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.