The reason why some people dislike vegetables such a broccoli and sprouts is due to the genetic defence mechanism to potentially harmful compounds in the plants, a result that deepens our understanding of taste.
"Taste has a great impact on food acceptability and choice," said lead author Mari Sandell from the Monell Chemical Senses Center. "A comprehensive understanding how food components contribute to taste is necessary to develop modern tools for both nutritional counselling and food development."
Such research may also help explain why children and adults like and dislike foods and could be important for the understanding of eating problems, such as obesity with over 22 million children under five are severely overweight.
Scientists have long assumed that bitter taste evolved as a defence mechanism to detect potentially harmful toxins in plants. And the research, published in the September 19 issue of the journal Current Biology (Vol. 16, R792-R794), claims to be the first paper to provide direct evidence in support of this hypothesis.
"The sense of taste enables us to detect bitter toxins within foods, and genetically-based differences in our bitter taste receptors affect how we each perceive foods containing a particular set of toxins," explained lead researcher Paul Breslin.
The researchers state that they have established that variants of a bitter taste receptor called TAS2R38 can detect glucosinolates, a class of compounds with potentially harmful physiological actions, in natural foods.
Glucosinolate-containing vegetables include watercress, broccoli, bok choy, kale, kohlrabi, and turnip. Many of these vegetables contain compounds that have been linked to lower incidences of certain diseases, most notably cancers, and form an integral part of the 'five-a-day' regime. However, studies have shown that many people are falling short of their fruit and vegetable quota, perhaps due to dislike of certain vegetables.
The researchers recruited 35 healthy adults classified according to their hTAS2R38 bitter taste receptor genotype: PAV/PAV (sensitive to the bitter-tasting chemical PTC), AVI/AVI (insensitive), and PAV/AVI (intermediate).
The participants were then asked to rate the bitterness of different vegetables, some glucosinolate-containing, other were non-glucosinolate-containing.
Sandell and her co-workers report that subjects with the sensitive (PAV/PAV) form of the receptor rated the glucosinolate-containing vegetables as 60 per cent more bitter than did subjects with the insensitive (AVI/AVI) form. The non-glucosinolate-containing vegetables were rated equally bitter by the two groups.
"The findings show that our taste receptors are capable of detecting toxins in the natural setting of the fruit and vegetable plant matrix," said Breslin.
These results also demonstrated, said the researchers, that variations in the hTAS2R38 gene affect bitter perception specifically of foods containing glucosinolate toxins, and that individuals respond to actual foods is multi-level, and depends on evolutionary, genetic, receptor, and perceptual factors.
Glucosinolates act as anti-thyroid compounds, and inhibit the uptake of iodine by the thyroid, increasing risk for goiter and altering levels of thyroid hormones. The ability to detect and avoid naturally-occurring glucosinolates would confer a selective advantage to the over 1 billion people who presently have low iodine status and are at risk for thyroid insufficiency.
"The contents of the veggies are a double-edged sword, depending upon the physiological context of the individual eating them," said Breslin. "Most people in industrialized cultures can and should enjoy these foods. In addition to providing essential nutrients and vitamins, many are reported to have anti-cancer properties."
The research echoes a recent study, published in the American Journal of Clinical Nutrition (Vol. 84, pp. 245-251) that also reported that genetic variations in the TAS2R38 gene might indeed affect a kid's acceptance and preference of bitter tasting foods.
The research was funded by the National Institute on Deafness and Other Communication Disorders, National Institutes of Health