January 2008 Issue

Taste Perception and Eating Behavior — They’re in the Genes
By Diane Welland, MS, RD
Today’s Dietitian
Vol. 10 No. 1 P. 38

There’s no accounting for taste, as the saying goes. Or is there? Nutrigenetics is helping researchers understand the complexities of taste and unravel the mysteries behind our food habits.

Imagine two preschoolers, each given a piece of broccoli. One toddler takes a bite and grimaces, pushing it aside; the other takes a bite, carelessly chewing as if barely aware of the taste. Lacking environmental stimuli and the psychosocial pressure adults face, children choose their food based primarily on taste perception, which varies so greatly among individuals that researchers surmise there’s only one explanation for such obvious differences: It’s in the genes.

The fact that genes influence our taste perception is nothing new. The discovery of genetic variations in taste dates back to the 1930s when a chemist trying to synthesize a very bitter compound called phenylthiocarbamide (PTC) accidentally released some crystals into the air. Surprisingly, he found that some of his lab partners could barely tolerate the bitterness while others (himself included) hardly sensed anything.1,2 This finding led him to divide people into two categories—nontasters (or those with taste blindness) and tasters—and the science of taste genetics was born.

Today, the study of genetic influences on what we eat, as well as how much we eat and even our eating behavior, has branched into a new field called nutrigenetics, the influence of genes on diet. Nutrigenomics, another buzzword in the industry, also refers to the influence of diet on genes, mainly related to health and disease states such as obesity, cancer, and heart disease.3,4

Spurred by technological advances, a surge in public interest, and innovative scientific projects such as the Human Genome Project (the mapping human genes), nutrigenetics has grown in leaps and bounds. Despite this progress, the field is still slow to catch on among dietitians.

Many dietitians work on the premise that eating is strictly a behavioral issue. While what we choose to eat, when we eat, and even how much we eat are largely determined by our culture, ethnic background, religion, and individual experiences, we now know that the delicate interplay of genes plays just as important a role as environmental factors. As new research emerges, dietitians are primed to be on the cutting edge in this field. “Who better to evaluate how your genes affect nutrition than a dietitian?” asks Ruth DeBusk, PhD, RD, a private practice dietitian in Tallahassee, Fla., and coauthor of It’s Not Just Your Genes! “Genetics is something we all should know about.” In the not-too-distant future, DeBusk and others like her believe evaluating gene profiles will become standard practice among dietitians; simply another tool in the arsenal of nutrition strategies used to help patients.

Tasteful Research
Getting up to speed in genetics—a field dietitians generally know little about—takes time and energy, particularly since there have been so many recent advances. In 2006, scientists at the Monell Chemical Center in Philadelphia discovered that the bitter taste gene receptor hTAS2R38 detects glucosinolates, a class of bitter compounds naturally found in some fruits and vegetables, especially cruciferous varieties such as broccoli, watercress, kale, bok choy, and turnips.

The study tested bitterness ratings of these vegetables in 35 healthy adults expressing the three genotypes of hTAS2R38: PAV/PAV, PAV/AVI, and AVI/AVI (labeled after the three variable amino acids).5 Generally, people with PAV/PAV are considered supertasters, extremely sensitive to bitter tastes in foods such as coffee, tea, grapefruit juice, and vegetables and in man-made compounds such as PTC and 6-n-propylthiouracil (PROP), primarily used in research. People with PAV from one parent and AVI from another are intermediate or medium tasters, meaning they can taste bitter but not as intensely as the supertasters. Because they can taste some bitter, they are usually lumped together with supertasters in most studies. Finally, there are AVI/AVI, or nontasters, who are insensitive to bitter. To them, a PROP-saturated litmus paper tastes like paper.6

Results from the Monell study found that supertasters (PAV/PAV) rated the glucosinolate vegetables 60% more bitter than did nontaster subjects with the AVI/AVI form. Intermediate tasters fell somewhere in between. What’s more telling is that supertasters, intermediate tasters, and nontasters rated other vegetables not containing glucosinolates, such as endive, eggplant, and spinach, equally bitter.6

How does this relate to food choices? “The ability to taste [or not taste] some types of bitter or sour compounds probably does lead to differences in diet among people, but these differences are hard to pin down scientifically,” says Monell behavioral geneticist Danielle Reed, PhD. “One popular belief is that people who are genetically more sensitive to some types of bitter taste avoid vegetables like broccoli or kale.”

