They noted that the percentage of bacteria associated with obesity decreased, while bacteria associated with lean body mass increased.
Previous studies show that chemicals in green tea called polyphenols are absorbed and alter the energy metabolism in the liver.
The new study, published in the European Journal of Nutrition, shows that black tea polyphenols, which are too large to be absorbed in the small intestine, stimulate the growth of gut bacterium and the formation of short-chain fatty acids, a type of bacterial metabolites that has 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 persons well-being, researchers said.
"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 Susanne Henning, professor at UCLA.
"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," Henning said.
Researchers gave four groups of mice different diets - two of which were supplemented with green tea or black tea extracts.
The first group received low-fat, high-sugar diet, the second a high-fat, high-sugar diet, the third a high-fat, high-sugar and green tea extract, and the last group got a high-fat, high-sugar and black tea extract diet.
They noted that 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 team also collected samples from the mices 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, researchers said.