While specific food cravings are generally thought to be the product of temporary biological changes, such as hormonal fluctuations, our bodies may determine what we eat in other ways as well.
Biological differences in our sense of taste have such an influence on our diets that they may help determine which diseases we might be susceptible to, according new research by Linda Bartoshuk of Yale University. And, new insights into how diet has adapted to the course of human evolution have emerged from McGill University researcher Timothy Johns' observations of how indigenous peoples use plants for food and medicine.
A Matter of Taste
The "neon taste world" of people whom Dr. Bartoshuk calls "supertasters" is roughly three times as intense as the "pastel world" of the nontasters. That's because the tongues of supertasters have a higher concentration of taste bud-containing structures than the tongues of less taste-sensitive groups do.
"The ability to taste bitter substances has always been associated with poison detection, but now we have found all these health associations," Bartoshuk said. "We know people's whole diets are different, based on their taste sensitivity."
Because taste buds also detect the sensations of touch and pain, supertasters are also the most sensitive to the heat of chilis, for example, and the feel of fat. Women are more likely to be supertasters than men are.
Thus, being a picky eater isn't necessarily a sign of a difficult personality.
"Supertasters are picky eaters. They taste bitterness in food that other people don't notice. For some, the food world is just slashing bright and they opt out from many food choices," Bartoshuk said.
Supertasters also tend to avoid very sweet, high fat foods, according to Bartoshuk, but are also generally averse to vegetables, which taste unpleasantly bitter to them. As a group, supertasters tend to be thinner and healthier, but their veggie aversion may lead to a higher risk of certain cancers, Bartoshuk has found.
Studying the colonoscopies of a group of older men, Bartoshuk and Marc Basson at Wayne State University School of Medicine found a correlation between the number of polyps and the ability to taste bitterness. The men with more polyps reported eating the fewest vegetables and were heavier, both risk factors for colon cancer.
Bartoshuk and her colleagues have also found a correlation between weight and a history of ear infections that seems to be related to taste sensitivity. She thinks this relationship may be related to the ability to appreciate the sensation of eating fat, which can be increased by upper respiratory infections.
The men in her study that reported histories of ear infections were heavier, which Bartoshuk thinks may be related connections between taste and the perception of fat. Taste and fat stimulate different nerves that normally inhibit one another. When the taste system is damaged, fat produces more intense sensations. In males this tends to increase their liking for and intake of high fat foods, leading to increases in weight, according to Bartoshuk.
One puzzle yet to be solved is why females tend to have the opposite response, liking high fat food less after becoming more sensitive to fat.
Bartoshuk mentioned that her colleague, Valerie Duffy, has found that supertasters are at less risk for cardiovascular disease, likely because of their lower fat intake.
The genes responsible for nontasting, medium tasting, and supertasting may have emerged during evolution because they provided each group with certain health advantages, related to their food preferences, Bartoshuk proposed.
Evolutionary Trends in Diet
Based on his investigations of the diets of indigenous communities today, Timothy Johns believes that humans have long used plants to counter the effects of diets rich in animal products.
Around two million years ago, as their brains and body sizes increased, our ancestors began to eat more animal foods and fewer plants. Cooking and eating more meat--and doing it for longer, as longevity increased--meant that early humans were consuming more fatty acids and cholesterol.
The body's method for turning these substances into energy produces harmful free radical molecules, which can, over time, damage tissue through a process known as "oxidative' stress." Many plants provide antioxidant molecules, which can prevent this type of damage.
The popularity of dietary supplements in modern western cultures seems to be part of an ancient and widespread practice, according to Johns.
"Everybody sort of looks at using natural health products as really unusual, but from the point of view of someone who has worked with traditional people, it seems to be a perfectly natural thing to do," he said.
The Masaii of East Africa and the Tibetan Highlanders, for example, both have diet very high in fat and low in fruits and vegetables, but don't have particularly high rates of heart disease. Johns has found that the Masaii add different roots and barks to their soups and milk, and chew other plant materials as gum.
Tests in the lab have revealed that some of these plants contain high levels of antioxidants, compounds that may lower cholesterol and lipid levels in the blood, and other compounds that may help fight gout and related conditions.
Many of the spices and medicinal plants most commonly prescribed in Tibetan medicine for heart-related diseases contained powerful antioxidants.
How did indigenous populations develop these dietary practices?
"That's the million dollar question," Johns said. "There's no obvious answer, but ultimately there must be a strong evolutionary basis for this."
Natural selection may have led to certain human behaviors involving the ingestion of substances that coincidentally turned out to have beneficial effects, Johns suggested, such as chewing gums or drinking tea. In other cases, we may have begun using some plants specifically for their health benefits.
"You don't need to be a scientist to know when you feel good and bad, so there may have been conscious learning," Johns said. "There may also have been some conditioned learning. We learn by past experiences what makes us feel better, without always being aware of it."
Advance interviews possible upon request.
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MEDIA NOTE: A newsbriefing on this research will take place at 10:00 a.m. Mountain Time, Friday, 14 February, during the AAAS Annual Meeting in Denver, in Rooms C-110-112, Colorado Convention Center. Further, these and other speakers will take part in a symposium titled, "The Changing View of Human Biodiversity," at 2:30 p.m. Friday, 14 February, in Room 112 on the Main Level of the Colorado Convention Center. Press registration is located in the AAAS Press Center in Room C-101 of the Colorado Convention Center.