Cholesterol-lowering effect of garlic extracts and organosulfur compounds:
human and animal studies.
Abstract
The medicinal use of garlic dates back thousands of years, but there was little scientific support of its therapeutic and pharmacologic properties until recently. In the past decade, the cancer-protective effects of garlic have been well established by epidemiologic studies and animal experiments. However, the cardiovascular-protective properties of garlic are less well understood. In particular, despite the reported hypocholesterolemic effect of garlic, the mechanism of the effect is unclear. In a recent randomized, double-blind, placebo-controlled intervention study, we showed that aged garlic extract (AGE) supplementation was effective in lowering plasma concentration of total cholesterol by 7% and LDL cholesterol by 10% in hypercholesterolemic men compared with subjects consuming a placebo. Supplementation of AGE in animal diets similarly reduced plasma concentrations of total cholesterol and triacylglycerol by 15 and 30%, respectively. In subsequent experiments using cultured rat hepatocytes, we found 44--87% inhibition of cholesterol synthesis by the water-extractable fraction (WEF), methanol-extractable fraction (MEF) and petroleum ether-extractable fraction (PEF) of fresh garlic, and Kyolic (liquid form of AGE). These observations suggested that hydrophilic and hydrophobic compounds of garlic are inhibitory to cholesterol synthesis. Because S-allylcysteine (SAC) alone was less potent than Kyolic, which contains SAC and other sulfur compounds, a maximal inhibition appears to require a concerted action of multiple compounds of garlic. In a series of experiments, we further characterized the inhibitory potency of individual water-soluble and lipid-soluble compounds of garlic. Among water-soluble compounds, SAC, S-ethylcysteine (SEC), and S-propylcysteine (SPC) inhibited cholesterol synthesis by 40--60% compared with 20--35% by gamma-glutamyl-S-allylcysteine (GSAC), gamma-glutamyl-S-methylcysteine (GSMC) and gamma-glutamyl-S-propylcysteine (GSPC). Lipid-soluble sulfur compounds (i.e., diallyl sulfide, diallyl disulfide, diallyl trisulfide, dipropyl sulfide and dipropyl trisulfide) at low concentrations (0.05--0.5 mol/L) slightly (10--15%) inhibited cholesterol synthesis but became highly cytotoxic at high concentrations (1.0--4.0 mol/L). All water-soluble compounds, except S-allylmercaptocysteine, were not cytotoxic, judging from the release of cellular lactate dehydrogenase into the culture medium. Taken together, the results of our studies indicate that the cholesterol-lowering effects of garlic extract, such as AGE, stem in part from inhibition of hepatic cholesterol synthesis by water-soluble sulfur compounds, especially SAC.
The use of garlic as an aid for improving health is older than its use as a food. In ancient times, the garlic bulb was considered a common treatment for deafness, dropsy (abnormal accumulation of fluid in the body), intestinal parasites, leprosy, respiratory illnesses, and loss of appetite. Today, garlic is more commonly used as a food or food seasoning.
It is available in its natural bulb form and as a tablet, capsule, dried powder, and aged extract. However, raw garlic, which has a high concentration of a sulfur-containing compound called allicin, is more medicinally powerful than cooked garlic. Research on garlic's cholesterol-lowering effects has been inconsistent, mostly due to poorly designed studies and differences in the garlic preparations used. In the early 1990s, evidence suggested that garlic reduced cholesterol 9 to 12 percent, but in 2000, a review of the evidence found that garlic reduced cholesterol by only 4 to 6 percent. A study that used dried garlic powder over 8 to 12 weeks showed significant reductions in total cholesterol, LDL cholesterol, and triglycerides, but the effect didn't last beyond six months of treatment, suggesting that garlic has only a short-term effect on cholesterol. If you choose to include more garlic in your diet, the worst outcome may be strong breath or an upset stomach. But remember that garlic, in whatever form, is just one way to help lower your cholesterol. Your top priority should be a balanced, healthy diet that includes vegetables, fruits, whole grains, beans, low-fat dairy, lean protein, and unsaturated fats.
One surprisingly healthy source of unsaturated fat is nuts. Read on to explore the benefits of including nuts -- in moderation -- to a healthy diet.
As with any comprehensive treatment program to treat bad cholesterol, modifying what you eat remains one of the cornerstones. Several herbs have been thought to improve cholesterol levels, and more specifically, the makers of several herbal supplements have claimed benefit with their product. Garlic is one of the most commonly consumed herbal supplements. Among its many purported health benefits, which include lowering blood pressure, preventing blood clots, killing bacteria and fungus, and repelling both ticks and bloodsucking creatures of the night, garlic has long been thought to improve cholesterol.
Garlic (Allium sativum), otherwise known as the “stinking rose,” contains a chemical called alliin. When garlic is crushed, a chemical reaction occurs and alliin becomes allicin. Allicin has been well documented to inhibit the formation of cholesterol in several laboratory studies. Furthermore, over a hundred animal studies have been performed that showed a positive effect of allicin on cholesterol.
Unfortunately, the results seen in the laboratory and in animals have not been consistently seen in humans. Earlier studies shown mixed results and produced more criticism concerning study designs than answering the question concerning the benefits of garlic. Much criticism focused on the form of garlic (raw vs. supplemental form) and the dose given. Therefore, a well-designed and powerful study was performed by Stanford University last year to answer the garlic cholesterol connection.
In this study, almost 200 people with moderately elevated cholesterol were given 3 forms of garlic: raw garlic (blended and given in a sandwich as a condiment), Garlicin (powdered garlic supplement), and Kyolic-100 (aged powdered supplement). Each garlic product was eaten 6 days of the week for a total of 6 months. The amount of allicin and garlic in each product was fairly similar and the supplement doses were actually 2-3 times the manufacturer’s recommended dose.
Besides bad body and breath odor reported by over half of those eating the raw garlic, there was no significant improvement in cholesterol levels in any of the 3 treatment groups. No serious side effects were reported either. Does this lay the garlic-cholesterol connection to rest? Perhaps. Critics of this study point out that a lower dose of the Kyolic-100 supplement was used in this study as opposed to earlier positive studies. Furthermore, not all forms of garlic supplements (i.e. garlic oil) were investigated. Nevertheless, not even a hint of cholesterol benefit was seen over the 6 months, a pretty negative conclusion if you ask me.
Based on this information, I don’t recommend that people start taking garlic as the sole therapy to treat high cholesterol. Eating a heart healthy diet, exercising, and taking prescribed medicines if indicated still remain the cornerstone of treatment. If you still want to take garlic for heart health, that’s OK with me. Just because there is not a proven garlic cholesterol connection doesn’t necessarily mean that garlic may not have overall heart benefit. Garlic has several other favorable effects on the body that may reduce a risk of a heart attack independent of cholesterol. All I ask is that you wait till after your appointment to eat your daily dose of garlic.
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