Background: Green tea consumption is reportedly associated with various health-promoting properties. For example, it has been shown to promote fat oxidation in humans at rest and to prevent obesity and improve insulin sensitivity in mice.

Objective: We investigated the effects of acute ingestion of green tea extract (GTE) on glucose tolerance and fat oxidation during moderate-intensity exercise in humans.

Design: Two studies were performed, both with a counter-balanced crossover design. In study A, 12 healthy men performed a 30-min cycling exercise at 60% of maximal oxygen consumption (VO2max) before and after supplementation. In study B, 11 healthy men took an oral-glucose-tolerance test before and after supplementation. In the 24-h period before the experimental trials, participants ingested 3 capsules containing either GTE (total: 890 +/- 13 mg polyphenols and 366 +/- 5 mg EGCG) or a corn-flour placebo (total: 1729 +/- 22 mg).

Results: Average fat oxidation rates were 17% higher after ingestion of GTE than after ingestion of placebo (0.41 +/- 0.03 and 0.35 +/- 0.03 g/min, respectively; P < 0.05). Moreover, the contribution of fat oxidation to total energy expenditure was also significantly higher, by a similar percentage, after GTE supplementation. The insulin area under the curve decreased in both the GTE and placebo trials (3612 +/- 301 and 4280 +/- 309 microIU/dL . 120 min, respectively; P < 0.01), and there was a concomitant increase of 13% in insulin sensitivity.

Conclusions: Acute GTE ingestion can increase fat oxidation during moderate-intensity exercise and can improve insulin sensitivity and glucose tolerance in healthy young men.