November 10, 2015
After my Upshot column on the potential health benefits of coffee, the No. 1 request I got was to look into the potential benefits — or harms — of tea.
Unlike coffee, tea does not seem to generate negative perceptions. I know many more people who think that tea is beneficial, much more so than coffee. (That is, until my coffee column, I hope.)
As with coffee, a fairly large number of studies have looked at associations between tea and health. Most of the studies don’t have the rigor of randomized control trials and don’t prove causality. But so many studies were available that I was able to focus on systematic reviews and meta-analyses, or “studies of studies.”
Nine prospective cohort studies, three retrospective cohort studies and four cross-sectional studies including more than 800,000 participants have looked at the association between tea and liver disease. Those who drank tea were less likely to have hepatocellular carcinoma, liver steatosis, liver cirrhosis and chronic liver disease. This confirmed the findings in a previous systematic review published in 2008.
Tea has been associated with a lower risk of depression. A 2015 meta-analysis of 11 studies with almost 23,000 participants found that for every three cups of tea consumed per day, the relative risk of depression decreased 37 percent.
Tea was also associated with a reduction in the risk of stroke, with those consuming at least three cups a day having a 21 percent lower risk than those consuming less than a cup a day. A more recent meta-analysis examined 22 prospective studies on more than 850,000 people and found that drinking an additional three cups of tea a day was associated with a reduction in coronary heart disease (27 percent), cardiac death (26 percent), stroke (18 percent), total mortality (24 percent), cerebral infarction (16 percent) and intracerebral hemorrhage (21 percent).
A 2014 meta-analysis of 15 published studies of more than 545,000 participants found, as with coffee, an inverse relationship between tea consumption and the risk of developing Type 2 diabetes. For each additional two cups per day of tea consumed, the risk of developing diabetes dropped 4.6 percent.
What is tea not associated with? It does not seem to be linked with a reduced risk of fracture. And a systematic review from 2015 found that black tea was not linked to a reduced risk of endometrial cancer. But increasing green tea consumption by one cup a day could reduce the relative risk by 11 percent. A 2011 meta-analysis found that green tea, but not black tea, was associated with lower rates of prostate cancer. A 2013 meta-analysis could not find a significant association between tea consumption and the risk of glioma, a form of brain or spinal tumor.
The science is even more equivocal about cancer prevention. A Cochrane systematic review examined all the studies, regardless of type, that looked at associations between green tea and the risk of cancer incidence or mortality. They found 51 studies containing more than 1.6 million participants. Only one was a randomized control trial, however. Results were conflicting.