OVERVIEW

A great deal is written on diet and all forms of cancer and recently researchers have suggested that we may be able to prevent about 35% of cancer cases by altering our diets. Each year 10.9 million people worldwide are diagnosed with cancer and there are 6.7 million deaths from the disease and, of these, nearly one million are in the EU (1). It is understandable when we see such figures, to expect that any possible relationship between what we do, what we eat and drink, and the risk of developing cancer, has been extensively studied. Coffee, as a popular beverage, has been included in such studies.

In 1991 the International Agency for Cancer Research (IARC), which is part of the World Health Organisation, classified coffee as being “possibly carcinogenic to bladder cancer” This classification was given because there was insufficient data available at the time to be more specific (2). However the report also stated that drinking coffee may even help to protect against some forms of cancer. This resulted in many further studies being undertaken.

Since 1991, the results of those studies have shown data about four main cancer sites that would be of interest to coffee drinkers. They are bowel (colorectal), pancreatic, bladder and liver cancers.

Bowel cancer

The latest estimate is that about 2 out of 3 bowel cancers may be preventable by changes in diet and lifestyle, but there is no one diet that can guarantee you will not get bowel cancer (1).

Several studies have found that coffee consumption is related to a lower risk of bowel cancer. According to one study in Canada, the risk reduced as coffee drinking rose to five cups a day (especially evident in men) (3). Another group of studies noted there was a 28% reduction in the risk of developing colorectal cancer for those drinking four or more cups of coffee daily as compared to those drinking less than one cup (4).

This lower risk of colorectal cancer, among moderate and regular coffee drinkers, was observed consistently in over a dozen studies undertaken in a variety of settings in Asia, Northern and Southern Europe, and North America.

However there are a number of studies that did not reach this conclusion, but allowances were not made in those studies for other factors, such as smoking and alcohol consumption, which are known to have an impact on an individual's cancer risk.

How does it work?

Apart from increasing movement of food in the bowel, and thereby reducing the exposure time to cancer inducing substances in the colon(5), coffee is believed to help reduce the output of bile acids, which are known to play an active part in promoting cancer in the colon(6). Coffee also contains several compounds with strong antioxidant properties, such as caffeic acid and chlorogenic acid (7, 8), and others - such as cafestol and kahweol - with anticarcinogenic activity (9). Antioxidants are thought to play a significant role in protecting our cells and tissues from oxidative damage.

Pancreatic cancer

Most experts do not think there is a link between coffee and pancreatic cancer. Since the 1991 IARC Report, results of seven major studies have been published. No association emerged in a study of 17,633 American men (10), or in another Norwegian study (11). Three other studies in the US involving 14,000 retired residents (12), the Health Professionals Follow-up Study (13) and the Nurses' Health Study cases (13), all confirmed no association between drinking coffee and increased risk of cancer of the pancreas.

Some years ago some studies showed a possible link, but in one of those studies, when smoking was allowed for, the association with coffee was not considered statistically significant. (14). One further study even found an inverse relationship showing that the risk of developing pancreatic cancer was reduced in coffee drinkers (15).

Bladder cancer

Since the publication of the IARC Report in 1991, several studies have been undertaken on coffee and bladder cancer. One aspect of these studies was to try to establish whether the reported small association with coffee was the cause, or whether cigarette smoking, known to be a risk factor for bladder cancer, had been misclassified in those studies.

In an analysis of 10 European studies restricted to 564 non smokers, there was no excess risk in coffee drinkers (16). In a Norwegian study no significant association was seen between a high coffee consumption (greater than or equal to 7 cups per day) and the risk of developing bladder cancer, and these results applied to both men and women participating in the study (11). More recently a study in the Netherlands identified 569 bladder cancer cases in a study population of 3,123 men and women (17).

After making allowances for other possible causes, no significant association between coffee consumption and the risk of developing bladder cancer was seen in men, but in women there was a significant inverse association i.e. a lower risk in those who consumed coffee.

So the large amount of data from a number of population studies on coffee and bladder cancer risk clearly excludes, after making allowances for other possible causes, a strong association between coffee consumption and the risk of developing bladder cancer. Where a moderate association is sometimes observed, it is possibly due to links with cigarette smoking and other factors.

Liver cancer

Coffee drinking has been shown to be inversely related to the risk of cirrhosis in studies from North America and Europe (18) and, as cirrhosis is often a starting point for liver cancer, this is significant.

Several studies on a potentially favourable effect of coffee on liver cancer have also been published over the last two decades and the relationship between coffee drinking and the risk of primary liver cancer has been examined in at least six studies.

In one Japanese study of 334 subjects, who consumed coffee on a daily basis, results showed they had a lower risk of developing liver cancer than those who almost never drank coffee, and the risk decreased with the amount of coffee consumed. Compared with non-drinkers, the relative risk for those drinking 1-2 cups per day was a 50% reduction, whereas for those drinking more than 5 cups per day, it was a 75% reduction in risk (19). In a recent Greek study (20) of 333 subjects, results showed that those who drank around 3 cups of coffee a day (20 cups a week) had a 30% reduction in risk compared to non drinkers.

In another hospital based study in Italy, involving 250 cases and 500 controls, similar conclusions were reached i.e.compared with non coffee drinkers, those drinking 1 - 2 cups per day reduced their risk by 20% whereas those drinking 3 - 4 cups reduced their risk by 60% and those drinking 5 or more cups per day had a 70% less risk of developing liver cancer (21). Finally, the findings of these studies were further endorsed when a pooled analysis consisting of over 60,000 people was undertaken (22). The findings confirmed a significant inverse association between coffee consumption and the risk of liver cancer.

How does it work?

Various components of coffee have been related to such a positive effect, including caffeine, coffee oils kahweol of cafestol, and antioxidant substances from coffee, but no definite evidence is available.

However most scientists agree that it is probably the antioxidants in coffee, working in conjunction with caffeine, which results in this positive effect. Caffeine is already known to help protect against liver cirrhosis and given the weight of evidence from population studies, coffee appears to have a real effect in reducing the risk of liver cancer.

Other cancers

The IARC Report included data on coffee and gastric cancer from five studies (2). There was no evidence of association in any and further studies confirm that coffee is unlikely to have any major effect on gastric cancer.

Six studies providing data on cancers of the mouth, throat and oesophagus were considered in the IARC Report. There was no evidence of association with coffee consumption in any of them. (11, 23-26).

With relevance to breast cancer, again the IARC Report found no association with coffee consumption (2). This was more recently confirmed, in several studies including an Italian study involving nearly 6000 cases (27).

Likewise, no consistent relation was observed between coffee with ovarian, laryngeal, lung, prostate, cervical, endometrial and thyroid cancers, Hodgkin's and non Hodgkin lymphomas, sarcoma, and skin melanoma (28, 29).

Conclusions

The large amount of data on bladder cancer allows us to exclude a strong association with coffee intake and, likewise, other scientific studies also strongly suggest no association between coffee and pancreatic cancer risk. For bowel (colorectal) cancer, most studies reported an inverse association i.e. a positive effect resulting from coffee consumption whereas evidence from population studies concerning liver, and probably oral and oesophageal, cancers consistently showed a positive effect with coffee consumption.

In an earlier major study involving over 15,000 men and women (30) the authors concluded “The most important findings reported here are the lack of positive association between coffee drinking and any major cause of death and coffee not significantly increasing the incidence of any common cancer”

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