A busy schedule in recent weeks has kept me from diving
deeply into research the way have been able to in the past, but this week I‘ve
got a bit of extra time and some research to share, so am excited to dig in and
talk about evidence.
Earlier this week, I posted a great graphic that presented evidence about supplements in a clean and easy to read format. Though I think
the graphic was great, I thought that I might put it to the test, just to see
if the assessments bore out. The subject I chose was garlic as a treatment for
hypertension. Though I’d previously reviewed garlic for its use in lowering cholesterol, I didn’t run into any significant references for garlic’s use in
hypertension – to be sure, it’s often mentioned as an herb for high blood
pressure, but as I hadn’t seen any data, I was surprised to see it listed so
highly.
The article the author of the graphic referenced was this one, a recently updated meta-analysis originally published in 1994. The
researchers, working at the University of Adelaide in Australia found that, according
to data from the eleven studies included in the meta-analysis, garlic
preparations produced approximately an 8 point drop in both systolic and
diastolic blood pressures in patients with hypertension. While that number is
on the one hand statistically significant, it’s also clinically significant as
well – in combination with lifestyle changes which can generate some pretty
important reductions in blood pressure themselves, an extra eight points can
help bring the blood pressure under control.
While this study was the one cited in the graphic, it’s not
the only one that showed positive results. Another meta-analysis, which looked
at many of the same studies as the first study mentioned, found that the effect
of garlic on blood pressure was quite strong in patients who had elevated blood
pressure in the first place, but not on those who did not. Those of you who
read my blog post on hawthorn a hypertension may recall that hawthorn appeared
to have a similar effect, i.e. normalizing elevated blood pressure, but not
causing hypotension in those with normal blood pressure. A variety of
individual studies have shown positive effects, but these two meta-analyses,
which compiled data from these smaller studies, throw some pretty strong weight
behind garlic’s use in hypertension.
However, not all studies have raved about the benefits of
garlic. One systematic review, done by the well-respected Cochrane Database,
found that, while research indicated very positive trends, and that garlic held
promise for future research, current research was lacking, and that it was
impossible to make firm statements regarding the efficacy of garlic. Likewise,
another study, published in the Netherlands, also found that there was too
little evidence to make firm statements about the effect of garlic on blood
pressure, and that garlic could not yet be recommended as a treatment for
hypertension. Neither of these studies were negative, pe se, but both said, ‘We
have to wait and see.’
Before I go on to discuss the conclusions that I’m drawing
from this evidence, I’d like to mention an issue in research that these studies
bring up.
Conflicting evidence exists on just about every topic in
medicine, and that it takes the human brain of a physician to interpret and
apply evidence correctly. In some cases, the conflict between pieces of
evidence is due to problems in one of the study’s design – mistakes in design
can result in incorrect results. In others, it’s because the studies asked
slightly different questions, and found differing answers about a common topic.
These conflicting results are an issue not only with research into
‘alternative’ treatments, but even commonly prescribed medications. Without
being flippant, I think it’s safe to say that, given all the research that
exists in the world, there’s some piece of evidence against nearly everything
we do, but we do these things anyway, because the job of a physician isn’t to
be a slave to data, but to pay attention to the patient, and use data (positive
and negative) to aid the patient in their return to health. When the evidence is overwhelming, this job becomes a little easier, but frequently it's a careful balanced approach taking into account conflicting pieces of data.
So here’s my take on the data. Sometimes, when data is
conflicting, it’s because the positive studies were just outliers, and the bulk
of the research is negative. Other times it’s the opposite. However, sometimes,
when research conflicts, it’s because there’s something going on that we didn’t
previously know. Based on these articles, it would appear to me that garlic
modulates/normalizes blood pressure, but doesn’t act exactly like a hypotensive
drug – this might explain why many of these studies found little or no effect,
but that the most specific study (that which looked at the effect of garlic on
blood pressure in hypertensives) was so strongly positive. It’s not yet known
how garlic lowers blood pressure, but it may be that it does not do so
directly, but rather secondarily, as a ‘side effect’ of a primary action.
