According to the study's authors, components in green tea known as polyphenols are smaller than those found in black tea, which is why they get absorbed through your body's tissues, thereby impacting energy metabolism in the liver. However, black tea's polyphenols are too large to get through the small intestine and into the rest of your body, which is why it was previously unclear whether or not they would have a beneficial effect on weight loss.
"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, the study's primary author and an adjunct professor at UCLA's Center for Human Nutrition. "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."
That particular mechanism appears to be one which changes the bacterial ratio in the intestine by increasing the microbes connected to lean body mass and reducing those connected to obesity. While both black and green teas act as prebiotics in such a way, it appears that black tea might have a leg up over its green partner.
The study required 4 groups of mice to be fed different diets. One group ate high-fat, high-sugar meals, while another had low-fat, high-sugar foods. The remaining two groups of mice were both on high-fat, high-sugar diets, but one of them received a black tea extract, while the other one got given a green tea extract.
After a month had passed, both groups which were given tea extracts had weights which were in line with those on the low-fat diets. It was found that for both of those groups, intestinal samples showed lower levels of obesity-related bacteria and higher levels of lean-related bacteria. However, only the mice which were given the black tea extract showed heightened levels of a bacteria known as Pseudobutyrivibrio, which, the researchers believe, is the secret behind the extract's success.
These mice were also found to have a heightened level of short-chain fatty acids in their stomachs, compounds which have previously been linked to a beneficial effect on energy metabolism.