The Ohsaki National Health Insurance Cohort Study

This study began back in the ’90s in Japan, following over forty thousand adults over the age of 40 without any history of major chronic disease (heart disease, cancer, that sort of nasty stuff).  Individuals were followed up on over the course of the next eleven years to see how many had died from any cause. Participants filled out a questionnaire that asked some basic questions about daily nutrition habits, including the consumption of green tea.  This makes this study very similiar to the Nurse’s Health Study, a study that has followed over 100,000 nurses over the course of decades (except, since it’s an American study, they don’t specifically ask about green tea).

For the Ohsaki study, participants were asked if they drank green tea never, occasionally, 1-2 cups per day, 3-4 cups per day, and 5 or more cups per day.  The authors noted that the typical “cup” of green tea is about 100 milliliters (about 3.5 ounces).

By the way, one of the nice things about this study is, the researchers took the time to see if their questionnarie was at all accurate.  They picked a sample of people and had them do a more comprehensive food diary for three days on four seperate occasions.  They found that there was a moderate to strong correlation between reported intake and actual intake.

The Results: Green Tea Makes You Live Longer

After accounting for potentially confounding variables like age or physical activity, the researchers found that the more green tea people drank, the less likely they were to die from any cause.  The effect was obviously most noticeable in the 5 or more cups of tea per day group; if you used non-tea drinkers as a baseline, the hazard ratio was 0.88 for the 5 cups a day drinkers.  This means that for every 1 non-tea drinker, 0.88 tea drinkers died… which basically means green tea makes you live longer.

The effect was even MORE pronounced among women; the hazard ratio was 0.77 for the heavy tea-drinkers.  So for every 100 non-tea drinkers who died, only 77 tea drinkers died of any cause.  The researchers followed these effects for eleven years.

Then, they took a look at more specific causes of death (they reviewed data for seven years for this). They found that green tea consumption significantly reduced the risk of death from cardiovascular disease, but didn’t reduce the risk of death by cancer.  After looking even closer into the data, they found that when it came to cardiovascular disease, the greatest protection was from stroke.  Once again, the effect was most pronounced in women, who had a 42% less chance of death by stroke if they were drinking 5 or more cups per day of green tea (compared to one or less).

This shouldn’t be a big shock to readers of this website- we’ve talked before about how antioxidants make arteries more elastic and pliable (and therefore healthier).  Since strokes are caused by either blockage or rupture of an artery in the brain, healthier arteries are going to be less prone to those kinds of problems.  And I don’t know about you, but I have no deep and burning desire to experience a stroke, so I’m going to keep drinking green tea and live longer than I would have otherwise!

Stay healthy!

By the way, the full text of this study is available online for free at this link:  http://jama.ama-assn.org/content/296/10/1255.long