UCLA researchers have demonstrated for the first time that black tea may promote weight loss and other health benefits by changing bacteria in the gut. In a study of mice, the scientists showed that black tea alters energy metabolism in the liver by changing gut metabolites. The study found that both black and green tea changed the ratio of intestinal bacteria in the animals – the percentage of bacteria associated with obesity decreased, while bacteria associated with lean body mass increased.
Previous studies indicated that chemicals in green tea called polyphenols are absorbed and alter the energy metabolism in the liver. The new findings show that black tea polyphenols, which are too large to be absorbed in the small intestine, stimulate the growth of gut bacteria and the formation of short-chain fatty acids, a type of bacterial metabolites that have been shown to alter the energy metabolism in the liver.
“The results suggest that both green and black teas are prebiotics, substances that induce the growth of good microorganisms that contribute to a person's well-being,” says Susanne Henning, PhD, adjunct professor at the UCLA Center for Human Nutrition in the David Geffen School of Medicine at UCLA.
In the study, four groups of mice received different diets: low fat and high sugar; high fat and high sugar; high fat, high sugar and green tea extract; and high fat, high sugar and black tea extract. After four weeks, the weights of the mice that were given green or black tea extracts dropped to the same levels as those of the mice that received the low-fat diet throughout the study.
The researchers also collected samples from the mice's large intestines, to measure bacteria content and liver tissues to measure fat deposits. In the mice that consumed either type of tea extract, there was less of the type of bacteria associated with obesity and more of the bacteria associated with lean body mass. However, only the mice that consumed black tea extract had an increase in a type of bacteria called pseudobutyrivibrio, which could help explain the difference between how black tea and green tea change energy metabolism.
The study also concluded that green tea and black tea have different effects on liver metabolism. Dr. Henning says the molecules in green tea are smaller and can more readily be absorbed into the body and reach the liver directly, while black tea molecules are larger and stay in the intestine rather than being absorbed. When black tea molecules stay in the intestinal tract, they enhance the growth of beneficial bacteria and the formation of microbial metabolites involved in the regulation of energy metabolism.
“Decaffeinated Green and Black Tea Polyphenols Decrease Weight Gain and Alter Microbiome Populations and Function in Diet-induced Obese Mice,” European Journal of Nutrition, September 30, 2017