Several scientists at various laboratories are studying the bioactivity of tea compounds. Some have studied the relationship between lea and metabolism, and some have looked at the effect of tea on blood glucose and cholesterol levels. Others are studying tea's impact on the ability of the body's cells to handle oxidative stress. And some are looking at the effect of green tea on slowing development of abnormal blood vessels in lab mice. In certain diseases, including cancer, angiogenesis-growth of new blood vessels-becomes excessive. These new vessels not only provide nourishment to tumors, but they also serve as portals through which rumor cells can escape into the body's circulatory system and spread to other organs. Compounds found to be anti-anglogenic may prove therapeutic by "starving" the tumors.

Differences in Tea Types

The age-old sinensis plant Camellia is the source of all non-herbal teas. Today, about 75 % of the tea produced worldwide is black; about 23 percent is green; and about 2 percent is oolong.

Consider that coffee beans are green before roasting turns them brown and ready for market. Tea leaves are also green at harvest. To achieve a variety of taste profiles, manufacturers carefully control whether, and for how long, tea leaves are exposed to air, a process called fermentation. When fermentation is completely arrested, the tea stays "green" or yellowish brown. When fermentation time is long, the leaves darken and become "black" tea. Somewhere in between these two extremes, "oolong" tea is created.

Over the centuries, as Camellia sinensis plants grew in the sun, they protected themselves against photo-syntheric stressors by forming chemicals known as polyphenols. This group of beneficial compounds includes flavonoids-the same class of compounds that give many fruits and vegetables their antioxidant boost. It is perhaps because of tea's high antioxidant activity that tea research is taking such a variety of turns.

The Metabolics of Tea

Physiologist William Rumpler is investigating the ancient Chinese belief that oolong tea is effective in controlling body weight.

To measure how tea influences energy expenditure (EE), Rumpler and colleagues gave each of 12 male volunteers 4 separate beverage formulas for 3 consecutive days - Before the study, the volunteers refrained from consuming caffeine and had their 24-hour EE measured. EE was measured again on the third day of each formula treatment. The treatments consisted of full-strength tea, colored water with caffeine equal to full-strength tea, half-strength tea, and colored water.

The results showed that the EE of volunteers was about 3 percent higher after they drank either the caffeinated water or the full-strength tea than after they drank the coloured water.

On average, the volunteers burned an additional 67 calories a day when they drank tea instead of an equal amount of water. Perhaps most interesting was that fat oxidation was a significant 12 percent higher after the full-strength tea treatment than after the water treatment.

"Anecdotal evidence over time, particularly in China , points to a relationship between green tea consumption and weight loss," says Rumpler. "But until we do a really comprehensive" study in which we have humans drink tea and see whether they lose weight, we can't actually say that green tea makes people lose weight. What we can say is that it raises metabolic rates and increases fat oxidation rates. Those are two things that are predictive of weight loss."

Coping with Confounding Variables

In September 2002, ARS and other researchers met at the U.S. Department of Agriculture in Washington , D.C. , for the Third International Scientific Symposium on Tea and Human Health. The symposium was hosted by organizer Jeffrey B. Blumberg, associate director of the Jean Mayor USDA Human Nutrition Research Center on Aging at Tufrs University in Boston, Massachusetts . Blumberg is also director of HNRCA's Antioxidants Research Laboratory.

One Issue discussed there was inconsistency among early studies of tea. "Confounding"-a situation in which findings are affected by a variety of uncontrolled factors-can occur. "Some studies are simply not sensitive enough to eliminate confounding factors," says ARS chemist Joseph- Judd, who is with DHPL. In the case of tea studies, it could be as simple as a volunteer's getting the same flavonoids that are in tea from other foods consumed during the study.

Blumberg notes that flavonold concentrations differ in tea beverages, depending on whether the preparation was blended, decaffeinated, brewed, or iced. Milk protein, for example, when added to tea, had previously been reported to possibly bind to, and therefore reduce, the flavonoid concentrations.

"There has been only one study that showed that adding milk decreased the bioavallabtlity of catechins in tea," says Blumberg. "Those results were not replicated in any of several subsequent studies." Many factors can affect the way tea compounds are absorbed, metabolized, and excreted, according to Blumberg.

A Low-Down on Lipids

Judd, who is with the DHPL, is the lead author of a recent study that found that drinking tea lowered cholesterol and, therefore, could possibly reduce the risk of coronary heart disease

(CHD). Judd points out that while several epidemiological studies found that green and black tea consumption is associated with reduced risk of CHD, experimental studies had not confirmed this. "The experimental studies did not control the background diet of the volunteers," says Judd. "Other foods or nutrients consumed during the studies could very well have affected the risk factors."

Judd's recent study assessed the effects of black tea consumption on blood lipid and lipoprorein concentrations in adults with mildly high cholesterol. He carefully controlled the volunteers' diet and weight. Seven men and eight women were given five servings of black cea a day for 3 weeks and a tea-flavored water for another 3-week period. In a third study period, caffeine was added to the tea-flavored water in an amount similar to that found in the tea.

"Overall, we found a 6 to 10 percent lowering of blood lipids in drinkers of black tea in Just 3 weeks," says Judd. What's more, the study showed no effect on high-density lipoprotein, or "good," cholesterol.

The study's authors concluded that drinking black tea-along with following a prudent diet moderately low in fat, cholesterol, and saturated fatty acids-reduce total and LDL cholesterol by significant amounts and may, therefore, reduce the risk of CHD.

Enhancing Insulin Activity

ARS chemist Richard Anderson found that regularly brewed tea, when added to the fat cells of laboratory rats, increased insulin activity by more than 15 times. Anderson is with the BHNRC's Nutrient Requirements and Functions Laboratory. He noted that this increased insulin activity was found with green, black, and oolong teas, regardless of whether caffeinated or decaffeinated.