by Matthew Denos, PhD
(The Best Years in Life) Green tea, one of the most popular beverages in Asian countries, has been regarded as a medicine and healthful beverage since ancient times. It was around 2700 BC when the legendary Chinese emperor, Shen Nung, discovered the detoxifying and health-maintaining effects of green tea. Since then it has been traditionally used for the prevention and treatment of a broad range of illnesses.
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The earliest report that green tea has a weight loss promoting effect comes from the Chinese pharmacist Wang Ang (1615-1695) who observed that drinking tea for a long period of time can eliminate fat. Yet, it was not until 1995 when the systematic medical evaluation of green tea as anti-obesogenic agent began.
In the last 15 years, the health-promoting effects of green tea have been extensively investigated. Evidence from epidemiological and laboratory studies confirms traditional observations and suggests that green tea can protect against heart disease, cancer, diabetes, and obesity.
Obesity is a great public health burden costing the US more than $100 billion annually in medical spending. This is because obesity has increased at an alarming rate and is a risk factor for many chronic diseases. Obesity is the result of positive energy balance where energy intake is higher than energy expenditure. Weight loss requires negative energy balance. This can be achieved by either reducing energy intake or increasing energy expenditure.
How Does Green Tea Help You Lose Weight?
Green tea contains polyphenols, referred to as catechins, which have various physiological effects. Experiments have shown that tea catechins can suppress the excessive accumulation of body fat in humans.
An epidemiological study of 1103 Taiwanese adults found that tea drinkers, who consumed tea at least once per week for more than 10 years, had 20% lower body fat and 2% lower waist-to-hip ratio (slimmer waistline) compared to non-habitual tea drinkers [1]. A population study of 4280 Netherlanders found an inverse relationship between catechin consumption and Body Mass Index (BMI) over a 14-year period [2].
A number of studies have examined the effect of green tea consumption on body weight. In the majority of studies, participants who consumed a high-catechin green tea beverage for 12 weeks experienced a significant decrease in body weight, waist and hip circumference, body fat mass and LDL cholesterol. It was suggested that ingestion of green tea high in catechins can contribute to the prevention from and improvement of various lifestyle-related diseases, particularly obesity. The participants in these studies did not change their usual dietary intake or normal physical activity.
You Store Less Fat
The ability of our body to store fat depends on the size and number of its fat-storing cells, called adipocytes (fat cells). If the number of fat cells is low, the room available for fat storage is limited. Adipocytes are generated by pre-adipocytes through a process called adipogenesis.
Once adipocytes are generated by pre-adipocytes they proliferate to form more adipocytes (fat tissue) and increase our body’s storing capacity. Adipogenesis, the formation of fat tissue, is an ongoing process in our body. Fat cells are being created to replace those that die.
Cell studies have shown that green tea inhibits not only the proliferation of adipocytes but also their formation from pre-adipocytes (adipogenesis) [3]. It is speculated that people who consume green tea have fewer adipocytes and therefore a lower fat storing capacity.
You Spend More Energy
Resting Energy Expenditure (REE) is the amount of calories you spend in one day at rest. It is the energy you spend if for one day you do nothing but sleep. REE is also known as basic metabolism. Part of this energy is expended in the form of heat production—thermogenesis—as your body tries to keep its temperature constant. Some types of food, called thermogenic foods, have the ability to increase your body’s heat production, and thereby stimulate your basic metabolism.
A French study investigated whether daily administration of capsules containing green tea extract could stimulate thermogenesis and increase REE in humans [4]. In this study, 10 healthy men ingested green tea extract, caffeine or placebo along with their breakfast, lunch and dinner. Treatment with the green tea extract resulted in a significant 4% increase in REE, which was higher than that with both caffeine and placebo.
In a similar study, 31 men and women consumed 3 servings a day of a green tea beverage [5]. On the third day, REE had increased by 4-8%. This is equivalent to an extra 100 calories/day burned by basic metabolism. The study suggested that when consumed regularly as part of a healthy diet and exercise regime, green tea can provide benefits for weight control.
You Can Maintain Your Lower Weight
The effect of green tea on metabolism is particularly important for people who want to lose weight and maintain it. Weight reduction is associated with a decrease in metabolic rate or REE. This is because human metabolism depends on body weight. Lower weight usually means lower metabolism.
This decrease in metabolic rate during weight loss is responsible for the weight loss plateau that dieters encounter soon after they lose their first pounds. It also accounts for the difficulty in maintaining weight loss. By increasing the REE, green tea counteracts the decrease in metabolic rate during weight loss and leads to sustained weight reduction. This is supported by studies showing that habitual consumption of green tea aids weight maintenance following weight loss [6].
You Absorb Less Fat From Food
The fat content of the food we consume cannot be absorbed in the intestines unless it undergoes two processes:
1. Emulsification of lipids by bile salts. This is an indispensable process for lipid intestinal absorption because the intestines can absorb only water-soluble macronutrients. Lipids are water-insoluble until bile salts transform them into water-soluble particles called micelles (emulsified lipids).
2. Digestion of lipids by lipases. Lipids are broken down by certain enzymes called lipases which are secreted in our stomach and the small intestine.
Green tea and its catechins lower the intestinal absorption of lipids. In particular, EGCG, the most abundant and strongest bioactive catechin in green tea, blocks both the emulsification of lipids (formation of micelles) and the function of gastric lipase and pancreatic lipase, two key enzymes involved in fat digestion. This way, green tea inhibits the intestinal uptake of lipids and therefore limits the caloric intake of fatty foods. The effect of green tea on lipid digestion is manifested as increased fecal lipids [7].