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 qualities of green tea is its ability to limit the absorption of fat intake from the diet. This process occurs because Green tea catechins – especially EGCG – interfere with the lipase – fat-digesting – enzymes in the small intestine and stomach. Since the fats are not completely digested, lipid droplets are produced that cannot enter intestinal cells, and so they remain unabsorbed. This chain of action has produced a significant decrease in the absorption of dietary fats by rats consuming green tea. Any interface with the efficient absorption and digestion of dietary fats could figure prominently in any effort to manage weight effectively, even though it is not proven how effective green tea is in blocking fat absorption in humans.
Green tea catechins not only decrease the efficiency of absorption of fatty acids from the diet, they also interfere with the production of fat for storage in adipose tissue depots. Green tea leaf extract that is both purified and rich in EGCG decreases the activity of fatty acid synthase, which is an enzyme that regulates how quickly the body produces fat for storage. This process is consistent with a body of literature recently analyzed in the journal Molecular Nutrition and Food Research, which shows how EGCG inhibits new fat formation – “lipogenesis” – and fat storage within adipocytes. Tea aids in the absorption of less fat from the diet, and can also interfere directly with the storage of fat in adipocytes, a dual mechanism for supporting healthy body weight.
The dominant green tea catechin, EGCG, also entices the body to shift some of its manner of producing energy from glucoseburning to fat-burning. There are two ways to accomplish this and EGCG seems to do both. First, if the amount of glucose available to tissues, especially the skeletal muscles, is reduced, then more fat must be metabolized to carbon dioxide and water in order to satisfy energy needs. During times when glucose is in short supply, the liver synthesizes glucose from a variety of precursors, including amino acids released by muscle cells. The first enzyme in this synthetic (“gluconeogenic”) pathway, phosphoenolpyruvate carboxykinase (PEPCK), is inhibited by EGCG. Blocking this enzyme reduces the rate of formation of new glucose, requiring cells to switch to burning fat for 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.
Because under the conditions of this experiment all energy expenditure was essentially “resting” energy expenditure, the catechin-induced increase in resting energy expenditure reflects enhanced thermogenesis. That is, more heat production as a “byproduct” of energy production. Since increased heat production to satisfy the same energy demand means that the efficiency of energy production decreased, more stored energy needed to be “burned” – accelerating the rate at which energy stored in fat depots would become depleted. Of course, as stored fat becomes depleted, both body weight and fat depot size decrease. The increase in fat utilization in this experiment, which was minimized by keeping the subjects in a “resting” state, could result in the loss of one pound of excess body weight in 1 to 2 months and a loss of 6 to 12 pounds in a year. Consistent with this rough prediction, overweight adults consuming 270 mg of EGCG daily for 3 months experienced an average loss of 4.6% of total body weight, with an average decrease in waist circumference of 4.5%. This thermogenic effect of green tea catechins, when combined with a healthy diet and exercise, could be extremely beneficial for those 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 Abdominal fat and Stress?
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 – L-theanine. This amino acid comprises up to 2.5% of the total dry weight of unfermented green tea leaves, is absorbed efficiently and can enter the brain from the blood. Within the brain, L-theanine exerts relaxing physiologic effects. In so doing, L-theanine may act to reduce perceptions of stress with possible beneficial effects on abdominal fat formation. For example, mice fed L-theanine have gained less weight and accumulated less abdominal fat. By supporting the body’s stress response, green tea and green tea extracts containing L-theanine can make important contributions to healthy weight maintenance.