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Garlic and Cardiovascular Disease
By Dónal P. O'Mathúna, PhD, Dr. O'Mathúna is a lecturer in Health Care Ethics, School of Nursing, Dublin City University, Ireland; he reports no consultant, stockholder, speaker's bureau, research, or other financial relationships with companies having ties to this field of study.
Cardiovascular disease (CVD) is a complex group of heart-related conditions that is by far the leading cause of death in women.1 More women die from CVD than die from all forms of cancer, including breast cancer. In the United States, about 42 million women have CVD, with 460,000 dying from it each year. Both numbers are higher for women than men. A woman dies from CVD almost every minute. One in five U.S. women has some form of CVD, and almost two-thirds of those who die suddenly from it had no previous symptoms. The risk of heart disease is two to three times higher after menopause than among women of the same age before menopause, and the risks among African American and Mexican American women are higher than among white women. Finding safe and effective strategies to prevent and treat CVD is a major initiative in women's health.
Many factors contribute to the development of CVD. Epidemiological studies have noted the role of elevated serum lipids, including cholesterol and triglycerides, elevated blood pressure, increased platelet aggregation, increased plasma fibrinogen and coagulation factors, alterations in glucose metabolism, and smoking.2 Improvements have been associated with increased serum levels of high-density lipoprotein (HDL)-cholesterol, normalization of abnormal lipid levels, inhibition of platelet aggregation, and increased antioxidant status.
The latter is tied to the role of diet in CVD, including proposals that garlic may have a role in the prevention and treatment of cardiovascular disease. Garlic was the most popular herbal remedy sold in single-herb formulations in the United States in 2004.3 It is also by far the most popular herb used by patients with CVD.4
Over the centuries, many cultures have viewed garlic as an important dietary supplement with beneficial health effects. Ayurvedic medicine in ancient India refers to the beneficial effects of garlic for blood flow and strengthening the heart.5 The Egyptian Codex Ebers (1500 BC) recommended garlic for treating heart disease and also for tumors, worms, bites, and many other conditions. The ancient Greek physician Hippocrates (400 BC) and the Roman authority Pliny the Elder (77 AD) similarly recommended garlic for the cardiovascular system. During the ancient Olympics, athletes were encouraged to consume copious quantities of garlic to increase their stamina.6
Clinical work as early as 1926 found garlic to have beneficial effects on cardiovascular disease. These effects were rediscovered in the 1960s and 1970s when a number of studies noted reductions in serum cholesterol and triglycerides levels.5 However, these early studies were conducted with raw garlic administered at very high doses: between seven and 28 cloves per day. This amount of raw garlic has serious social ramifications, regardless of any health benefits.
Mechanism of Action
A number of mechanisms are believed to be involved in garlic's cardiovascular effects reflecting the presence of several compounds with biological activity. Some of these inhibit liver enzymes involved in making cholesterol, including HMG-CoA reductase (the enzyme inhibited by the statin drugs). Others lower plasma cholesterol and triglyceride levels via mechanisms that are as yet unclear.7 Garlic also contains antioxidants that reduce the oxidation of low-density lipoprotein (LDL)-cholesterol, thus giving rise to beneficial effects that can counteract the development of atherosclerosis. Other constituents in garlic cause smooth muscle relaxation that can lead to reduced hypertension. Some garlic preparations have antiplatelet properties and other effects that counteract blood-clotting mechanisms.
Because of the odor problem, much work has been conducted to find more palatable and less odorous formulations of garlic. However, this generates further problems in attempting to review the effectiveness of garlic. Garlic's cardiovascular effects are believed to be caused by sulfur-containing compounds.3 An intact clove of garlic contains almost all its sulfur in one storage compound called alliin (a name coming from garlic's botanical name, Allium sativum). Raw garlic also contains an enzyme called alliinase, which rapidly converts alliin to allicin. The distinctive aroma and taste of garlic is due to allicin, but this is very volatile and unstable, breaking down either in a few hours at room temperature or after 20 minutes of cooking. Raw garlic can be consumed as whole cloves, but usually it is crushed or cut into slivers, and more commonly, it is cooked. However, depending on whether it is cooked in water, oil, or alcohol, different sets of compounds are formed.
As allicin decomposes, dozens of other more stable sulfur compounds are formed. Many of these are biologically active. To complicate matters even further, garlic supplements are prepared in different ways, resulting in different ingredients. The two most common powered formulations are dried garlic powder and aged garlic extract (AGE). During the aging process, the volatile components are lost, thus leading to AGE being called odorless garlic. Garlic oil also is available, made using three different methods, with each leading to different mixtures of the sulfur compounds. The most commonly used dosage form in clinical trials is a standardized garlic powder extract called Kwai® (200-400 mg three times daily).
This raises an important issue for clinical studies: Different preparations contain different compounds in different ratios, which may impact the effects the garlic preparations have on people.
Many laboratory and animal tests have demonstrated that garlic and its constituents have biological activities related to CVD. However, controversy continues over the clinical significance of these results. An editorial in the February 2007 issue of Archives of Internal Medicine asked whether garlic prevents CVD and concluded, "The jury is still out."8 Results of trials have been contradictory. Another problem has been that while earlier studies often found beneficial effects, more recent trials have not. Often, the more recent trials were larger, longer, and of higher methodological quality.
Cholesterol and lipid levels. Two meta-analyses published in 1993 and 1996 generated much interest in garlic because they reported 9% and 12% reductions in total cholesterol levels, respectively.9 Since 1993, 25 randomized controlled trials of garlic have been published, 14 showing no effect on cholesterol but 11 showing some reduction. All the trials showing effectiveness had participants with elevated cholesterol levels, while those that were not effective used participants with normal or mildly elevated cholesterol levels.2 However, a 2002 review noted that in spite of earlier beneficial results, "in the last five years, no randomized, double-blind, placebo-controlled study could be found in which the results indicated a clear beneficial effect of a garlic preparation alone on blood lipids."9 The most recent controlled trial randomly allocated 192 adults with moderately elevated cholesterol levels to receive either raw garlic, one of two garlic supplements, or placebo for six months.10 No significant differences were found between the cholesterol or other lipid levels measured.
