Update: Green Tea & Health

Green Tea Research Expands

Over the past decade, many experimental and population-health studies have linked green tea to a variety of health benefits such as the prevention of cardiovascular disease, cancer, and the decline in brain function that occurs with aging. A 2007 review indicates that, while not all the findings have been consistent, cardiovascular and metabolic health benefits can be obtained by regular consumption of 5-6 or more cups of green tea daily (1).

Buoyed by these findings, researchers are now studying green tea or green tea extracts in a more systematic manner by conducting controlled intervention trials. Three of these clinical trials, highlighted below, suggest that green tea may indeed have a beneficial effect on heart health.

Green Tea Supports Blood Vessel Health

Scientists studied the effects of 6 grams of green tea (the amount typically in 3 tea bags), 125 mg of caffeine (the amount contained in 6 grams of tea), or hot water on flow-mediated dilation of the brachial artery in healthy subjects (2). Flow-mediated dilation is a measure of the artery's ability to relax and expand, and is considered to be an independent predictor of cardiovascular risk.

Artery dilation increased significantly after tea, while it did not change with caffeine or the hot water placebo. This indicated that green tea's positive effects were not related to caffeine, rather that green tea's polyphenols were more than likely responsible for its effects on blood vessel health.

Response to Insulin Improved by Green Tea

Green tea has been shown to promote fat oxidation (fat burning) in humans, and to prevent obesity and improve the response to insulin in mice. This led researchers at the Human Performance Laboratory, University of Birmingham, to examine the effect of high dose green tea extract on glucose tolerance and fat oxidation in healthy men undergoing moderate intensity exercise (3).

Healthy young men underwent a glucose tolerance test and fat oxidation was measured before and after 30 minutes of cycling. In the 24 hour period before these tests, participants were given green tea extract containing about 366 mg of EGCG (the main polyphenol in green tea) or a placebo pill.

Average fat oxidation rates were 17% higher after the men took green tea extract compared to the test when they took the placebo. The scientists also observed an increase of 13% in insulin sensitivity after taking the green tea extract and cycling. Insulin sensitivity is a measure of how well the body responds to insulin's job of getting sugar into cells and out of the blood stream. Insulin sensitivity is one measure of one's risk for heart disease. The more insulin sensitive one is in general, the lower the risk for diabetes and heart problems.

Green Tea Protects LDL Cholesterol

In this study, researchers evaluated the effect of antioxidant green tea catechins (polyphenols) on LDL cholesterol in healthy adults (4). When LDL particles become damaged by oxidation they contribute to the process of atherosclerosis or clogged arteries.

The subjects were randomly divided into two groups, a control group and a group that received 500 mg of catechins, equivalent to about 6 or 7 cups of green tea. After 4 weeks, the green tea group had significantly lower levels of oxidized or damaged LDL cholesterol.

References

1. Wolfram S. Review. Effects of green tea and EGCG on cardiovascular and metabolic health. Journal American College of Nutrition 26:373S-88S, 2007.
   
2. Alexopoulos N, et al. The acute effect of green tea consumption on endothelial function in healthy individuals. European Journal of Cardiovascular Prevention Rehabilitation 15:300-5, 2008
   
3. Venables MC, et al. Green tea extract ingestion, fat oxidation, and glucose tolerance in healthy humans. American Journal of Clinical Nutrition 87:778-84, 2008.
   
4. Inami S, et al.Tea catechin consumption reduces circulating oxidized low-density lipoprotein. International Heart Journal 48:725-32, 2007.