By Evra Taylor levy & Eddy Lang, HealthWatch, The Montreal Gazette, January 14, 2009
Garlic has long been praised as having multiple medicinal uses, many of which focus on treating cardiovascular conditions. However, many of these claims, touted by herbal therapists and the supplement industry, have been anecdotal and not particularly well-supported by science. Over the past decade, some more rigorous studies have taken a serious look at garlic's antihypertensive properties, and in this edition of HealthWatch, we examine an overview of this research.
What is blood pressure?
Blood pressure refers to the force on the walls of arteries that blood produces as it flows through the circulatory system, very much like the pressure in your household plumbing. The higher the pressure, the more strain on the piping and the more it squirts out if there's a leak or an open faucet. The blood pressure cuff, which measures this, yields two numbers: the higher systolic measure (the first number) is the pressure in the arteries when the heart is squeezing; the diastolic (second number) is the pressure when the heart is relaxing.
How common is high blood pressure?
Normal pressure is considered to be under 120 for systolic, and under 80 for diastolic and uses a unit of measurement called millimeters of mercury (mm Hg). Anything above "140 over 90" is considered high, while diabetics are given "130 over 80" as their "high" threshold. Unfortunately, 22 per cent of Canadians are hypertensive, but only 13 per cent have been given this diagnosis, and of those age 18 through 75 whose condition has been diagnosed, only one-third are treated and well-controlled.
Why is high blood pressure bad for you?
Untreated for many years, high blood pressure can lead to serious problems, including stroke, heart attack, heart failure and kidney failure. Unfortunately, one can have high blood pressure and be symptom-free, which is why it is often called "the silent killer."
What are the effects of garlic supplements?
Garlic, the edible bulb of a species of lily, has been used as both a food and medicine for more than 5,000 years. On a more contemporary note, the National Institutes of Health observes that garlic has been studied, sometimes inconclusively, for the treatment of high blood pressure, high cholesterol, boosting the immune system and for certain types of cancer.
How do garlic supplements compare to raw or cooked garlic?
Standardized garlic supplements are probably more effective for medicinal purposes than raw or cooked garlic. One of the ingredients believed to be beneficial in raw garlic is affected by the digestive system and heat, so what you are consuming via that piece of garlic bread is not particularly well absorbed by the body. As a result, a process of aging and fermentation of the garlic extract is used in the manufacture of garlic supplements to increase delivery into the bloodstream.
What about bleeding and the dreaded bad breath associated with garlic supplements?
Garlic may promote excessive bleeding in people already taking blood-thinners. Furthermore, as is well-known, the bulb can cause unpleasant mouth or body odour and can lead to indigestion, which is why manufacturers have touted "odourless" supplements that keep you from losing friends and don't upset your stomach! Odourless garlic capsules are readily available from pharmacies and health food stores and are relatively inexpensive: a bottle of 250 capsules (500 mg) costs roughly $10 to $15.
The study.
Reinhart KM, Coleman CI, Teevan C, Vachhani P, White CM. Effects of garlic on blood pressure in patients with and without systolic hypertension: a meta-analysis. Ann Pharmacother 2008 Dec;42(12):1766-71.
What kind of study was this?
As is often the case in the HealthWatch series, the focus of this column is not a single clinical trial, but a type of research known as a systematic review, whereby scientists scour the world's medical literature for all the high-quality studies relevant to a specific question. These investigators were interested in what effects garlic would have on blood pressure in normal patients and in those with hypertension. They identified a total of 10 small studies, three of which involved patients whose systolic blood pressure was greater than 140 mm Hg.
What were the results?
While garlic supplements had no blood pressure lowering effect in normal subjects, there was an impressive reduction in both the average systolic and diastolic readings in the hypertensive subjects. Their blood pressure was reduced by an average of 16 mm Hg, in systolic, and 9 mm Hg in diastolic blood pressure.
How good a reduction is this?
Although these estimates are very preliminary, if confirmed, they would make for a very impressive effect, rivalling any prescription anti-hypertensive available. In fact, based on what we know about the benefits of blood pressure reduction, it could diminish the risk of a heart attack or stroke by as much as 40 per cent.
Is there any reason to doubt this study?
Yes, there are actually lots of reasons to maintain healthy skepticism about garlic and its effect on high blood pressure; the details may even leave a bad taste in your mouth. Chief among these is the fact that the three studies of patients with high blood pressure recruited a measly total of 139 patients, a very small number on which to base any definitive recommendations. Second, the focus on blood pressure readings alone as opposed to more important health measures, like rates of stroke or heart attack, is a limitation of the studies performed so far and can only be remedied with bigger studies that would need to recruit thousands of patients. Finally, all three key studies that showed a benefit used a particular commercial brand of garlic supplement (Kwai), and it is conceivable that corporate sponsorship of this work may have biased the trials in ways that might be difficult to detect.
What should patients do?
Definitely don't stop your high blood pressure medications and replace them with garlic. The suggested benefits are intriguing but far from a slam dunk. Also, taking garlic in raw or supplement form is not an excuse to back off lifestyle modifications such as a salt-restricted diet, weight loss and exercise, that are proven to be beneficial and help keep your blood pressure down. Despite the caveats, patients with high blood pressure may derive benefit and probably are unlikely to experience harm as a result of taking daily garlic supplements. Patients with hypertension who are willing to check this out should keep close tabs on their blood pressure with a home monitor and report any changes to the doctor who is treating them.
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The CPR Instructor's Network offers lectures on healthy lifestyle, including information sessions of blood pressure. The CPR Instructor's Network also provides blood pressure clinics, and corporate complete wellness programs.
To assess your blood pressure and your risk for heart attack and stroke, please click on the link for 'heart and stroke risk assessment' on our news and links page.