As regular readers of this blog will know, I’m a big fan of Camellia sinensis, the tea leaf. Sometimes I hardly know where to start in singing its praises, but today I’m keeping things limited to a fairly small area of green tea’s activity. Few people think of flu prevention when they think of green tea, but several studies in Japan have looked into that exact problem.
A few years ago, some researchers did some in vitro studies on green tea and influenza virus, and found that green tea inhibits the ability of influenza virus to replicate within cells. The mode of action is the following – the catechins found in green tea prevent the influenza virus from fusing with human cells in the first place, thus preventing the virus from being able to replicate. The exact mechanism isn’t fully understood, but it’s believed that his happens through a combination of membrane protein changes, as well as altering the properties of the viral membrane itself (1, 2). As a side note, these catechins are also responsible for the antioxidant properties of green tea.
This is all well and good if you’re a Madin-Darby canine kidney cell swimming in a sea of supraphysiologic concentrations of green tea catechins, but if you’re a living breathing human, you’re probably more interested in clinical results. What can green tea do to help you?
In 2006, a group of Japanese researchers found that elderly residents of a nursing home were less likely to catch the flu if they gargled daily with a catechin and theanine solution during the peak flu season. While the idea of gargling with green tea may seem unusual to us, these researchers were drawing on the previous research, which had shown that green tea catechins inhibited the flu virus by direct contact – the research was done to see if the in vitro results translated into clinical results by preventing the flu virus from infecting cells right in the mouth and throat. Though the means of delivery seems odd, these same results ought to translate to a similar effect garnered from the act of drinking green tea.
A further study showed that drinking green tea does indeed seem to prevent the flu. The results of a survey done on schoolchildren showed that those who drank at least one cup of green tea per day were 40% less likely to get the seasonal flu. On the one hand, this was only a survey, and not a clinical trial, so we can’t necessarily draw major conclusions from it, but on the other hand, the survey group was fairly large – about 2000 schoolchildren – which makes us take this result more seriously. Were the survey done on a smaller number of children, we could easily chalk the results up to chance, and the problems inherent counting on accurate self-reporting by children. However, because of the large group size, these problems become less likely to affect results, and so we can draw a reasonably reliable conclusion that drinking green tea does prevent flu infection.
Finally, a randomized control trial (the highest level of medical evidence) showed that healthcare workers who consumed a daily capsule of green tea catechins and theanine were significantly less likely to catch the flu compared to those consuming a placebo. What differentiates this study from the previous studies is not only the high level of evidence seen in the study, but also the mechanism. The prior studies showed that green tea prevents flu, but probably because of direct inhibition of the flu virus in the throat and mouth – when you’re taking a capsule, that’s not a factor in how it works. Rather, this study implies that the green tea nutrients help prevent the flu after they’ve been absorbed into the blood. We can only speculate as to how this happens – it may be that the catechins continue to inhibit viruses in the same way we saw in Petri dishes, it may be that they boost the immune system in some way. Either way, this study shows that green tea not only prevents the flu by stopping the flu virus in the mouth, but also that the body somehow uses the nutrients in green tea to fight the flu.
Taken together, it’s clear that green tea helps prevent the seasonal flu. We see that in both the lab, and in clinical settings. Interestingly, we see that green tea does this by two modes of action: the direct inhibition of flu virus in the mouth and throat, and also by supplying the body with nutrients it uses to conduct its own fight against the flu. Fortunately for all of us, there is a convenient, user-friendly way to apply both methods of prevention – just boil up some water, brew up some tea, and enjoy!