Brewing the latest in tea research.

Tea is the most-consumed beverage next to water. For some 5000 years, people have been getting a lift from brewing tea leaves. But it's only been in the last three decades that researchers have immersed themselves in the science behind tea's purported health benefits.

Several USDA-ARS scientists are studying the bioactivity of tea compounds. Some have studied the relationship between tea and metabolism, and others have looked at the effect of tea on blood glucose and cholesterol levels. Investigators are studying tea's impact on the ability of the body's cells to handle oxidative stress.

ARS 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 four separate beverage formulas for three consecutive days. Before the study, the volunteers refrained from consuming caffeine and had their 24-hour EE measured. The 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 EE of volunteers was about 3% greater after they drank either the caffeinated water or the full-strength tea than after they drank the colored water. On average, the volunteers burned an additional 67 calories a day when they drank tea instead of an equal amount of water. Fat oxidation was 12% greater after the full-strength tea treatment than after the water treatment. Data suggested that a component of tea other than caffeine might have promoted preferential use of fat as an energy source, but the information is tentative, and researchers need to undertake additional studies to confirm this.

It is universally accepted that caffeinated tea raises the metabolic rate because caffeine is a stimulant. Some scientists speculate that caffeine and EGCG--a highly active catechin in tea--may act together to increase fat oxidation. "Anecdotal evidence over time, particularly in China, points to a relationship between green tea consumption and weight loss," says Rumpler.

Scientists have found that drinking tea lowers cholesterol levels, and could possibly reduce the risk of coronary heart disease (CHD). While several epidemiological studies found that green and black tea consumption is associated with reduced risk of CHD, experimental studies had not confirmed this. A recent ARS study assessed the effects of black tea consumption on blood lipid and lipoprotein concentrations in adults with mildly high cholesterol. Scientists carefully controlled the volunteers' diet and weight. Seven men and eight women were given five servings of black tea a day for three weeks and a tea-flavored water for another three-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, there was a 6% to 10% lowering of blood lipids in drinkers of black tea in just three weeks. What's more, the study showed no effect on high-density lipoprotein, or good cholesterol. The study's authors concluded that drinking black tea, and following a prudent diet moderately low in fat, cholesterol and saturated fatty acids, reduce total and LDL cholesterol by significant amounts and may also reduce the risk of CHD.

Others have determined that regularly brewed tea, when added to the fat cells of laboratory rats, increased insulin activity by more than 15 times. This increased insulin activity was found with green, black and oolong teas, whether caffeinated or decaffeinated. Research showed that in green and oolong teas, EGCG was largely responsible for the results. In black tea, active ingredients included tannins and theaflavins, in addition to EGCG. Confirmational studies in humans are required before the results can be applied to people.

Other ARS investigators completed a pilot study during which eight volunteers with type II diabetes lowered their blood sugar levels by 15% to 20% after drinking six cups of tea per day for eight weeks. Researchers have since launched a 24-week, randomized, double-blind study involving 40 male and female volunteers with type II diabetes, not taking insulin. They want to examine the effect that green and black teas have on the glucose levels of the volunteers.

Since nearly 95% of tea's polyphenol compounds are flavonoids, tea ranks among the plants with the highest total flavonoid content. Green tea contains more simple flavonoids--catechins--while black tea contains more complex varieties--the thearubigins and theaflavins. Some polyphenols have been determined in test-tube studies to be more potent antioxidants than the well-known vitamins A, C and E. But results from such tests cannot be applied to humans because they do not account for factors such as bioavailability, metabolism and excretion.

Generally, consuming a variety of teas and preparations adds nutritional benefits to the diet. Researchers hope that future studies will be designed to accurately assess tea's polyphenol levels and to measure tea's role in lowering the risk of chronic diseases. Further information. Rosalie Bliss, USDA-ARS, 5601 Sunnyside Ave., Beltsville, MD 20705; phone: [phone]; fax: [phone]; email: [e-mail].