"Physiologist William Rumpler is investigating the ancient Chinese belief that oolong tea is effective in controlling body weight. Rumpler is with ARS' Diet and Human Performance Laboratory (DHPL), one of seven laboratories that make up the Beltsville Human Nutrition Research Center (BHNRC), Beltsville, Maryland.

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 colored 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. 'Our data suggested that a component of tea other than caffeine might have promoted preferential use of fat as an energy source,' says Beverly Clevidence, a study coauthor and head of the DHPL. 'But the information is tentative, and we need more studies to confirm it,' she adds.

It is universally accepted that caffeinated tea raises metabolic rate because caffeine is a stimulant. 'The interesting part of our study, which agreed with findings from a similar study in England, was that when you drink tea you turn on the fat-burning spigot a little bit more than when you drink caffeinated water..."

The article also notes that tea leaf fermentation which turns green tea into oolong or black tea changes the metabolic effects, and that consuming a variety of teas gives the broadest benefits.