The Durk Pearson & Sandy Shaw®
Life Extension NewsTM
Volume 5 No.
6 • December 2002
Tea Enhances Insulin Activity
tea has been reported to have antidiabetic effects, but human studies have
failed to detect consistent effects in changes in blood glucose.
Streptozotocin mice, a model of human diabetes, treated with various types of
green, oolong, and black tea show lowered levels of blood glucose.
catechins inhibit alpha-amylase
active components in tea that cause the reduced glucose levels in animals are
not known, but epicatechin gallate was shown to have the greatest activity of
the catechins in lowering glucose uptake by Caco-2 cells. Epicatechin gallate
was also reported to inhibit glucose uptake in the brush border membrane
vesicles from rabbit small intestine. Most interesting, however, is the fact
that catechins have been shown to inhibit enzymes that hydrolyze carbohydrates,
including alpha-amylase, a major starch-digestive enzyme that converts starch to
glucose. This may explain at least in part why a mixture of green tea catechins
suppressed increases in blood glucose and insulin following carbohydrate
ingestion in rats. Moreover, in another study cited in this paper,
humans ingesting 50 grams of starch following consumption of 200–500 mg of tea
catechins had a suppression of the elevation of glucose and insulin levels.
Intestinal glucose uptake is reported to be markedly inhibited by green tea
polyphenols, especially those polyphenols having galloyl residues.
authors performed a rat study on the effects of green tea on insulin
activity. They evaluated the insulin-potentiating activity of approximately 40
black, green, and oolong teas and found all to enhance insulin activity in the
insulin-potentiating epididymal fat-cell assay. Instant teas were not found to
have such activity except for one brand (not named). They found the most active
green tea constituent in potentiating insulin action to be epigallocatechin, but
tannins, theaflavins, and epicatechin gallate were also found to have
insulin-enhancing activity and to account, the authors stated, for the number of
fractions of black tea that had insulin-enhancing activity.
added to tea inhibited in vitro insulin potentiation
(2%) was found to inhibit the tea insulin potentiation when used at about 1
teaspoon per cup (237 ml), decreasing activity by roughly 33%. Nondairy creamers
also decreased insulin-enhancing activity. The decreased insulin-enhancing
effects of milk and nondairy creamers was due to the precipitation of
epigallocatechin, gallocatechin gallate, and epicatechin gallate. However, the
researchers also cited another paper in which it was reported that
drinking a mixture of tea and milk did not reduce the bioavailability of the tea
catechins. They suggest that perhaps the catechin-milk complexes dissociate and
allow the tea catechins to be absorbed. Yet another paper reported
that the antioxidant potential of tea alone or tea plus lemon was greater than
that observed when milk was added to tea.
authors propose that one possible reason why previous studies of humans drinking
tea have not found reduced glucose levels is that more effective insulin could
lead to lower levels of insulin with no changes in glucose. They note that in a
recent study they performed using chromium, they observed no changes in glucose
clearance but a very significant effect on circulating insulin. They also note
that polyphenols are very rapidly cleared from the blood; hence, measuring
glucose levels after an overnight fast would likely yield no effect of tea
consumption because the half-life in humans for epigallocatechin gallate is less
than 6 hours, and those for epigallocatechin and epicatechin are less than 4
- Anderson, Polansky. Tea enhances insulin activity. J Agric Food Chem 50:7182-6
- van het Hof et al. Bioavailability of catechins from tea: the effect of milk. Eur
J Clin Nutr 52:356-9 (1998).
- Tewari et al. Comparative study of antioxidant potential of tea with and without
additives. Indian J Physiol Pharmacol 44:215-9 (2000).