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If onions and garlic can't shield us from sour skin and are only slightly discouraging to bacteria, what of their vaunted powers to lower blood pressure and lower cholesterol? Alas, the juries are out.

Garlic, in large doses (or one of its active ingredients, allicin), is said to lower blood pressure. One doctor describes its effects as "modest but measurable." An herbal medicine Web site says that garlic "gently" lowers blood pressure. (When you hear that a substance does something "gently," do the math -- often it means that the effects are laughably small.) Some studies show garlic lowering blood pressure by small amounts. Other studies show nothing of the kind. Several meta-analyses have looked at garlic and blood pressure, some finding a small but significant reduction on both systolic and diastolic blood pressures. Others find nothing. "Increased amount of garlic and onions have not been found to affect blood pressure," sighs the National Heart, Lung & Blood Institute. Everyone agrees it would be nice to do a few more studies with more people and better controls.

Garlic is often suggested for lowering cholesterol, and dozens of studies are said to show that around one clove of the raw stuff per day (or 600-900 grams of the processed stuff) can lower cholesterol. Gently. By about 10 percent. But one recent study at the University of Bonn found no improvement at all in 25 people with high cholesterol who ate the equivalent of three to four cloves of garlic a day for six months. (They took the garlic in pills containing garlic oil. This is why I never sign up for these studies. They're always about the extract or the pill or the powder or the puree -- never about the french-fried onion topping or the garlic mashed potatoes.) Critics say the study was too small and the pills were the wrong kind -- steam distilled! They argue that the process could destroy the allicin.

British researchers gave dried garlic pills (or placebos) to 115 patients with high blood pressure and found no effect on cholesterol levels. They then did a meta-analysis of previous studies, which gave a modest drop in cholesterol levels -- but they discredited even this, saying that the earlier studies weren't good enough to be meaningful. Once again, they used a form of garlic supplement that others find suspect. Once again, more and better studies are called for.

Both onions and garlic are said to "clear the blood," a mystifying phrase beloved of amateur herbalists. Sometimes they say "thin the blood." This is woo-woo talk for increasing clotting time, a thing that is usually considered good for cardiac health. You might not want to schedule your surgery right after competing in a garlic- eating contest, however. This is one of the effects of onions and garlic that is not in dispute.

The evidence that garlic and onions decrease the incidence of stomach and colon cancer also looks pretty good. (It seems to have no effect on the incidence of breast cancer and lung cancer.) Most of these studies looked at thousands of people in Iowa, the Netherlands or China, asked people how much garlic they ate, and they compared stomach and colon cancer rates in people who ate lots of garlic or onions and people who didn't. The Iowa study, which looked at 40,000 women, found a 35 percent lower risk of colon cancer in those who ate garlic at least once a week.

At one meeting an Italian researcher also reported that people who cooked with lots of garlic in the area where he lives, around Genoa, have lower stomach cancer rates, specifically praising pesto. (God, I want to believe him.) And a Dutch study found that those who ate half an onion a day had half the risk of stomach cancer, ascribing this to an antioxidant called quercetin, found in onions, especially red and yellow onions.

Some enthusiasts happily declare that garlic has no side effects -- then they make jokes about the smell. Which I can see I'm going to have to discuss. The active ingredients in onions and garlic are mostly sulfur compounds, to which our noses are quite sensitive. Odorless garlic preparations, many authorities suggest, are useless garlic preparations. But then, they all disagree about whether you should eat raw or cooked onions, and whether garlic should be eaten raw, cooked, fresh, aged or in a capsule.

In one study, researchers made a fresh garlic extract by whirling about 40 cloves in a blender, and filtering the sludge -- but the resulting extract was so powerful that it caused nausea and burning sensations in the throat.

Many sources recommend chewing parsley after eating garlic or onions, to freshen the breath. I suspect it's propaganda from the Parsley Board.

A recent German study that claimed powerful garlic pills could not only reduce the rate of plaque formation in arteries, but actually make existing plaque go away, has been criticized on several counts. The Berlin university where the principal researcher works is investigating the allegation that the study (which was commissioned by the makers of a powerful garlic pill) uses suspicious photos purporting to show the inside of a carotid artery before and after four years of garlic therapy, but which in fact look as if they were taken at the same time.

Another complaint is that the statistical analysis ignored the fact that many patients quit the study because they couldn't stand the ferocious reek of the high-dose pills.

While onions and garlic may not be the answer to every medical problem, they are beneficial as well as enjoyable, providing you stay away from zealots with blenders. The only things to watch out for in your pursuit of the allium vegetables are people who are just trying to fortify you for building pyramids, defending Shiloh or conquering Asia Minor.
salon.com | Oct. 29, 1999

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