The study findings, published in the European Journal of Nutrition , demonstrate for the first time that black tea may promote weight loss and other health benefits by changing bacteria in the gut.
Led by researchers from the UCLA Center for Human Nutrition the study found that both black and green tea changed the ratio of intestinal bacteria -- resulting in a decrease in percentage of bacteria associated with obesity and an increase in bacteria associated with lean body mass.
"It was known that green tea polyphenols are more effective and offer more health benefits than black tea polyphenols since green tea chemicals are absorbed into the blood and tissue," said study lead author Susanne Henning. "Our new findings suggest that black tea, through a specific mechanism through the gut microbiome, may also contribute to good health and weight loss in humans."
"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," she said.
Black Tea Study
In the study, four groups of mice received different diets, two of which were supplemented with green tea or black tea extracts: low-fat, high-sugar; high-fat, high-sugar; high-fat, high-sugar and green tea extract; 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) -- finding that in the mice that consumed either type of tea extract, there was a change in the ratio of two key microbiome family groups.
Indeed, Henning and colleagues reported a decrease in the presence of Firmicutes bacteria that are associated with obesity and a higher number of Bacteroidetes that have been associated with lean body mass.
"The relative proportions of Blautia, Bryantella, Collinsella, Lactobacillus, Marvinbryantia, Turicibacter, Barnesiella, and Parabacteroides were significantly correlated with weight loss induced by tea extracts," wrote the team.
However, only the mice that consumed black tea extract had an increase in a type of bacteria called Pseudobutyrivibrio, which the team suggest could help explain the difference between how black tea and green tea change energy metabolism.
The study also concluded that both green tea and black tea have different effects on liver metabolism.
According to Henning, 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.
"We hypothesize that black tea polyphenols increased pAMPK through increased intestinal SCFA production, while green tea polyphenols increased hepatic AMPK through green tea polyphenols present in the liver," wrote the team.