Garlic linked with reduced risk of colon cancer

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Garlic is an herb that has gained a reputation for enhancing the health of the heart and circulation (see below). However, perhaps what is less well known is that garlic has been implicated as a cancer-preventive agent too. Within the body, garlic is metabolised into a variety of sulphur-containing compounds that may have anti-cancer properties. It is thought that eating garlic may protect against cancer through a several mechanisms which include ‘antioxidant’ activity, enhanced repair of DNA and protection of chromosome damage [1].

The most recent study to assess the relationship between garlic and cancer was published this week in the Journal of Nutrition [2]. Specifically, it looked at the evidence linking garlic with cancer of the colon.

As part of this review researchers looked at eight ‘epidemiological’ studies which examined whether or not garlic eating is associated with lower colon cancer risk. Five of these studies found a seeming protective association. Also, when seven of these studies were amassed together in the form of what is known as a meta-analysis, garlic consumption was found to be associated with a 30 per cent reduced risk of colon cancer.

The review also highlighted 11 studies conducted in animals which demonstrated that garlic and/or its active constituents had a significant anti-cancer effect. This review appears to provide quite strong evidence that garlic consumption may reduce the risk of cancer, including cancer of the colon.


1. Khanum F, et al. Anticarcinogenic properties of garlic: a review. Crit Rev Food Sci Nutr. 2004;44(6):479-88

2. Ngo S N T, et al. Does Garlic Reduce Risk of Colorectal Cancer? A Systematic Review. Journal of Nutrition 2007;137:2264-2269

The Many Benefits of Garlic – 10 February 2002

Last week garlic was thrust into the spotlight following the publication of a study which suggests that regular consumption of this herb could halve the risk of stomach cancer and cut the incidence of colon cancer by two-thirds. Actually, garlic has long had a reputation as a bit of a cure-all. With references to its medicinal use stretching as far back as the ancient Egyptians, garlic has been touted as an effective treatment for heart disease, high blood pressure, high cholesterol, colds, flu and other infections. So, do the proposed benefits of garlic stand up to scientific scrutiny, or are they based more in folklore than fact? Over the last 20 years, garlic has been the subject of numerous scientific studies. While not all of this research supports the claims often made for it, it turns out that there is indeed a wealth of evidence which points to garlic as one of the most versatile natural substances we have for maintaining and promoting our health.

Garlic (botanical name: Allium sativum) is rich in a substance known as alliin (pronounced al-ee-in). Once garlic is chopped or crushed, an enzyme present in the herb called allinase converts alliin into another compound known as allicin. While alliin is relatively odourless, allicin is responsible for garlic’s characteristic pungent odour and taste. Allicin is also thought to be behind many of garlic’s therapeutic properties in the body. Once absorbed by the gut, allicin is eliminated via the lungs and the skin. This is why garlic taints the smell of the breath and sweat of those who eat it.

It is during its passage through the body that allicin seems to exert its health-giving effects. Much has been made of garlic’s ability to reduce the risk of heart disease. For a long time it had been thought that at least part of garlic’s heart-protective action comes from an ability to reduce the level of cholesterol in the bloodstream. However, in recent years, garlic’s reputation as a natural remedy has been tarnished by a well-publicised study which suggested that taking garlic supplements has no effect on cholesterol. Just last week, while on a lecture assignment in America, I was in discussion with a doctor specialising in the prevention of heart disease. She cited this negative study as reason to dismiss garlic as a heart disease protector. However, I respectively pointed out to her that two meta-analyses (a meta-analysis combines the results of several studies) exist which show garlic has the potential to reduce cholesterol levels by about 10 per cent. Besides, even if it had no effect on cholesterol, garlic has a number of other effects in the body which are likely to reduce the risk of heart disease and stroke.

Another beneficial effect of garlic on the cardiovascular system concerns the blood pressure. Some recent research reported in the Journal of Hypertension showed that garlic consumption could reduce blood pressure by 5 – 10 per cent. The authors of this study concluded that just this effect alone might reduce heart disease and stroke rates by up to 40 and 35 per cent respectively. Another study showed that individuals who consumed more garlic were less likely to develop unhealthy stiffening of the body’s main blood vessel (the aorta).

Garlic also has the ability to reduce the tendency for the formation of blood clots (thrombi) within the circulation. As thrombi formation can trigger problems such as heart attack and stroke, this tendency to thin the blood is quite likely to help prevent these conditions. Garlic has also been shown to help break down clots in the body. For this reason, I tend to recommend garlic to sufferers of thrombophlebitis – a painful condition characterised by inflammation and thrombus formation in the veins of the legs.

While I am an enthusiastic advocate of garlic, there are a couple of instances when I tend not to advise its use. Because it thins the blood and can increase bleeding tendency, I usually advise individuals on blood-thinning drugs such as warfarin to avoid garlic. For the same reason, I recommend anyone undergoing surgery to stop garlic 10 days before the operation, commencing again a week after the procedure.

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