After our episode on the health benefits of coffee, the number one request I received from all of you was to look into the potential benefits or harms of tea. That's the topic of this week's Healthcare Triage...

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Unlike coffee, tea doesn't seem to have the negative connotations in general that coffee often does. I know a lot of people that think that tea is beneficial much more so than coffee, that is until, I hope the episode we did on coffee. Like with coffee, there are a fairly large number of studies that have looked at associations between tea and outcomes we might care about. So many studies were available that I was able to focus on systematic reviews and meta-analyses (or studies of studies) exclusively.

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As with coffee, tea has been found to be associated with improvements in various forms of liver disease. Nine prospective cohort studies, three retrospective cohort studies, and four cross-sectionional studies including more than 800,000 participants have been analyzed together. Only one of the cross-sectional studies though was conducted in the United States. That study showed that tea drinkers 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 the lower risk of depression as well. 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%. Tea was also associated with the reduction of the risk of stroke with drinkers of at least three cups a day having a 21% lower risk than those drinking 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 three more cups of tea a day was associated with the reduction in coronary heart disease 27%, cardiac death 26%, stroke 18%, total mortality 24%, cerebral infarction 16% and intracerebral hemorrhage 21%.

A 2014 meta-analysis of 15 published studies including more than 545,000 participants found, as with coffee, an inverse relationship between tea consumption and risk of developing type 2 diabetes. For each additional 2 cups per day of tea consumed, the risk of developing diabetes dropped 4.6%.

Tea does not seem to be associated with a reduced risk of fracture, though. A 2014 meta-analysis of 15 studies including almost 200,000 participants could not detect a significant or convincing link between tea consumption and a lower risk of him fracture. Another meta-analysis that included studies of all fractures, analyzed 9 including 150,000 participants. It also failed to find a link between ea consumption and a reduced risk of fractures.

A systematic review from 2015 found that black tea did not reduce the risk of endometrial cancer, but that increasing green tea consumption by 1 cup a day could reduce the relative risk by 11%. A 2011 meta-analysis found that green tea, but not black tea, was associated with lower rates of prostate cancer, but only in less robust study designs. A 2013 meta-analysis could not find a significant association between tea consumption and the risk of glioma.

A comprehensive study was even more equivocal about cancer prevention. A Cochrane systematic review examined all prospective, controlled interventional and observational studies 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. But only one was an randomized controlled trial. Results were conflicting. Moreover, most of the studies were done in Asia, where tea consumption is much higher than that in the United States. Regardless, the authors felt that there was insufficient evidence to give any firm recommendations. A more recent study agrees.

But these are all data from observational studies, and as such, they can't prove causality. We've been burned many times before by assuming that what we see as associations in cohort studies will translate into causality in randomized controlled studies. So everything I just said has to be taken with a grain of salt.

Finally, there seems to be less of a dose response than in the studies we saw with coffee. It often takes drinking 3 cups or more of tea a day to see positive relationships. There are some randomized controlled trials, however, which don't have most of these limitations.

Green tea has been claimed to help people lose weight. Enough people believe this that 18 randomized controlled trials with 1945 participants can be reviewed. Half of these trials took place in Japan. Only one took place in the United States. The sum total of evidence found that green tea produced a small, but non-significant, weight loss in overweight and obese adults. It also didn't hep with the maintenance of weight loss previously achieved.

Green tea catechins had no effect on HDL cholesterol, triglyceride levels or C-reactive protein concentrations. Two more meta-analyses confirmed these findings. But 11 randomized controlled trials, including 821 patients, found that green tea, black tea, and tea extract can reduce other cardiovascular risk factors.

Both were found to reduce low-density lipoprotein an average of 0.5 mmol/L, systolic blood pressure 2.3 mmHg, and diastolic blood pressure 2.8 mmHg. These results should be interpreted with caution, however, as they focus on risk factors and not necessarily outcomes. There were also few studies contributing to each of these findings, so the results may not stand up to further scrutiny or replication.   At the end of all of this, I’m a little less impressed with the body of evidence than I was with coffee. I own that this is an interpretation though, and others may disagree. The lack of a dose response in many of these trials, coupled with so many being performed in countries with massively different tea consumption than our own, makes these less generalizable than those of coffee might be. I'm also a coffee drinker.   But the conclusions I make are similar. I wouldn’t strongly advocate that anyone take up tea based on these findings. But there seem to be few harms, and some potential benefits. Drink it if you like it. It, too, seems to be a completely reasonable addition to a healthy diet.
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