It comes as no surprise to any adult who once entertained childhood fantasies of eating cotton candy and chocolate bars for breakfast that tastes change as we age. At a time when unhealthy eating habits and obesity are problems for both adults and children, choosing a diet with fewer sweet and fatty foods could be a biological battle at any age but is especially challenging for children.
"Everybody is telling us to reduce sweet and salty foods and increase fruits and vegetables. This advice is difficult for adults to comply with, let alone kids," said Julie Mennella, PhD, a scientist at the Monell Chemical Senses Center in Philadelphia, PA. Recent research into the innate flavor preferences of children could one day play a role in promoting healthy eating and, researchers hope, increasing acceptance of oral medications.
Children are born with an evolutionary preference for sweet tastes and avoidance of bitter shaped in utero. "This learning occurs early in life. By learning about how children learn, we can help them get on the right path," Dr. Mennella told ADVANCE. "As you get older, there are other cognitive influences. We don't always eat what we like, although it is a primary driver of consumption. Childhood is a time in life when taste is particularly salient."
Researchers are working to understand how children learn individual preferences for basic tastes of sweet, salty, sour, bitter and umami and avoidance of others. "How a food tastes is really a combination of taste and smell and a third chemical sense that detects sensations such as the coolness of mint, the burn of hot chili peppers," she explained. "All these perceptions arise and combine in the mouth to form the impression of flavor."
The basic senses of taste and smell are "gatekeepers" that decide what foreign substances are taken in or rejected by the body, she said. The same senses function as the sources of extreme pleasure and pain, a particularly important point for babies and young children. "The senses of taste and smell function in utero," Dr. Mennella stated. "The receptors are there, and the machinery is there for babies to detect taste and smell."
Babies are born with the ability to detect sweetness, the predominant taste quality of human milk; and the preference for high-intensity sweetness continues through late adolescence. Though babies do not develop the ability to detect salt until about 4 months of age, a penchant for higher intensity levels of salt stretches through the teen years. "Between periods of maximal growth, children may be attracted to sugar because it's a signal to the body of energy and salt because it's a signal for minerals," she suggested.
Sweet preferences are particularly powerful during early development because they serve as an analgesic. Dr. Mennella's research has shown that sweets make children feel better by reducing pain. "The processing of sweet stimuli involves brain areas common to other pathways that reward and pleasure," she said. "In fact, many drugs of abuse often co-opt these pathways that were designed for sweet. We now live in an environment where foods are plentiful, and with refined sugar there are foods that taste more sugary than our ancestors ever experienced."
The taste of sugar also is a signal that a food has calories and, therefore, nutrition. This most likely underlies a newborn's initial preferences for sweet flavors.
As much as children are drawn to sweetness, they have an innate avoidance of bitter flavor. "Bitter is a taste signal for poisons," Dr. Mennella noted. Bitter taste tolerance can be learned through repeated exposure to foods. The predisposition to dislike bitter flavors can present difficulties for children who must take oral medications. "The No. 1 reason for noncompliance with oral medication is taste," she said.
Recent flavor research increasingly has focused on understanding the biology of bitter taste. "It's really important for us to detect bitter and reject it," she noted.
Scientists are working to block bitter receptors so oral medications will taste better to children. "Hopefully, with some of the discoveries in taste, we can learn how to make medications better tasting so children comply," Dr. Mennella explained. "Think of the child who has AIDS, the baby or young child who's taking not just one but many medications a day. This becomes a big issue."
Taste and flavor preferences differ not only between adults and children but among individuals, families and cultures as well. "Not only are children living in different sensory worlds from adults, but each child and adult differs from the one next to them," she said. Flavor preferences vary widely. "There is a lot of variation in some bitter receptors. You may be very sensitive to bitter, while the person next to you may not be able to detect it."
Children and adults can come to like a bitter-tasting food after repeated exposure. Vegetables are a prime example. Since flavor and taste learning occurs so early in mammals, mothers who eat more fruits and vegetables during pregnancy have children who are more accepting of those foods, research has indicated.
This signals the need for a more individualized research approach to how children learn taste acceptance, Dr. Mennella said. "A child is a member of a family. When you look at the earliest of learning, whether it's in utero or breastfeeding, an infant learns about the foods that mom eats."
In one of her studies, babies who were fed protein hydrolysate formula before 3 months of age were more apt to accept them readily when they were older. In addition, they had a higher preference for sour, bitter and umami, she reported. "The types of table foods moms choose to feed infants, such as cheese or snack foods, translate into higher salt preferences even early on."
Dr. Mennella hopes basic taste research eventually can be expanded to include children who were tube fed as infants and may present with different taste experiences and flavor learning than their peers who were not. "Clearly, there's learning going on with the absence of certain experience," she said. "Many who are tube feeding also have an experience that is associated with negative consequences such as pain. We have to step back and look at how we can facilitate the transition from tube feeding to oral feeding. That's an important gap in knowledge."
• Beauchamp, G., Mennella, J.A. (2009). Early flavor learning and its impact on later feeding behavior. Journal of Pediatric Gastroenterology and Nutrition, 48: S25-30.
• Mennella, J.A. (2009). Flavour programming during breast-feeding. Advances in Experimental Medicine and Biology, 639: 113-20.
• Mennella, J.A., Nicklaus, S., Jagolino, A.L., et al. (2008). Variety is the spice of life: Strategies for promoting fruit and vegetable acceptance in infants. Physiology & Behavior, 94: 29-38.
For More Information
• Monell Chemical Senses Center, online: www.monell.org
Alyssa Banotai is a Senior Associate Editor at ADVANCE. She can be contacted at firstname.lastname@example.org.