Environmental and lifestyle factors are estimated to play a part in 80% of all common cancers. So when a breast cancer survivor with a scientific background publishes a book about her own illness and her views on the role of diet and other lifestyle factors in helping to control it, it is inevitable that it produces interest from both the media and the public.
Jane Plant, the author of Your Life in Your Hands, is a geophysicist whose breast cancer relapsed in lymph glands in her neck. She sensibly underwent conventional chemotherapy - but at the same time, she withdrew dairy produce from her diet. In trying to understand why she had developed breast cancer, she began reading widely on the subject and noticed that women in China and Japan - where dairy consumption is low - have a significantly lower risk of developing breast cancer than women in western countries. So she had her number one cause: dairy produce. And having found it, she came up with a detailed theory, involving bovine hormones, about why cow's milk might cause breast cancer.
But this is far too simplistic. The fact is that, in 20 years of study, countless doctors, environmental scientists and nutritionists have failed to find simple relationships between diet and cancer risk around the world. Dietary fat content has been given particular attention, as some countries traditionally consume much less animal produce than others. Even research involving large numbers of women has not clarified a role for animal fat or dairy produce in breast cancer, but by studying women from ethnic populations with low rates of cancer, researchers have identified factors that may help cut down cancer.
One factor is a diet high in soy produce, a food that is rich in chemicals called isoflavones, which mimic some of the activity of the female hormone oestrogen and may act to block the stimulatory action of oestrogen on the breast. Women in these populations also tend to be significantly less overweight than their western counterparts, partly due to lower overall calorie intake and higher levels of physical activity, both factors also known to reduce breast cancer risk. They also drink little or no alcohol, do not smoke and on average start their families a decade earlier than modern, westernised women - all recognised risk factors for breast cancer.
Another key point is that some cultures with low consumption of dairy produce have similar breast cancer rates to those in the west. The Indian state of Kerala is a good example of the complexity of interactions of risk factors. There, women eat virtually no dairy or animal produce, but very high levels of female education mean the average age of marriage is 23 and women have their first pregnancies 10 years later than women in some other Indian states, where dietary habits are very similar but risks of breast cancer are much lower.
Hormonal factors remain the most significant and best understood of the metabolic risks for breast cancer. Early menstruation, late menopause and late or no pregnancy are all known to increase risk, but by relatively small levels compared with the "average" woman. It is equally likely that these factors also interact with other lifestyle factors, such as exercise levels, smoking and alcohol consumption, to modify risk over many decades.
I am concerned by this new book for two reasons: it is not based on proper research and in western diets, dairy produce is a major source of nutrients such as calcium. We don't want cancer patients making radical changes to their diets without proper nutritional advice from experts.
Gillian Ross is a senior lecturer in oncology at the Institute of Cancer Research and a consultant in clinical oncology at the Royal Marsden Hospital's breast unit. For more information, call Cancer BACUP on 0808 800 1234.
