A biological urge to pick carrot cake over carrots

Gary K. Beauchamp

is director and president of the Monell Chemical Senses Center in Philadelphia, where he explores topics related to taste, olfaction, and chemesthesis

'Hey, Mom, pass the brussels sprouts, please."

What parent wouldn't like to hear that? Yet no matter how many times children - or adults - are told we need to consume more healthy foods and avoid foods high in sugar, salt, and fat, we show no inclination to change our behavior. Why?

Scientists believe that the main reason rests on the biological imperatives that underlie human food likes and dislikes. We evolved in an environment where the primary challenge was to obtain enough sweet, salty, and fatty nutrients to grow, survive, and reproduce. It was even more important to avoid the potentially deadly bitter poisonous compounds abundantly found in plants. As a result, humans - especially young ones - are predisposed to choose exactly those foods that nutritionists label "bad" and to avoid those that are "good."

A thorough understanding of biology is needed to help us understand why we eat what we eat. The same knowledge can help us identify solutions to our nutritional crises. This is precisely why biology matters.

When nutrients were scarce, our sensory responses helped us survive. Sweet-tasting substances, a source of needed calories, are inherently attractive to humans. Research has shown that children like even higher levels of sweetness than adults, presumably to promote calorie intake during the rapid growth of childhood and early adolescence. And sweet taste releases natural opiates in the brain - sweet makes kids feel good.

A liking for salt evolved to help many animals, including humans, detect and consume sodium, a mineral necessary for life. It turns out kids like higher levels of salt, too. Finally, there is some tantalizing new evidence that the pleasing taste of fat may also be part of our biology.

Millennia have passed, but we continue to love foods that are sweet, salty, and fatty - even though these inborn responses now are counterproductive and even detrimental in our modern society of plentitude.

Just as our fundamental biology predisposes us to seek out and eat sweet, salty, and fatty foods, it works against our acceptance of vegetables. One of the predominant flavor characteristics of many of these prototypical healthy foods is their bitterness.

Humans are programmed to avoid bitterness because in nature it usually signals poison. Indeed, many of the health benefits of consuming vegetables come precisely from natural bitter compounds, which at low levels are healthful but at higher levels can be harmful. For a young child just learning to select foods, much less an adult, avoiding bitterness is fundamentally wise.

Thus the human's basic biology does not favor low-sugar, low-sodium, low-fat, vegetable-rich diets. How can we help overcome these biological imperatives to promote healthy eating?

Biology does not have to be destiny; experiences also mold our food preferences. A growing body of evidence suggests that certain dietary experiences, particularly during early development, can increase liking for the flavors of healthy foods such as vegetables, thereby helping to overcome the natural rejection based on bitter taste.

Consider the dislike for vegetables. Due to research, we now know that flavors consumed by pregnant and nursing women enter their amniotic fluid and breast milk. We also know that infants exposed to these flavors in utero and during nursing learn to like them. From an evolutionary perspective, the foods that a woman eats when she is pregnant and nursing are precisely the ones that her infant should prefer. Thus, it is imperative for the pregnant and nursing woman to increase the amount and variety of vegetables she consumes.

Research is less clear on how much we can change the established liking for sweets, salt, and fat. Some studies indicate that reducing exposure to these tastes over time may help reduce intake. The best evidence concerns salt: Studies show that when adults go on a reduced-salt diet, they gradually come to prefer lower levels of salt in their food. A similar phenomenon may occur for fat and perhaps even for sweet, although more research is needed to verify this. These effects of experience may be particularly salient early in life. Thus, we may be able to moderate the inherent desire for sweet, salty, high-fat foods by reducing the exposure of infants and young children to them.

In the end, biology helps us understand why we like to eat what we eat. Of course, the biology of flavor liking is not the entire basis for our food choices; cost and availability also are important determinants. Nevertheless, biology can and should help us identify solutions to our nutritional crises and their health consequences.