I’d better declare a conflict of interest straight away: I am rather taken by anything to do with garlic, have researched it for more than 30 years, and am particularly fond of it when it is served with spaghetti as aglio e olio. This article, however, is almost entirely focused on its medicinal usage.
The history of medicinal garlic is almost as long as that of medicine itself. Hippocrates, Galen, Pliny the Elder, and Dioscorides all mention garlic for therapeutic or prophylactic reasons. More recently, researchers have identified the plant’s pharmacologically active components and have studied their biological actions in some detail.
The most important of these components is alliin, which is quickly transformed into allicin when garlic is crushed or chopped. Like most of the other components (and there are many) allicin contains sulphur, which explains garlic’s famously pungent smell — and indeed that of people who eat a lot of the stuff.
The list of diseases that were once believed to be curable by garlic now seems remarkably diverse. Even so, garlic’s active compounds have a wide range of actions that are of interest to modern medicine — from its antimicrobial properties to its effects on the cardiovascular system and on cell proliferation and division. As a result, research has concentrated mainly on three areas:
1. Garlic’s alleged effects against the common cold.
2. Its benefits for the cardiovascular system.
3. Its potential to reduce the cancer risk.
1. COMMON COLD
Garlic supplements are frequently recommended for colds. The evidence, however, is flimsy. The only rigorous study suggested that regular garlic supplementation for 12 weeks does indeed reduce the chances of getting a cold by about half. But it also showed that, once you’re suffering from a cold, garlic supplements don’t shorten it. Nor is one trial usually enough to base therapeutic decisions on. So if you ask me, garlic supplements might reduce the risk of colds, but I’d want more evidence before being convinced.
One study claims that garlic cuts the chances of a cold by half
2. CARDIOVASCULAR DISEASE
Garlic’s cardiovascular effects have been much better studied. Numerous test-tube and animal experiments, as well as clinical and epidemiological investigations, have been published. Their findings are complex and by no means free of contradictions. My reading of this sizeable chunk of evidence is that regular garlic consumption or supplementation has multiple potentially positive effects. Each of them on its own is fairly small and seemingly insignificant; but their totality might well have important consequences on cardiovascular health.
• Garlic has been shown to normalise elevated blood lipids.
• Some studies suggest garlic lowers high blood pressure.
• Other studies have demonstrated that garlic inhibits blood clotting and promotes the dissolution of clots that do form.
• Garlic might keep our arteries flexible in old age.
Garlic reduces blood pressure and normalises blood lipids
As I say, none of these actions is very pronounced: synthetic drugs developed for specific purposes often have much stronger effects — statins, for instance, lower blood cholesterol levels more dramatically than garlic. Yet all of garlic’s effects working together do have the potential to impact on our health. To be certain, we’d need very large, long-term studies — which so far don’t exist. They’d be prohibitively expensive and so might never be initiated.
My conclusions must therefore remain tentative. Multiple effects have been described with garlic’s potential to lessen the risk of heart attacks, strokes and other cardiovascular diseases. But whether this is truly the case is still an open question.
Dozens of epidemiological studies have compared large populations of people who regularly consume considerable amounts of garlic with those who do not. The results have been somewhat contradictory, but it seems that garlic consumption does lower the risk of certain cancers, particularly those of the intestinal tract, e.g. gastric and colorectal cancers.
Research suggests that regular garlic intake ‘significantly reduces’ the risk of intestinal cancers
Sadly, epidemiological investigations are rarely conclusive about the value of a therapy, and large long-term clinical trials would be needed to be sure. To date, only four such studies have become available — and again their findings aren’t uniform. Nonetheless, the biggest study by far has suggested that regular garlic intake does significantly reduce the risk of gastric cancer.
Even those of us who like garlic have to admit that there’s a limit as to how much of it one can enjoy. About 4g of fresh garlic a day would be enough for achieving the pharmacological effects described above, but that might be a tall order for regular consumption.
People who don’t want
to stomach such heroic amounts of the real thing might prefer to buy a garlic supplement — not that this would avoid the body odour, but it is easier to swallow. My advice is to purchase a high-quality supplement that’s standardised to about 1.3% alliin content, and to take 300mg three times a day.
The notion that everything natural must be entirely safe is as popular as it is misleading. Most herbal remedies can cause side effects, and garlic is no exception.
No matter whether one consumes garlic as food or as a supplement dosed correctly for medicinal purposes, garlic will produce a smell that emanates not just from the mouth but also from the skin. Garlic fans might well put up with it, but it’s advisable to make sure your partner and close friends love garlic too; if not, you might find life a lonely experience.
Some of the more serious adverse effects of regular garlic intake include nausea, diarrhoea, vomiting, allergic reactions and anaemia — which can occur in predisposed people, particularly those on high doses. Moreover, garlic can interact with a range of prescribed drugs such as anti-coagulants — it would be prudent to inform your doctor before you embark on a self-prescribed garlic regimen.
No discussion of garlic would be complete without mentioning its unbelievable power to protect us from blood-sucking vampires. I can confidently vouch for its effectiveness in this particular application: I have never seen or heard of anyone falling victim to a vampire while wearing a wreath of garlic around their neck.
Professor Edzard Ernst is emeritus professor of complementary medicine at Peninsula medical school, University of Exeter.