THIS HAS been a confusing week. First we heard that grapefruit increases breast cancer risk. Next we were told that eating fruit and veg does not boost chances of beating breast cancer. Both stories seemingly demolished assumptions about the benefits of a healthy diet.

The cancer and diet study was particularly troubling. The Women’s Health Eating and Living study reported that boosting fruit and veg consumption way beyond recommended five-a-day intakes in 3,000 women with breast cancer did not make a difference in terms of whether the cancer came back and how soon they died.

So what is the real story on diet and cancer? First, there are many types of cancer. Although it seems common sense that a diet rich in fruit and veg will prevent them all, the evidence for breast cancer and prostate cancer is weak. The growth of these cancers is primarily driven by hormones: oestrogen (which may be boosted by grapefruit intake) and testosterone respectively. So diet may not be as important as other factors in the cause, growth or recurrence of these types.

But for other cancers, especially those affecting the gut, and to a lesser extent lung cancer, there is strong evidence that eating fruit and veg has a protective effect. A big continuing study of diet and cancer, with half a million participants thoughout Europe, has shown that high-fibre intakes reduce bowel cancer and that there is a clear link with bowel cancer and a high intake of red or processed meat.

But it’s difficult to be definitive about all types of cancer, or to know exactly why fruit and veg may be beneficial. That’s because the research itself isn’t definitive. Studying the effect of diet is always tricky, relying on participants remembering what they ate (can you remember what you ate yesterday, let alone a year ago?). You also need to involve huge numbers of people to get results that are statistically valid.

One thing is clear. Scientists are increasingly convinced that we should be concerned about our general diet. Individuals vary immensely, although women have markedly better dietary patterns than men. How much you eat is also vital; obesity is linked to cancer, particularly endometrial and kidney cancers, and breast cancer after menopause.

What you really need to know is this, and the evidence is cast-iron: a diet high in fruit and veg reduces the risk of dying early from any cause by 20 per cent. Even a single extra portion of fruit and veg a day could make a difference. So eat up your greens. They really are good for you.

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