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!
