We have always been told that fruit is good for us, but some new research may make you look twice at your breakfast grapefruit.
We're always being told by the Department of Health to eat five portions of fruit and vegetables a day. But it's not clear where the evidence for this comes from and a large study by the European Prospective Investigation Into Cancer and Nutrition (Epic) two years ago of the dietary intake of more than 400,000 people found only a weak link between eating fruit and vegetables and a reduction in overall cancer risk. There's no evidence it reduces breast or prostate cancer.
Even so, how could anyone argue against eating more fruit and vegetables? Well, it depends what fruit you're eating. A review article in the Canadian Medical Association Journal warns that a chemical in grapefruits can interact lethally with certain medicines. Even apple juice, the staple of many breakfast tables, may reduce our absorption of some drugs. Parents also often think fruit juice is a healthy alternative to fizzy drinks but juices contain sugar and calories too - as much as a glass of Coke (160 calories) - so should be drunk in moderation.
Grapefruit has also been linked to an increase in breast cancer. A study in the British Journal of Cancer (BJC) of 500,000 post-menopausal women found that eating a quarter of the fruit a day increased the risk by 30%.
There is one good reason for not eating grapefruits - they taste sour. But they are unlikely to increase your risk of breast cancer: further research in the BJC showed no increase of breast cancer in pre- or post-menopausal women who tuck into grapefruits or drink juice.
Grapefruit contains a type of chemical called furanocoumarin (also found in seville oranges and limes but not valencia or other sweet oranges), which, by inhibiting the enzyme CYP3A4, stops the breakdown of some prescription drugs. So the concentrations of these drugs rise and can have serious side-effects, including kidney damage, heart block (where no electrical impulses pass through the heart and it can stop beating), and deep vein thrombosis (which occurred when an oral contraceptive was taken by a woman who also ate grapefruit three days in a row).
Patient information leaflets should tell you what foods to avoid with which drugs. If in doubt, ask your doctor. So you don't need to eat less fruit, and may still need to eat more. Another paper from the Epic study found a 22% reduction in deaths from heart attacks in people who ate eight portions of fruit and vegetables a day compared to those who ate three or fewer. The way the study is designed, however, means that it can only suggest a link - it can't prove one.
Grapefruit has been linked to an increase in breast cancer. Photograph: Amanda