Monday November 5 2007
More than six cups a day was linked to lower risk
Drinking six cups of coffee a day can reduce the risk of some types of skin cancers by 30%, reported the Daily Express. The benefits “are only seen with caffeinated coffee – decaffeinated does not have the same impact”, the newspaper said.
This story is based on a large US study, which showed an association between higher consumption of coffee and fewer non-melanoma skin cancers. However, it cannot prove that the higher levels of coffee drinking are responsible for reducing the risk. This is because it looked at the association between coffee and non-melanoma skin cancer in a way that cannot determine whether the coffee drinking took place before the development of skin cancer and cannot determine whether one causes – or prevents – the other.
Where did the story come from?
Dr Ernest Abel and colleagues from Wayne State University, and universities in Tennessee, New York, and North Carolina carried out this research. The study was funded by the National Heart, Lung, and Blood Institute, US Department of Health and Human Services and was published in the peer-reviewed medical journal European Journal of Cancer Prevention.
What kind of scientific study was this?
This was a cross-sectional analysis of 77,373 postmenopausal Caucasian women aged 50-79 years from across the US, who enrolled in the Women’s Health Initiative (WHI) study between 1993 and 1998. Only women who provided information about their coffee and tea consumption and who had a history of skin cancer were included in these analyses.
When the WHI study began, women answered questionnaires about many areas of their lives, including their lifestyle and whether they had ever had skin cancer. The questionnaire asked them how many cups of coffee and tea they drank daily over the previous three months, and whether these drinks were caffeinated. The researchers then compared the rates of non-melanoma skin cancer in women with different levels of coffee and tea consumption. Researchers took into account factors that might affect the risk of developing non-melanoma skin cancer in these analyses, including age, ethic origin, how far north or south the women lived, body mass index, smoking, alcohol consumption, and their use of hormone replacement, among other things.
What were the results of the study?
About eight in every 100 of women in the WHI study reported having had non-melanoma skin cancer. Researchers found non-melanoma skin cancer was less common in women who drank coffee at least once daily than in women who drank no caffeinated coffee. The more coffee a woman drank daily, the less likely she was to have had non-melanoma skin cancer. About seven in every 100 women who drank six or more cups of coffee a day reported having had non-melanoma skin cancer, compared with 10 in every 100 women who drank no caffeinated coffee. There was no significant association between drinking tea or decaffeinated coffee and having had non-melanoma skin cancer.
What interpretations did the researchers draw from these results?
The researchers concluded that drinking coffee is associated with a reduction in the risk of non-melanoma skin cancer in Caucasian women, and that greater coffee consumption is associated with a greater reduction in risk. They suggest that their conclusions should be tested in further prospective studies.
What does the NHS Knowledge Service make of this study?
Although this study is large, and does show a statistical association between coffee drinking and self-reported non-melanoma skin cancer, it cannot show that coffee drinking causes a reduction in a person’s risk of developing cancer. This is because of a number of limitations, which the authors acknowledge:
- The main reason why this study cannot prove a causal link between coffee and cancer is that it uses cross-sectional analyses, which cannot establish the sequence of events (in this case, drinking coffee and developing non-melanoma skin cancer) and therefore cannot prove that one thing causes the other. If researchers think that drinking coffee may reduce risk of cancer, they need to assemble a group of women who do not have skin cancer, assess their coffee drinking, and then follow them up over a period of time to see who develops skin cancer and who does not.
- This type of study, because it does not randomise people into groups, is always susceptible to having imbalances between the groups compared (in this case coffee drinkers and non-drinkers), and these differences may be contributing to the association with cancer, rather than the coffee drinking. The researchers did try to take this possibility into account, but it is not possible to account for all potential known or unknown contributing factors.
- Women reported on their coffee consumption in only the previous three months; this may not be representative of their coffee consumption over the rest of their lives, including the period before they got skin cancer.
- The paper did not report whether they had confirmed the women’s self-reported skin cancers in any way, for example whether they checked their medical records. If women’s records were not checked, it may be that some cases of non-melanoma skin cancer were missed, or that some cases of skin cancer were misclassified because women may not have been sure whether what they had was skin cancer, or what type of skin cancer they had. If there were many inaccuracies of this type, it could affect the results of the study.
- This study only included Caucasian women; the results may not apply to people of different ethnic backgrounds or to men. Women from different ethnic backgrounds were excluded from this study because they had a much lower rate of non-melanoma skin cancer than Caucasian women.
The authors of this study note that some previous studies have suggested that coffee drinking is associated with an increased risk of some other types of cancer (e.g. bladder or colorectal cancer), although this link has not been found in all cases. Based on all of this information, it would be unwise to suggest that people should start increasing their coffee intake to prevent non-melanoma skin cancer.
These are intriguing results which will no doubt lead to renewed efforts to identify underlying biochemical and physiological mechanisms underlying the association.
Sir Muir Gray adds...
Association does not mean causation; I will not change my coffee consumption because of this study.