What is Flavnoids?
As researchers probe the once-hidden depths of foods, they're discovering that vitamins and minerals are merely the tip of the nutritional iceberg.
Perhaps the largest group of "other" nutrients are the flavonoids, also known as bioflavonoids. Researchers have identified more than 4,000 of them in plants.
What Are Flavonoids?
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 Wine...or the Grape?
One clue to the health benefits of flavonoids comes from studies of the "French paradox." The paradox is that the French eat almost four times more butter and three times more lard-and have higher cholesterol levels and blood pressures-than do Americans. Yet the French are 2.5 times less likely than Americans to die of coronary heart disease.
Many people have suggested that the liberal French consumption of red wine protects against coronary heart disease, apparently by lowering cholesterol levels or preventing abnormal blood clots.
In fact, at least eight medical studies have found that a glass or two of wine daily protects against heart disease. But some studies have reported that red wine is better than white wine, suggesting that some of the benefits might be unrelated to the alcohol.
To better understand the potential benefits of moderate wine drinking, a team of Israeli researchers led by Alexendra Lavy, Ph.D., compared the effects of red and white wine on 20 healthy men.
Half of the subjects were given 400 milliliters (roughly two glasses) of either red or white wine daily for two weeks. Both of the wines contained 11 percent alcohol.
Lavy and her colleagues examined how the wines affected blood fats. The most dramatic effect was the increase in high-density lipoproteins (HDLs), the so-called "good" cholesterol, among men drinking red wine.
Their HDL levels rose 26 percent and their apolipoprotein A-1 levels, closely related to HDL, increased 12 percent, according to Lavy's report in Annals of Nutrition and Metabolism (Sept./Oct. 1994;38:287-94). The white-wine drinkers had no change in HDL.
On the negative side, red-wine drinkers had a 26 percent increase in triglyceride levels, a type of blood fat associated with risk of heart disease. Furthermore, Lavy found no decrease in blood clotting among either the red- or white-wine drinkers.
As it turns out, the color of the grapes may be more important than the wine itself. John D. Folts, Ph.D., director of the coronary thrombosis laboratory at the University of Wisconsin, Madison, recently fed dogs red wine, white wine, or unsweetened "purple" grape juice and then measured their blood-clotting factors and blood flow. Red grapes are particularly high in two flavonoids, quercetin and rutin, which are absent in white grapes.
Folts found that blood clotting decreased and blood flow increased in the dogs given red wine and purple grape juice, but not in the animals fed white wine. "It is therefore possible to speculate that the cardioprotective effects of red wine consumption observed in the French and other populations may be attributed in part to the ethanol content of the wine and in part to the antioxidant and platelet inhibitory properties of other naturally occurring compounds in the wine the consumption of flavonoid-containing foods and beverages may retard atherogenesis and prevent thrombosis on a daily basis," Folts wrote in Circulation (Feb 15, 1995;91:1182-8).
Another dietary factor likely influences the French paradox. In addition to their wine consumption, the French eat a high-flavonoid diet. In an analysis of the eating habits of people in 40 nations, William E. Connor, M.D., an expert in blood fats at Oregon Health Sciences University, failed to find a strong relationship between wine consumption and heart disease risk. Instead, Connor determined that the French eat large quantities of vegetables, rich in vitamins, carotenes, and flavonoids, according to his article in Circulation (Dec. 1993;88;2771-9).
Flavonoids in Tea:
Common green and black tea leaves consist of about 25-30 percent flavonoids, including quercetin and gallic esters. Like the flavonoids in grapes and other fruits and vegetables, they also protect against heart disease.
For five years, Dutch researchers Michaël Hertog, M.Sc., and Edith Feskens, Ph.D., followed the dietary and lifestyle habits of 805 men ages 65-84 in the town of Zutphen. After accounting for the men's physical activity, smoking habits, and intake of vitamins C and E, and beta-carotene, the researchers found that men eating a lot of flavonoids-in tea, onions, and apples-were far less likely to suffer heart disease or heart attacks than men eating few flavonoids.
"There is evidence that free-radical oxidation of LDL plays an important part in atherogenesis," Hertog and Feskins wrote in Lancet (Oct. 23, 1993;342:1007-11). "Flavonoids are scavengers of free radicals...It is possible that quercetin and other flavonoids reduce the rate of formation of oxidised LDL and thus inhibit the growth of atherosclerotic plaques."
The benefits of tea flavonoids were confirmed earlier this year by researchers at the Saitama Cancer Research Center, Japan. K. Imai, PhD, and K. Nakachi, PhD, studied 1,371 men enrolled in a 40-year study of eating habits and health. They reported that elderly men who drank 10 or more cups of green tea daily had lower blood levels of cholesterol and triglycerides than men who drank less of the tea.
"Consumption of green tea was significantly associated with lower serum concentrations of lipids and lipoproteins," they wrote in the British Medical Journal (March 18, 1995;310:693-6). "An increase in consumption substantially decreased serum total cholesterol and triglyceride concentrations, and this strong association remained almost unaltered even after age, cigarette smoking, alcohol consumption, and relative body weight were controlled for."
The heavy tea drinkers also benefited from higher blood levels of the HDL and lower levels of the "bad" low-density lipoprotein (LDL) form of cholesterol. In addition, they had lower levels of certain liver enzymes, suggesting a reduced risk of liver disease.
"Green tea has many advantages over chemical preventive agents-tea is non-toxic and thus readily available to the general population," Imai and Nakachi wrote.
Drinking either green or black tea also protects against skin cancer by ultraviolet (UV) light and hazardous chemicals. In an experiment, Allan Conney, Ph.D., of the College of Pharmacy, Rutgers University, brewed four types of teas in concentrations comparable to what people drink: black tea, green tea, decaffeinated black tea, and decaffeinated green tea. He gave one type of each tea to four different groups of mice as their sole source of drinking fluid, then exposed them to either cancer-causing UV radiation or chemicals. For comparison's sake, he gave another group of mice plain water to drink.
Conney reported in Cancer Research (July 1, 1994;54:3428-35) that the teas "markedly inhibited" the numbers and sizes of tumors. Black tea was most effective, reducing the number of tumors by 93 percent in comparison with mice fed water. Green tea was almost as good, resulting in 88 percent fewer tumors. The decaffeinated black and green teas resulted in 77 and 72 percent fewer tumors, respectively.
Conney wrote that the benefits of tea are probably related to the antioxidant effect of the flavonoids. Why were the decaffeinated teas less beneficial? It's very possible that the decaffeination process removes some flavonoids as well as caffeine.
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.
The flavonoids are an important reminder that the nutritional benefits of wholesome foods go beyond familiar vitamins and minerals. Nutrition research is ongoing and new discoveries are made every year.
The most convenient to reach for a high-potency flavonoid is tablet at your vitamin store.
Food Science's Bioflavonoids
Douglas Lab's FlavonALL