To prove this point, Rutgers University scientist Beverly J. Tepper, PhD, identified tasters and nontasters in a group of 65 preschool children. She then gave them five different kinds of vegetables, both bitter (black olives, cucumbers, and raw broccoli) and nonbitter varieties (red peppers and carrots), and allowed them to take as much as they wanted.

Now, think back to the broccoli-eating preschoolers discussed earlier. As expected, nontaster children consumed more bitter vegetables than taster children, and total vegetable consumption was also higher, with nontaster children eating approximately one serving of vegetables while tasters took in only one half that amount. Furthermore, only 8% of the nontaster children refused to eat any vegetables during the free choice compared with 32% of the taster children.7,8

“Being a bitter taster definitely influences food choices in children,” says Tepper. “But that doesn’t mean these taster children won’t eat these vegetables. Tasters still ate half a serving of vegetables. Most of these children were already exposed to these foods repeatedly, which we know increases preference.”

Considering that in North America roughly 70% of Caucasians are bitter tasters and 30% are nontasters, these are important findings.8 Other ethnic groups such as the Japanese, Chinese, and West Africans have much smaller numbers of nontasters—as little as 3% in some populations. The largest group is found in India, where taste blindness is found in more than 40% of the population.1,8 Why there is such wide variation between tasters and nontasters among different populations is unknown.

Scientists have long assumed that bitter taste evolved as a defense mechanism to detect potentially harmful toxins, and certainly there is evidence to support this. Bitter foods in nature are often poisonous. Even glucosinolates act as antithyroid compounds, blocking the thyroid from taking up iodine. For the 1 billion people who presently have low iodine, this can be a serious problem.5,9

But if that was the only aspect involved, nontasters would have died out long ago as part of natural human evolution.9 Rather, bitter foods do have some nutritional advantages and benefits and may increase variety in the diet. Furthermore, some glucosinolate vegetables, such as broccoli, protect against cancer and other diseases. Although this doesn’t explain the wide variations we find in different ethnic populations between tasters and nontasters, it does tell us why nontasters are still around.

Bittersweet Predictions
The ability to taste bitter foods isn’t the only difference between tasters and nontasters. “Although there are exceptions to the rule, supertasting is highly correlated with a greater density of tongue papillae than nontasting,” notes Tepper, “which means they have more taste buds.” On the tongue, papillae show up as tiny bumps, and each one houses several taste buds.

Researchers believe this may be one reason why supertasters and, to a lesser extent, tasters perceive a greater intensity of a whole range of sensory characteristics, including spicy, sweet, and salty stimuli, than nontasters.10,11 Even fat perception is amplified, which was confirmed by a Rutgers study showing that supertasters and medium tasters could distinguish differences in fat content in salad dressing composed of either 10% or 40% fat, whereas nontasters could not.10

How this pans out when it comes to actual food choices, however, is still up in the air. Consider sweetness, a taste humans are universally programmed to like, located on two sweet receptor genes—TAS1R3 and TAS1R4—and affected by factors such as age, culture, appetite, mood, and even race and sex.12

In a 2005 Monell Chemical Center study of bitter sensitivity and sweet preferences, both supertaster and taster children preferred higher concentrations of sugary drinks and cereal compared with nontasters and were less likely to choose milk or water as their favorite beverage, opting for carbonated drinks instead. Surprisingly, no preferences were observed in adults, making researchers suspect that forces other than taste, such as culture and experience, were at work.13

Even so, why would supertaster children prefer such sweet drinks? “It could be because those [children] who are most bitter sensitive use more sugar to mask unpleasant tastes in food and thus come to prefer it more,” says Reed.

