Leaf through a natural health magazine or stroll through a supplement aisle, and you'll get the impression that tea promotes weight loss. There’s some truth to that idea… though a recent evidence review suggests that tea and tea extracts may only bring modest weight-control benefits.
And those benefits seem to stem from tea’s caffeine, as much or more than from its unique array of polyphenol-type antioxidants, called catechins. Last year, researchers from the University of Connecticut reported the results of their review of the evidence from credible clinical trials testing tea for weight-loss and shape-altering effects.
Their analysis indicated that taking supplements containing only green tea’s antioxidant catechins (GTCs for short) doesn’t improve people’s body weight or measurements… at least in the short run.
In contrast, trial participants who consumed GTCs together with caffeine—as in brewed green tea—enjoyed reductions in BMI (body mass index), total body weight, and waist size. But while the changes were statistically significant, they were minor. As the UCONN team wrote, “…the clinical significance of these reductions is modest at best” (Phung OJ et al. 2009).
In other words, while a tea-drinking habit may enhance the effects of a generally healthful diet, it cannot overcome over-eating or under-exercising. That said, a new animal study from Japan suggests that black tea may rival green tea as a small but significant ally in weight control.
Black tea may match green for weight benefits: Study
Researchers working for Kirin Beverage Company—Japan’s famed brewery— conducted the new study in rodents fed a high-fat diet (Uchiyama S et al. 2010). Some of the rats also received either a whole black tea extract or isolated black tea antioxidants.
The scientists fed male rats a fat emulsion containing a black tea extract, at an intake level equaling either 500 mg or 1,000 mg per kilogram (2.2 lbs.) of body weight. After eight weeks, they measured the levels of fat in the blood and liver of the animals that got the tea, and in those that did not. Compared with the rats that got fat but no tea, the rats that got the black tea extracts had smaller increases in their blood fat (triglyceride) and liver fat levels.
The fact that the tea extracts limited rises in the animals’ blood fat levels in a “dose-dependent manner” lends great weight to the presumption that the tea blocked the rise in fat levels. In a separate study with female mice, the researcher supplemented their high-fat diets with a black tea extract for eight weeks. In that study, the female rats that got black tea showed no increase in body weight.
And while the showed increases in adipose tissue (belly fat) mass and liver fat, those increases were much less (56.9 and 81.7 percent lower, respectively) than the increases seen in the control mice.
Green versus black tea: Different antioxidants, similar fat-control benefits
Green tea contains between 30 and 40 per cent of water-extractable polyphenol antioxidants, while black tea (green tea that has been oxidized by fermentation) contains between 3 and 10 per cent. Oolong tea is semi-fermented tea and its antioxidant content lies between those of green and black teas.
The four primary polyphenols found in green tea leaves are catechins, and most studies have focused on green tea’s catechins… especially a major one called EGCG (epigallocatechin gallate). But green tea’s catechins now have a fat-control rival in the form of black tea’s unique antioxidants. As the Kirin Co. researchers wrote, “Although black tea extract contains only small amounts [of catechins], significant physiologic effects of administering black tea extracts were observed” (Uchiyama S et al. 2010).
Unlike green tea, the primary polyphenols in black tea are ones called theaflavins and thearubigins. It’s been unclear whether, like the catechins in green tea, black tea’s own polyphenols improve body shape slightly while reducing blood and liver fat levels more substantially.
Given the outcomes of this study, the Japanese team’s positive conclusion seems reasonable and plausible: “Our results are consistent with the possibility that black tea-derived polyphenols are responsible for the observed physiologic effects of the black tea extracts” (Uchiyama S et al. 2010).
Additional food factors that may help control weight gain and/or fat levels in the blood and liver include various other plant foods rich in polyphenol antioxidants (e.g., berries, cocoa, grapes, and cherries), the “resistant” starch in beans and cooked (and quick-cooled whole corn and pasta), caffeine, the omega-3s in fish, and the fibers in whole grains.