People worry about caffeine, and they’re right that drinking caffeine too late in the day can make it harder to sleep at night. But there is also some evidence linking tea, generally at least three cups a day, to a reduced chance of various illnesses, including heart and liver disease, depression, stroke, and diabetes.
The strongest evidence comes from studies of tea drinking and its effect on the heart. In a meta-analysis of studies involving a total of more than 850,000 people, researchers concluded that for every three cups of tea you drank each day, you could cut your chance of heart disease by 27 percent. You’d cut you chance of stroke 18 percent and of dying in a heart attack by 26 percent.
Some evidence supports green rather than black tea. Green tea-drinkers are less likely to get liver cancer and chronic liver disease, based on a 2015 review of studies with more than 800,000 participants. The results confirmed a 2008 overview.
Black tea doesn’t seem to lower the risk of endometrial cancer, but green tea may, according to a systematic review from 2015. Another meta-analysis supported green tea, but not black, as a way to protect against prostate cancer.
You’ll hear that green tea is generally good to prevent cancer. However, when a Cochrane systematic review considered every study of green tea and cancer risk, most of them conducted in Asia, the researchers concluded that the evidence wasn’t decisive.
Most of this data comes from looking at people’s diets and then seeing what illnesses they acquired. In this kind of research, there’s no way to determine whether any one aspect of diet or lifestyle leads directly to an illness, or protects against it. We’d need randomized controlled trials to be surer that tea-drinking actually had the desirable effect. Maybe tea drinkers are simply more health-conscious. Researchers try to rule out the effect of other habits that may be more common among tea drinkers, but it’s hard to eliminate every possibility.
Also, since most of the research was done with people in Asian countries where it’s ordinary to drink tea, the people who didn’t drink tea or drank less in those studies may be unusual in other ways.
Many people drink green tea or take green tea supplements to help them lose weight or stay thin. A 2012 Cochrane review found that some 18 randomized controlled trials —the gold-star type of study — with 1,945 participants have been conducted on that question, half of them in Japan. So far it looks like green tea doesn’t help people keep weight off and has only a slight, not necessarily significant, association with weight loss among the overweight and obese. Also, the famous antioxidants, green tea catechins, had no effect on bad cholesterol or triglyceride levels. Other meta-analyses confirmed these findings.
Altogether, if you like tea enough to drink three cups a day, you may well reap health benefits, but you can’t count on it based on the current research. You might do better with coffee, which is also linked to good health.
It should go without saying that drinking tea shouldn’t be your first line of defense against heart disease, depression, and diabetes. Eat your vegetables, keep your portions down, and be sure to get exercise and ample hours of sleep. If tea interferes with your sleep, you’ll most likely negate any health benefits.
Another warning: Processed, sugar-sweetened tea beverages are packed with extra calories. “If there are any health benefits to green tea consumption, it’s probably completely offset by adding sugar,” says Qi Sun, MD, assistant professor in the Department of Nutrition at the Harvard School of Public Health.
February 11, 2016
Janet O’Dell, RN