As green tea intake increases, the degree of body fat in men and women, whether expressed as percent body fat or the ratio of waist circumference to hip circumference, decreases. Green tea aids to the maintenance of healthy body weight in many ways.
One of the least appreciated properties of green tea is its ability to limit the absorption of the fat taken in from the diet. Green tea catechins (especially EGCG) interfere with the lipase (fat-digesting) enzymes in the stomach and small intestine. The resulting incomplete digestion of fats produces some lipid droplets that are not able to enter intestinal cells and that therefore remain unabsorbed. These effects have produced significant decreases in the absorption of dietary fats by rats consuming green tea. While it is not known how effective green tea is in blocking fat absorption in humans, any interference with the normally highly efficient digestion and absorption of dietary fats could figure prominently in any effort to manage weight effectively.
In addition to decreasing the efficiency of absorption of fatty acids from the diet, green tea catechins interfere with the production of fat for storage in adipose tissue depots. Green tea leaf extract rich in EGCG, as well as purified EGCG itself, reduces the activity of fatty acid synthase, the enzyme that controls how rapidly the body produces fat for storage. This effect is consistent with a body of literature reviewed recently in the journal Molecular Nutrition and Food Research that shows how EGCG inhibits new fat formation (“lipogenesis”) and fat storage within adipocytes. Tea helps to absorb less fat from the diet and can directly interfere with the storage of fat in adipocytes, a dual mechanism for supporting healthy body weight.
EGCG, the dominant green tea catechin, allures the body to change some of its ways of producing energy from glucose-burning to fat-burning. There are two different ways of doing this, and EGCG appears to do both. The first way, is if the amount of glucose available to tissues (especially the skeletal muscles) is reduced, then additional fat must be metabolized to carbon dioxide and water in order to satisfy energy needs. When there is a shorter supply of glucose, the liver is able to synthesize glucose from a wide array of precursors, such as amino acids that are released by muscle cells. The primary enzyme in this synthetic, gluconeogenic pathway, is phosphoenolpyruvate carboxykinase (PEPCK), which is inhibited by EGCG. The blockage of this enzyme reduces the formation rate of new glucose, requiring cells to switch to fat-burning for a source of energy.
In an example of exquisite biochemical coordination, EGCG also stimulates the conversion of fatty acids to energy. In cell culture studies, EGCG has increased the rate of utilization of fatty acid breakdown products instead of glucose to produce energy. In a series of experiments, mice, often studied because the way they obtain energy is pretty much the same as the way humans do, have responded to the addition of catechin-rich green tea extract to a high-fat diet with less weight gain and less fat accumulation within their bodies than mice fed the same high-fat diet but not fed catechins, despite eating just as much. This phenomenon has been studied in depth. In a recent study dietary supplementation of exercising mice with tea catechins forced skeletal muscles to switch from using their glycogen reserves as energy sources to increasing their reliance on burning fats from adipose depots. This “switch” is so reproducible that the researchers can predict when it will happen. The powerful phytonutrients (catechins) in green tea and green tea extract can recruit muscles to help stored fat get used up faster!
In humans, such a shift from glucose-burning to fat-burning will be seen as an increase in heat production (or thermogenesis). In a convincing demonstration of the fat-burning, thermogenic effects of green tea catechins, 24-hour heat production was measured in healthy lean to overweight young men during days in which they remained essentially at rest and consumed identical diets, no caffeine-containing foods or beverages, and either a placebo, 150 mg of caffeine alone or 150 mg of caffeine plus 270 mg of EGCG and 105 mg of other mixed catechins. These investigators observed that the consumption of placebo or 150 mg of supplemental caffeine alone during a 12-hour period failed to affect the utilization of fat or glucose to supply energy. In contrast, the consumption of green tea catechins during a 12-hour period increased same-day 24-hour total energy expenditure and heat production. This increase in energy usage was caused by increased fat-burning and decreased use of glucose for fuel.
The catechin-induced increase in resting energy expenditure reflects enhanced thermogenesis in this experiment, being that under the experiment’s conditions all energy expenditure was essentially “resting” energy expenditure. This means that there is more heat production as a byproduct of energy production. Being that increased heat production needed to satisfy the same energy demand means that the efficiency of energy production has decreased, additional stored energy must be burned, which accelerates the rate at which energy stored in fat depots would become depleted. Both body weight and fat depot size decrease as fat becomes depleted. In this experiment, the increase in fat utilization that was minimized by keeping subjects in a resting state, might result in the loss of 1 lb of excess body weight over a one to two month period, and a loss of 6 to 12 lbs over a period of one year. Consistent with this hypothesis, overweight adults who consumed a total of 270 mg of EGCG daily over a three – month period experienced a loss of about 4.6% of total body weight, with an average decrease of 4.5% of waist circumference. When combined with exercise and a healthy diet, this thermogenic effect of green tea catechins could potentially be very beneficial for those who are looking to support weight management efforts.
Beneficial results also were obtained in a “gold standard,” randomized placebo-controlled clinical trial, published recently in the American Journal of Clinical Nutrition. In this study healthy men supplemented their diets with either 22 mg or 690 mg of total catechins daily for 12 weeks. At the end of the experiment, the men who were consuming 690 mg of total catechins daily had lost more weight, more inches off their waist, more total body fat and more abdominal fat.
What about Stress and Abdominal Fat?
A substantial amount of evidence signifies that stress and mood issues are associated with increased abdominal fat storage and a larger waistline in men and women. Stress and belly fat are connected by hormones; stress can increase the secretion of a hormone called cortisol. This hormone increases the rate at which fat is accumulated by abdominal fat cells. Repeated episodes of stress-related cortisol secretion can cause an increase in abdominal fat, even in healthy individuals.
What Can Green Tea Do About It?
Green tea contains an unusual amino acid called L-theanine, which comprises up to 2.5% of the total dry weight of unfermented green tea leaves. This amino acid is absorbed efficiently and is able to enter the brain from the bloodstream. While in the brain, L-theanine releases relaxing physiologic effects. In this process, L-theanine may act to reduce stress perceptions, which in turn could have beneficial effects on abdominal fat formation. In one example, mice that were fed L-theanine accumulated less abdominal fat formation, and gained less weight. Green tea and green tea extracts that contain L-theanine can add to the support of healthy weight maintenance by supporting the body’s stress response.
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