Like their better known chemical cousins, the carotenes, flavonoids are plant pigments, creating a rainbow of colors. In addition, many flavonoids and carotenes function as antioxidants and protect plants from damaging free radicals. The big difference is that flavonoids are water soluble, whereas carotenes are oil soluble.
The flavonoids were first isolated in the 1930s by Albert Szent-Gyorgyi, Ph.D., the Nobel laureate who discovered vitamin C. Szent-Gyorgyi found that flavonoids strengthened capillary walls in ways vitamin C could not and, at first, they were referred to as vitamin P. But the chemical diversity of flavonoids precludes their classification as a single vitamin.
The major dietary sources of flavonoids include fruit and fruit products, tea, and soy. Studies have found that the flavonoids in these foods protect against heart disease and cancer.
The Benefits of Soy
The flavonoids in soybeans have also been attracting attention. In a recent analysis of 730 people and 38 medical studies, James W. Anderson, M.D., found that flavonoid-containing soy protein can dramatically lower blood levels of cholesterol.
Anderson, an endocrinologist and nutritionist at the University of Kentucky, Lexington, found that daily consumption of 47 grams of soy protein-one-tenth of a pound-significantly decreased total cholesterol, the “bad” low-density lipoprotein (LDL) form of cholesterol, and triglycerides.
Overall, substituting soy protein for about one-half of the meat protein in the diet reduced total cholesterol by an average of 9.3 percent, LDL by 12.9 percent, and triglycerides by 10.5 percent. People with very high cholesterol levels-above 335 mg per deciliter of blood-benefitted the most. On average, adding soy to their diet resulted in a 19.6 percent cholesterol reduction, according to Anderson’s article in the New England Journal of Medicine (Aug 3, 1995;333:276-82).
Although the amounts of soy protein consumed varied in the 38 studies, Anderson estimated that 25 grams daily would probably reduce blood cholesterol levels by an average of 8.9 percent and 50 grams by 17.4 percent.
According to Anderson, it would be very easy for people to increase their soy consumption. An 8-ounce glass of soy milk contains 4 to 10 grams of soy protein, 4 ounces of tofu contain 8 to 13 grams of soy protein, and a soy hamburger or hotdog contains about 18 grams of soy protein. Drinking two glasses of soy milk (instead of regular milk) and eating one soy burger daily would provide approximately 30 grams of soy protein.