Antioxidant effects. Although many of garlic's components have demonstrated an antioxidant effect, very few studies have been conducted on the clinical significance of this effect. The results of studies measuring serum antioxidant capacity for those taking garlic have been variable. The particular garlic preparation used here is significant. AGE products are made by soaking garlic slivers in alcohol for 20 months, which removes almost all allicin, but leaves other compounds with greater antioxidant capacity.6
Blood-clotting effects. In contrast to the above results, almost all trials examining garlic's impact on fibrinolysis have had positive effects. Fibrinolysis leads to the breakdown of blood clots and its impairment increases the risk of cardiovascular disease. Fibrinolytic activity, acute and chronic, has been increased with all types of garlic preparations in most of the studies examining this factor.6 Another aspect of blood clotting, platelet aggregation, also is affected by garlic. Seven clinical trials have examined this area since 1993, and all found beneficial effects.2 However, a review published in 2000 by the Agency for Healthcare Research and Quality concluded that these results must be taken as preliminary.11 While positive, all the studies found for this review were very small and of limited duration, and some had serious methodological flaws.
Antihypertensive effects. Several studies have examined the role of garlic preparations in lowering blood pressure. A 2001 meta-analysis concluded that the overall effect on blood pressure was insignificant.12 A 2002 review included almost 30 small, short studies, with most finding that various garlic preparations were of no greater benefit than placebo.9
Garlic is well-known for its adverse breath and body odor after oral ingestion. Eating raw garlic and high doses of some supplements also can cause mouth and gastrointestinal irritation and burning, heartburn, nausea, vomiting, and diarrhea.10 Some people also are susceptible to allergic reactions to garlic. The effects of garlic on platelet aggregation and fibrinolysis may increase the risk of bleeding, especially when combined with warfarin or other anticoagulants. Some case reports of postoperative bleeding have been reported. However, a randomized controlled trial found no change in adverse bleeding events among people taking warfarin when given either garlic (AGE formulation) or placebo.13
There is some evidence that allicin may stimulate the activity of a cytochrome P450 enzyme involved in the metabolism of many drugs, including oral contraceptives, calcium channel blockers, HIV protease inhibitors, and cyclosporine.14 Formulations containing alliin or alliinase are not believed to cause this type of drug interaction.
Overall, garlic preparations have some value as a complementary agent in reducing some risk factors associated with cardiovascular disease. The evidence at this stage points to limited beneficial effects for garlic as an anticoagulant and in lowering elevated cholesterol levels slightly for short periods of time. For example, when taken for up to six months, garlic lowers cholesterol levels 4-12%, which must be contrasted with statin drugs that typically reduce cholesterol levels by 17-55%.14
Garlic has been recommended as having other cardiovascular benefits. However, trials have been relatively small and subject to methodological problems. Results are also complicated by the diversity of formulations available. More rigorous studies of standardized preparations must be conducted before garlic can be used instead of conventional therapy.
Given the many associations between garlic and cardiovascular health, and the preliminary research results now available, garlic can be encouraged as part of an overall heart-healthy diet. Whether garlic supplements will provide significant cardiovascular benefits remains to be seen.
1. American Heart Association. Heart disease and stroke statistics: 2007 update. Available at: www.americanheart.org/presenter.jhtml?identifier=3000090. Accessed April 1, 2007.
2. Rahman K, Lowe GM. Garlic and cardiovascular disease: A critical review. J Nutr 2006;136(3 Suppl):736S-740S.
3. Amagase H. Clarifying the real bioactive constituents of garlic. J Nutr 2006;136(3 Suppl):716S-725S.
4. Pharand C, et al. Use of OTC and herbal products in patients with cardiovascular disease. Ann Pharmacother 2003;37:899-904.
5. Rahman K. Historical perspective on garlic and cardiovascular disease. J Nutr 2001;131(3S):977S-979S.
6. Banerjee SK, Maulik SK. Effect of garlic on cardiovascular disorders: A review. Nutr J 2002;1:4-18.
7. Allison GL, et al. Aged garlic extract and its constituents inhibit platelet aggregation through multiple mechanisms. J Nutr 2006;136(3 Suppl):782S-788S.
8. Charlson M, McFerren M. Garlic: What we know and what we don't know. Arch Intern Med 2007;167:325-326.
9. Brace LD. Cardiovascular benefits of garlic (Allium sativum L). J Cardiovasc Nurs 2002;16:33-49.
10. Gardner CD, et al. Effect of raw garlic vs commercial garlic supplements on plasma lipid concentrations in adults with moderate hypercholesterolemia: A randomized clinical trial. Arch Intern Med 2007;167:346-353.
11. Mulrow C, et al. Garlic: Effects on cardiovascular risks and disease, protective effects against cancer, and clinical adverse effects. Rockville, MD: Agency for Healthcare Research and Quality; 2000. AHRQ publication 01-E023. Available at: www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=hstat1.chapter.28361. Accessed Nov. 20, 2004.
12. Ackermann RT, et al. Garlic shows promise for improving some cardiovascular risk factors. Arch Intern Med 2001;161:813-824.
13. Macan H, et al. Aged garlic extract may be safe for patients on warfarin therapy. J Nutr 2006;136(3 Suppl):793S-795S.
14. Garlic. Natural Medicines Comprehensive Database. Available at: www.naturaldatabase.com. Accessed April 1, 2007.