“These early patterns of food acceptance lay the foundation for a lifetime,” says Susan Johnson, a professor of pediatrics and director of the Children’s Eating Laboratory at the University of Colorado’s Anschutz Medical Campus in Aurora. “Early exposure to flavor appears to translate into better acceptance over the long haul. You come to like what you know.”

Research by Dr. Johnson and colleagues has found that there is a critical window from between 6 to 12 months of age when children may be most receptive to new foods—including bitter ones. Scientists have also discovered that influences occur even earlier: Babies are exposed to the flavors of foods in their mother’s diet in utero via amniotic fluid and later through breastmilk. Some studies have found that babies seem to enjoy these foods more than others when they start eating solid food.

There’s a push to get children off to a healthier start. Babies and toddlers aren’t eating enough whole grains and are eating too much added sugar, says Kathryn Dewey, professor emerita in the department of nutrition at the University of California, Davis. Dr. Dewey was on a federal committee that issued dietary recommendations that the U.S. Department of Agriculture and the Department of Health and Human Services are now reviewing before finalizing guidelines by the end of the year.

Babies are primed to favor sweet tastes, says Julie Mennella, a biopsychologist at the Monell Chemical Senses Center, a nonprofit research institute in Philadelphia. “Sweet is our signal for calories, which growing children need,” she says, noting that breastmilk is sweet. This makes young children particularly vulnerable to foods with added sugars. Eating foods with added sugars can make a baby “develop sweet preference,” notes Steven Abrams, a professor of pediatrics at the Dell Medical School at the University of Texas at Austin and chair of the American Academy of Pediatrics’ committee on nutrition.

Sixty-three percent of babies 6 to 12 months old are eating added sugars on a given day, according to data analyzed by the committee. Toddlers age 12 to 24 months eat about a bit more than six teaspoons of added sugars per day. And 29% of them are consuming sugar-sweetened beverages. Studies have found that children who consume sugar-sweetened beverages have a higher risk of obesity.

Nearly 19% of 2- to 19-year-olds are obese, according to 2015-2016 data from the Centers for Disease Control and Prevention. That is up from 13.9% in 1999-2000. “An overweight baby becomes an overweight child becomes an overweight adult,” says Dr. Abrams. “We’re seeing lipid disorders in young teenagers that can include fatty liver and liver diseases. We’re seeing adult onset Type 2 diabetes in older children. Even among older toddlers and preschool kids, when we see growth falling outside the growth curve it needs to be addressed.”

Only 6% of babies 6 to 12 months old are eating dark green vegetables on any given day, according to data analyzed by the federal committee. But Dr. Johnson believes that should—and can—change. She conducted a study, which is unpublished, where 106 babies and toddlers were offered puréed kale by their caregivers (94% of whom were their mothers). Babies who were about 6 to 12 months old tended to eat the kale, even though many made faces. “Even if the children showed visual signs of not liking it, when the moms loaded up the spoons, they opened up their mouths and took” the next bite, says Dr. Johnson.

But toddlers who were about 1 and older were less likely to eat the kale. “It may be that period is a critical window for introducing those harder-to-like foods before rejecting behaviors become so problematic in later toddlerhood.” The study was funded by the Sugar Association, a trade organization that represents the sugar industry. In Dr. Johnson’s study, children were offered four versions of kale purée, one plain, two with small amounts of sugar and one with salt. Dr. Johnson said the children’s differing acceptance of the purée by age was true across all versions.

The federal committee has recommended that children under 2 not eat no added sugars at all. 

Children’s “neophobia” peaks between the ages of 2 and 5, says Catherine Forestell, an associate professor of psychological sciences at William & Mary in Williamsburg, Va. “Essentially what that comes down to is a hesitancy to try new foods,” she says. Dr. Forestell has tested interventions to encourage preschoolers to eat more fruits and vegetables by repeatedly offering children the foods and educating them about healthy eating. At the end of the interventions, children did eat more fruits, but not more vegetables. “I think that speaks to the fact that vegetables are not as palatable to children as fruit.”

For babies and toddlers, however, there’s growing evidence that repeated exposure to new foods, including vegetables, makes them more willing to eat them. A review paper published in the American Journal of Clinical Nutrition in 2019 found “moderate evidence” that when youngsters four to 24 months old tasted a particular fruit or vegetable each day for between eight and 10 days or more, they ate more of the food or ate it faster than before the exposures.

There’s also some evidence that acceptance can generalize to similar types of foods—say from green beans to peas. So Dr. Forestell encourages parents to keep offering foods even if babies make negative faces when trying it. (Don’t, however, force a baby to eat, or continue to try feeding if they indicate they are done by, say, turning their head away or spitting the food out.) Also, go for variety, she says. This way babies “will be more willing to accept new foods because they are not easily thrown off by a new flavor,” she says.

What Kids Eat, By the Numbers

Between 88% and 99% of children and adolescents eat less than the recommended amount of vegetables.

About half of children ages 4 to 8 are eating less than the recommended amount of fruit per day.

Only 6% of babies 6 to 12 months old are eating dark green vegetables on any given day.