However, a later study from England revealed very different findings, reporting that 67% of adult PROP supertasters disliked [concentrated] sweet taste compared with only 12% of nontasters.11 “We don’t know why this flip-flop occurs,” says Tepper. “It could be environmental influences or physiological or something else.”

A Fondness for Fat
Now, researchers are trying to make a connection between weight status and taste perception. “We know nontasters seem to want more of everything [in foods]—bitter, sweet, spicy, and especially fat. Supertasters tend to avoid very sweet, high-fat foods and bitter foods and generally eat less food overall,” says Tepper.

Several studies show that although nontasters are less adept at discriminating fat, they prefer higher-fat meats, cheese, and milk more than tasters do. So when Tepper measured body mass index (BMI) in a small study of 40 middle-aged women, it wasn’t a surprise that supertasters were 20% thinner than their nontaster counterparts, with a BMI of 23 vs. a BMI of 30. (Medium tasters had a BMI of 26.)14

But taste isn’t the only reason why people eat certain foods. When Tepper looked at women with high cognitive restraint (the conscious control of eating), the relationship between low BMI and supertasters was ruled out. “Food intake was motivated by other factors, not taste,” she says. This is important since experts say nearly 50% of adult women are restrained eaters; far fewer men fall into this category.

In fact, taste is only one small piece of the genetic puzzle. Researchers believe genes are involved in a whole host of eating activities, including caloric intake, meal size, and meal frequency. A number of family- and twin-based studies have even linked our fondness for specific macronutrients such as fat, as well as proteins and carbohydrates, to our genetic makeup.15

Now, researchers at the Tufts University Human Nutrition Research Center think they may have pinpointed one of the many genes responsible, a protein called apolipoprotein A-II (APOA2). In the recent Genetics of Lipid Lowering Drugs and Diet Network Study examining blood lipid levels, anthropometric measurements, and food intake of more than 1,000 men and women, scientists discovered three APOA2 gene variants, or alleles; two common variants, TT and TC, appeared in approximately 85% of participants; and one recessive, labeled CC, occurred in only 15% of the group. After examining dietary intake questionnaires, they noted that both men and women with the CC genotype ate more fat and protein and less carbohydrate than the T carriers. Furthermore, those with the CC allele took in an average of 200 more calories per day and were nearly two times more likely to be obese than the two more common genotypes. The findings suggest that APOA2 is involved in not only food preferences but also satiety signaling.16

Watch Your Behavior
Another area getting more attention is genes’ influence on eating behavior as it relates to cognitive restraint, disinhibition (loss of control), and susceptibility to hunger. “We know obesity runs in families, and high levels of disinhibition especially are associated with heavier weights,” says Tanya Agurs-Collins, PhD, RD, program director of the Health Promotion Research Branch of the National Cancer Institute. “High restraint is also related to obesity, though it can be associated with weight loss, too. Now we know people can inherit these behaviors.”

Although numbers vary, heritability based on family and twin studies can be as high as 28% for cognitive restraint, 40% for disinhibition, and 23% for hunger.15,17,18 Studies of genetic influence on macronutrient intake, caloric intake, and meal size show ranges from 20% to 40%.15,18 And now, thanks to genomewide scans, scientists think they have identified genetic linkages related to eating behavior.18

What’s a Dietitian to Do?
For dietitians, knowing that people are predisposed to a certain behavior can improve counseling strategies. “Eventually, we’d like to tailor intervention programs based on each person’s personal genetic profile,” says Agurs-Collins, “but we’re not there yet.” Still, understanding that our genes are involved in determining some variation in energy and macronutrient intake can help dietitians personalize dietary plans. “For example, one study suggested that African Americans may have higher sweet thresholds or prefer sweeter foods than Caucasians. Research is underway to determine how much is related to our genes and to the environment,” notes Agurs-Collins.

