to increase the palatability of foods — in other words, how appealing and tasty foods are perceived to be. The rest of the meal consisted of fish sticks, ketchup, rice, applesauce and milk. As young kids are very impressionable and can have a big influence on their peers, teachers helped steer the conversation away from food during the meal.

After everyone was finished eating, the team weighed the foods left on the plate. The children rated different versions of the vegetables by pointing at "yummy," "just okay," or "yucky" cartoon faces. Diktas explained that this method makes the study feel more like a game, and Rolls noted that kids are "a more captive audience than you'll find other places."

Children enjoyed both the plain and enhanced vegetables, and over 76% rated them favorably — either "yummy" or "just okay". "[Kids] liked the broccoli better than the corn," said Rolls. "That kind of surprised me because I thought they would like the more sweet, starchy vegetable more." In the paper itself, the authors speculated that "since both vegetables were already well-liked by the children, there may have been little room for improvement." Diktas said doubling the portion size was quite promising, as children ate 21 grams more, equivalent to 12% of their daily recommendation.

The team also drew some conclusions about "fussier eaters." Children who face pressure from their parents are less likely to eat a greater amount of vegetables when the portion size is increased, the authors found. Rolls said that, "The stealth approach, where you tuck [vegetables] into other foods, is very controversial but it works — it's probably the most effective strategy." On the other hand, openly forcing kids to eat specific foods has an unfavorable outcome.

Diktas mentioned that further research is needed on different demographics, as the parents in the current study were pretty homogenous: The families were 77.3% white and had an above-average income, with 85% of mothers holding a university degree. "Low-income populations have less access to vegetables as well as lower consumption," the authors explained, and many live in areas that are food deserts.

Rolls "suspects that the obesity incidence will be higher" after the pandemic, and stresses the need for prevention through dietary changes. Children, especially, "have more time to become adults with obesity," Diktas noted, and it is never too soon to introduce new vegetables into their diets.

The study, "Promoting vegetable intake in preschool children: independent and combined effects of portion size and flavor enhancement," published April 6 in Appetite, was authored by Hanim E. Diktas, Liane S. Roe, Kathleen L. Keller, Christine E. Sanchez and Barbara J. Rolls, The Pennsylvania State University.