Friday, June 3, 2011

Bitter for a Reason?

Can’t stand Broccoli? Research into the evolution of taste yields interesting food for thought. Our five taste senses– sweet, salty, umami (savory), bitter and sour – equip us for consuming the essentials for survival – energy, salt, and protein – as well as avoidance of poisonous or rotten food.1 The yen for umami makes us seek out protein-rich foods, while sweet drives selection of naturally sweet fruit and dairy foods. Salt is a highly valued seasoning, and our bodies require sodium. Dietary Reference Intakes (DRI) established by the Food and Nutrition Board of the Institute of Medicine define “Adequate Intake” as 1500 mg/day (age 9-50), decreasing to 1300 mg/day (age 50-70) and 1200 mg/day (age 70+). Sourness is associated with scurvy-preventing sources of Vitamin C, making sour a critical taste. It also helps us detect possible spoilage, such as sour milk. Interestingly, sourness is the one taste sensation which can substitute for saltiness – suggesting citrus juices, vinegars, and wines as salt replacements in cooking. The ability to taste bitterness varies. Some people more sensitive to bitter flavors will refuse vegetables with a bitter component. Groundbreaking research by Arthur L. Fox (working for Dupont Chemical Company in the 1930’s) found that people varied in their response to bitter chemicals – about 25% of the population are super-sensitive (these people are classified as “super-tasters”) and 50% of the population are sensitive (these are classified as “tasters”).1 The remaining quarter of the population is insensitive to the bitterness of certain plant chemicals. The bitter taste receptors respond mainly to plant secondary compounds –chemicals that make the plant unpalatable or even toxic to predators (in this case, plant-eaters!). Ability to taste bitter chemicals is related to genetic make-up. Humans reject foods that are excessively bitter, and this may be crucial to survival. Abnormal bitter flavor is associated with dietary danger – rancid fats, toxins and microbial fermentation result in bitter tasting compounds. The risk associated with plant toxins depends on the strength of the toxin and the dose. At moderate doses, secondary plant compounds may have beneficial effects. This brings me to my real topic – cruciferous vegetables! These plants have flower petals resembling a cross –making them “cross-bearing” plants. Members include broccoli, cauliflower, cabbage, Brussels sprouts, turnips, kale and other greens. Here is the sticky part – these cruciferous vegetables have some of those bitter secondary compounds, possibly toxic in large quantities, and it turns out that “super-tasters” usually dislike these vegetables in particular! The second point is that nutritionists recommend that people eat cruciferous vegetables, because these bitter compounds have cancer protective properties, and because they are rich sources of other nutrients. Another complication - bitterness is a quality that food producers and manufacturers try to minimize for more acceptable sensory qualities, possibly reducing the health protective benefits. Methods to produce milder versions include selective breeding, removal of the compounds in processing or modification through cooking or the use of additives.2 Cancer researchers, on the other hand, have proposed that selective breeding for heightened bitterness might be a plus for health!2 While the benefit vs. risk of consuming certain plant chemicals remains a hot topic of research, the best advice is moderation. At moderate levels, cruciferous vegetables may help inactivate precursors of cancer formation, while at high doses their beneficial effects may be lost. The bottom line, with guidance from USDA’s MyPyramid, is that 3 to 4 cups per week of varied cruciferous vegetables is safe and beneficial.
1. Krebs, J R; The gourmet ape: evolution and human food preferences; Am J Clin Nutr 2009; 90(suppl):707S-11S) 2. Drewnowski, A and Gomez-Carneros,C; Bitter taste, phytonutrients and the consumer: a review; Am J Clin Nutr 2000; 72;1424-1435

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