Taste perception is another area to consider. “Since nontasters want flavor and fat, a weight loss diet may be harder for them,” says Tepper. “So you may have to kick it up a notch to compensate.” Supertasters, on the other hand, tend to avoid strong flavors and healthy vegetables because they taste bitter. For them, Tepper suggests lightly steaming raw vegetables or adding a condiment to decrease bitterness.

Having a genetic tendency for something doesn’t mean you are doomed to that fate. “Just because your parents are obese doesn’t mean you will be,” says Agurs-Collins. “You may just need to monitor your weight more.”

When it comes to taste, personality may be the key. “Supertasters who like to try new things—I call them food adventurous—actually enjoy robust, pungent, and spicy foods,” explains Tepper. “I’m a supertaster, and I like briny olives, anchovies, broccoli, and brussels sprouts. It just may take a little longer to develop a taste for those foods.”

— Diane Welland, MS, RD, is a dietitian and a freelance food and nutrition writer based in Springfield, Va.

References
1. Nabhan GP. Why Some Like It Hot: Food, Genes, and Cultural Diversity. Washington, D.C.: Island Press; 2004.

2. Flaherty, J. The Supertasters. Tufts Nutrition. 2006:7-10.

3. DeBusk R, Joffe Y. It’s Not Just Your Genes! San Diego, Calif.: BKDR, Inc.; 2006.

4. DeBusk RM, Draper CF. Nutrigenomics, inflammation and obesity: A new paradigm for personalized prevention. Presentation at the American Dietetic Association Food & Nutrition Conference & Expo; September 30, 2007; Philadelphia, Pa.

5. Sandell MA, Breslin PA. Variability in a taste-receptor gene determines if we taste toxins in foods. Curr Biol. 2006;16(18):R792-R794.

6. Bufe B, Breslin PA, Kuhn C, et al. The molecular basis of individual differences in phenylthiocarbamide and propylthiouracil bitterness perception. Curr Biol. 2005;15(4):322-327.

7. Bell KI, Tepper BJ. Short-term vegetable intake by young children classified by 6-n-propylthiouracil bitter-taste phenotype. Am J Clin Nutr. 2006;84(1):245-251.

8. Tepper BJ. PROP sensitivity and food selection in children. Network Health Dietitian. 2006:10-11.

9. Tanaka T, Reed DR, Ordovos JM. Taste as the gatekeeper of personalized nutrition. In: Kok F, Bouwman L, Desiere F. Personalized Nutrition: Principles and Applications. Boca Raton, Fla.: CRC Press; 2007.

10. Tepper BJ, Nurse RJ. PROP taster status is related to fat perception and preference. Ann N Y Acad Sci. 1998;855:802-804.

11. Yeomans MR, Tepper BJ, Rietzschel J, et al. Human hedonic responses to sweetness: Role of taste genetics and anatomy. Physiol Behav. 2007;91(2-3):264-273.

12. Reed DR, McDaniel AH. The human sweet tooth. BMC Oral Health. 2006;6 Suppl 1:S17.

13. Mennella JA, Pepino MY, Reed DR. Genetic and environmental determinants of bitter and sweet preferences. Pediatrics. 2005;115(2):e216-222.

14. Goldstein GL, Daun H, Tepper BJ. Adiposity in middle-aged women is associated with genetic taste blindness to 6-n-propylthiouracil. Obesity Res. 2005;13(6):1017-1023.

15. Rankinen T, Bouchard C. Genetics of food intake and eating behavior phenotypes in humans. Annu Rev Nutr. 2006;26: 413-434.

16. Corella D, Arnett DK, Tsai MY, et al. The –256T>C polymorphism in the apolipoprotein A-II gene promoter is associated with body mass index and food intake in the genetics of lipid lowering drugs and diet network study. Clin Chem. 2007;53(6):1144-1152.

17. Genetics linked to diet patterns and fat fondness. Tufts University Health & Nutrition Newsletter. 2007;25(7):1-2.

18. Agurs-Collins T. Genetic influences on eating behaviors. Presentation at the Northern District Virginia Dietetic Association Seminar and Workshop Meeting; May 15, 2007; Alexandria, Va.

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