By Beth Azar
Monitor staff

Along with rational thought, human eating habits set us apart from all other animals. We don?t eat merely to gain nutrients. We eat to taste?combining meats, fruits, vegetables and seasonings to concoct a never-ending variety of flavor and taste sensations.

Biopsychologists and nutritionists have begun examining the factors that predict which foods people eat. Some are looking at how people?s earliest experiences with different flavors?through breast milk and perhaps even amniotic fluid?affect the food preferences they develop. Others are examining the human tongue to determine whether genetic differences in how people taste and experience food predicts which foods they eat.

Once researchers understand the social and biological factors that underlie food preferences, they may be able to design interventions that can help people learn to eat healthy diets, they say.

Early learning

Babies begin to learn about the flavors specific to their culture through breast milk, and maybe even amniotic fluid, research shows. When a woman eats garlic, for example, her baby will suckle longer, finds psychologist Julie Mennella, PhD, of the Monell Chemical Senses Center in Philadelphia. Babies don?t seem to ingest more milk. Rather they spend the extra time analyzing what they?re tasting, keeping the milk in their mouth, pausing and perceiving the flavors. Vanilla flavoring has a similar effect, studies show.

Amniotic fluid, which babies ingest inside the womb, also picks up a distinctive smell after a woman eats garlic, Mennella finds. And it?s likely the fetus detects the change in its living environment.

Many cultures believe that early exposure to traditional foods is critical for shaping a baby?s food preferences. Although this link between early experience with flavors and later food preferences hasn?t been established by research, several studies hint that it might be valid. For example, Leann Birch, PhD, now of Pennsylvania State University, and her colleagues found that repeated exposure to green beans and peas?in the form of baby food?helped babies overcome initial rejection of the vegetables. Also, several studies by Mennella and her colleagues find correlations between the diversity of a mother?s diet and her infant?s willingness to eat a variety of foods.

And studies of rodents, sheep and pigs show that, once weaned, young animals prefer flavors they were exposed to through their mothers? milk.

A bitter pill

Other researchers have begun to examine whether differences in how people perceive tastes?their perceptions of how bitter broccoli tastes or how salty a pickle seems?affect their food choices. In the early 1930s, researchers discovered an inherited taste trait that determines people?s sensitivity to bitter tastes. People can be classified as ?tasters? or ?nontasters? based on whether they are able to detect 6-n-propylthiouracil, or PROP, which tastes bitter to some people, but tastes as benign as water to others. Research has found that nontasters eat a larger variety of foods than tasters.

Knowing about genetically determined preferences may help people better understand their eating habits and work to reshape them.

Several years ago, Yale University psychologist Linda Bartoshuk, PhD, discovered that a subset of people find PROP even more offensive than regular tasters did. She named them ?supertasters? and soon determined that she could distinguish that group from tasters and tasters from nontasters by looking at their tongues. Supertasters have the most fungiform papillae?the rounded structures on the tongue that house the taste buds, which send information about flavor and texture to the brain. As their name implies, supertasters perceive tastes more intensely than tasters and nontasters: Bitter tastes more bitter, sweet tastes sweeter and salt tastes saltier, finds University of Connecticut assistant professor and dietician Valerie Duffy, PhD, RD, who collaborates with Bartoshuk. These findings confirm work by French physiologist Brillat-Savarin in 1826, who said that people with more papillae lived in different worlds of taste. Research by Bartoshuk and others finds that about 25 percent of the U.S. population are supertasters, 50 percent are tasters and 25 percent are nontasters. Also, women are more likely than men to be supertasters.

But eating is more than taste. The ?feel? of food can be just as important to people. In fact, 75 percent of the nerves coming out of the fungiform papillae lead to the trigeminal nerve, which connects to the pain and touch centers in the brain. So for super-tasters, an excess of fungiform papillae means they ?feel? foods more intensely too. Duffy finds that supertaster women rate fat as creamier than tasters or non-tasters, a sensation resulting from the ?feel? of fat rather than its taste. And they also feel more ?burn? from substances such as ginger, alcohol, the carbon-dioxide in soda, and capsaicin, the active ingredient in chili peppers, finds Bartoshuk and her colleagues. Research is beginning to find that these differences in taste sensations, in part, predict which foods people prefer. For example, University of Michigan psychologist Adam Drewnowski, PhD, finds that during a taste test, supertaster women are more likely than tasters or nontasters to reject bitter-tasting foods, including green tea and a range of soy products such as tofu, miso and soy milk. In other studies, supertaster women found the taste of naringin?a bitter chemical found in grapefruit juice? less appealing than nontasters. When asked to rate food preferences, supertasters gave lower ratings to coffee, cabbage, Brussels sprouts and spinach.

This research included a study of 123 college-age women as part of a grant from the National Cancer Institute to investigate whether being a supertaster affects people?s willingness to accept foods high in anti-oxidants?the compounds found in many vegetables that researchers believe ward off cancer.

So far, his results don?t prove that supertasters? distaste for bitter vegetables translates into a diet low in those foods, he says. He is now analyzing food consumption data from the women, including daily food diaries. And he is also collecting data from a sample of women with breast cancer to determine if taster status affects their willingness to switch to a cancer-fighting diet low in fat and high in fruits and vegetables.

Given the role of dietary fat in human obesity, researchers are also interested in whether taster status affects people?s preference for fats. In a study now in press, Duffy and her colleagues find that supertaster women report a lower preference for sweet and fats than tasters or nontasters. As with bitter tastes, supertasters seem to find sweets and fats too intense, says Duffy, whose research was funded by the U.S. Department of Agriculture. In another study in press, however, Drewnowski and his colleagues find no relationship between taster status and preference for sweetened dairy products.

The supertaster gene may be a remnant of our evolutionary past, which once acted as a safety mechanism to help humans avoid toxins and other unhealthy foods, researchers contend. Supporting that claim is evidence from Duffy and her colleagues that women become more sensitive to bitter flavors during the first trimester of pregnancy, a time when ingested toxins would most severely affect a fetus.