Raw Garlic and Garlic Supplements Offer No Effect on Lipids.

News Author: Shelley Wood
CME Author: Hien T. Nghiem, MD

Dashing hopes of a cheap, natural — some would even say delicious — means of lowering cholesterol, a new study has found no evidence that fresh garlic or garlic supplements can reduce cholesterol levels during a 6-month period.

"We really thought this would work," lead author Christopher D. Gardner, MD, of Stanford University in Stanford, California, told heartwire. "We thought raw garlic would work, and that one of these supplements would probably work; maybe both.... We are disappointed we couldn't find something that would help people more."

The National Institute of Health–funded randomized controlled trial flies in the face of more than a decade of research supporting the lipid-lowering effects of garlic, not to mention the traditional use of medicinal garlic "since antiquity," the authors, note. But while in vitro and animal studies have shown promising effects on lipids, previous clinical trial results have been inconsistent and plagued by problems of trial design and funding source. Combining these problematic studies in meta-analyses has only fueled the belief that garlic has cholesterol-lowering properties, Dr. Gardner noted.

"There have been five meta-analyses in the last couple of decades that pooled all of the trials and every one of them said that garlic worked and as a result, the labels of the supplements say that garlic lowers cholesterol on the basis of those studies," Dr. Gardner said.

30,000 Sandwiches; No Free Lunch

The study randomized 192 individuals with moderately elevated cholesterol (with low-density lipoprotein [LDL] cholesterol concentrations ranging from 130 - 190 mg/dL) to 1 of 4 treatment groups: raw garlic, powdered garlic supplement, aged-garlic–extract supplement, or placebo. According to Dr. Gardner, the 2 supplements were chosen because they have the most robust evidence behind them, and their active components were verified in biochemical testing prior to the study.

During a 2-week run-in phase, study participants consumed daily sandwiches prepared by the researchers. During the active phase of the study, participants continued to get sandwiches as well as tablets from the coordinating center. In the raw garlic group of the study, the sandwiches were prepared with garlic in the condiments, but the tablets were placebo. In the other groups of the study, people received sandwiches without garlic, but tablets containing 1 of the 2 supplements, or placebo.

During the course of the study, staff at the General Clinical Research Center made more than 30,000 "gourmet, heart-healthy sandwiches," Dr. Gardner noted. Blood samples were taken from the study participants every month for 6 months, when the study ended.

By the end of the study, there were absolutely no changes in LDL cholesterol level from baseline in any of the groups. "It's not like there was a small effect that was not statistically significant," Dr. Gardner emphasized. "There was no movement across the six months in any of the four groups. That's what was so disappointing."

The authors also saw no effects on levels of high-density lipoprotein (HDL) cholesterol and triglycerides or on total cholesterol:HDL ratios.

It is garlic's purported lipid-lowering effects that have been trumpeted the loudest, Dr. Gardner noted, adding that for the past 5 or 10 years, garlic has been in the top 5 list for herbal/botanical product sales — perhaps driven by the number of people hoping to lower their LDL levels.

But, Dr. Gardner was also careful to emphasize that the study did not evaluate other potential cardiovascular effects of garlic or health benefits beyond the heart. "It's still possible that garlic has an anti-inflammatory effect, or a blood pressure lowering effect, or an anti-cancer effect — all of which should be studied rigorously," he said.

In an accompanying editorial, Mary Charlson, MD, and Marcus McFerren, MD, of the Weill Cornell Medical College in New York, point out that there may even be effects of garlic on atherosclerosis, specifically, that were not picked up in the study. For one, they argue, the study by Dr. Gardner and colleagues was based on the premise that allicin is the putative active ingredient, and this may have hindered the ability to consider whether multiple, interactive compounds within garlic might have other effects on atherosclerosis progression.

"I think this is a well-designed trial with many things taken into account in terms of the availability of allicin, but atherosclerosis is a complex phenomenon that includes a number of other drivers other than lipid levels," Dr. Charlson told heartwire. "This study is predicated on the thought that allicin is the driver of effect, and I think they've shown that if allicin is the driver of effect, it doesn't affect LDL levels.... If you have an elevated LDL and you want to bring it down, garlic is not the solution. But does garlic have any role in preventing atherosclerotic disease? I think the jury is still out."

A Role for Publicly Funded Trials

The study by Dr. Gardner and colleagues also speaks to the value of publicly funded research into supplements — a multimillion dollar industry, when the bulk of health claims being made by their manufacturers has not been rigorously studied. "The FDA really wants to regulate supplements, but they get beat down by the lobbyists," Dr. Gardner explained. "The studies are expensive and there are so many things out there, we can't test them all. But if the FDA could go after some of the more popular things, like garlic, that have the least substantiation, that would serve us well," he said.

Gardner was also careful to say that garlic may still play a role in heart-healthy diets, if indirectly.

"You just can't go out and have an Egg McMuffin for breakfast, a Big Mac for lunch, a clove of garlic later and think you're okay. That's not the way it works," Dr. Gardner told heartwire. "I really hope the take-home message from this is, if you're going to use garlic, use it in humus on whole-wheat bread, or in an Asian stir-fry full of vegetables, all the power to you. That's where garlic is really good for you: do that."

Arch Intern Med. 2007;167:125-126, 346-353.

The complete contents of Heartwire, a professional news service of WebMD, can be found at www.theheart.org, a Web site for cardiovascular healthcare professionals.

Clinical Context

Garlic (Allium Sativum) is one of the top-selling dietary supplements in the United States. It is widely promoted to prevent cardiovascular disease, specifically as a cholesterol-lowering agent. Crushing garlic triggers the formation of allicin through the action of alliinase enzyme on the precursor alliin. Allicin is a bioactive compound in garlic that inhibits cholesterol synthesis in vitro. It is this single compound by which most supplements are standardized. Other bioactive compounds in garlic include ajoene, allixin, erubosides, S-allyl cysteine, and N-acetyl S-allyl cysteine.

Currently, the efficacy studies supporting a hypocholesterolemic effect of the different forms of garlic have produced conflicting results. The aim of this study is to evaluate the effect of raw garlic and 2 commonly used garlic supplements on cholesterol concentrations in adults with moderate hypercholesterolemia.