May 23, 2017

expert reaction to report on alcohol, diet, exercise and breast cancer

The World Cancer Research Fund has published a new report: ‘Diet, nutrition, physical activity and breast cancer.’

Prof Sir David Spiegelhalter, Winton Professor for the Public Understanding of Risk, University of Cambridge, said:

“The report does not provide absolute risks, but their results suggest that if around 110 older women consumed an extra drink every day, we would expect one extra case of breast cancer.  Whether this risk is worthwhile is a matter of judgement, but does not seem a firm basis for recommending abstention.

“The Chief Medical Officer’s report says that older women who drink moderately have lower death rates than similar women who don’t drink at all.  But this study focuses only on the risks of breast cancer and so gives a very different impression.”

Prof. Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“This thorough report makes a good job of reviewing available evidence from across the world on possible associations between diet, nutrition, physical activity and breast cancer.  Inevitably, though, the conclusions are complicated, and it’s not straightforward to pick out clear implications from them on how you might reduce your risk of breast cancer.  Like many cancers, breast cancer isn’t generally caused by something simple like getting an infection or having an injury – different factors have to act together.  Also, most of the factors that were studied in this report are themselves linked in complicated ways.  For example, the researchers concluded that being overweight or obese throughout adult life increases the risk of breast cancer after the menopause.  But if you were overweight and wanted to change that, you’d have to do it by changing your diet, cutting out or reducing alcohol, taking more exercise, or some combination of all of those.  So sorting out the separate effects of overweight, diet, alcohol and exercise, and deciding what is cause and effect, is not easy.

“Anyway the patterns of association are complicated.  The press release mentions the association between being overweight throughout adult life and post-menopausal cancer, but it does not mention other findings, such as that being obese or overweight when one is younger (18-30) is associated with a decreased risk of cancer both before and after the menopause, or that being obese or overweight before the menopause is associated with a reduced risk of cancer before the menopause.  So how body fat is associated with breast cancer depends on when you’re carrying the extra weight and when in your life the cancer might occur.  How do you balance this out?  It’s certainly true that most breast cancers occur after the menopause, so in one sense things that affect cancer risk after the menopause might be considered more important.  But in any case breast cancer is not the only disease that has evidence of links to overweight or to aspects of diet and exercise.  Decisions on whether and how to change one’s lifestyle shouldn’t be made on the basis of breast cancer risk alone, but evidence of other risks might well point in the same direction.

“It’s interesting that the press release leads on effects of alcohol on breast cancer risk, because actually there isn’t much new here.  Evidence that drinking alcohol increases breast cancer risk has been around for a long time, and though this report does take into account some new evidence, fundamentally not much has changed.  Indeed WCRF appear to have downgraded their assessment of the evidence that alcoholic drinks increase the risk of pre-menopausal breast cancer.  In 2008, and again in 2010, they characterised the evidence as ‘convincing’.  In this latest report they conclude the evidence is ‘probable’, which is one step down on their scale of strength of evidence.  And, taking their data at face value, the effect on breast cancer, while it looks real to me, isn’t huge.  Breast cancer is one of the commonest cancers, and according to Cancer Research UK (CRUK), of 100 UK women, about 12 or 13 will develop a breast cancer at some point in their lives.  Imagine that these 100 women all drank an extra small glass of wine or half a pint of beer every day, compared to what they drink now.  On WCRF’s figures, that would lead to 1 more of them developing a breast cancer during their lifetime.  Any increase is a bad thing, but it’s only one more out of the 100 women, and that has to be set against whatever pleasure the women might obtain from their drinking.  Drinking alcohol has a greater effect on the risks of several other cancers (such as cancers of the mouth, oesophagus and bowel) than it does on the risk of breast cancer, so there are other reasons to give up or cut down, but that just shows the importance looking at the whole picture and not just at breast cancer.”

* ‘Diet, nutrition, physical activity and breast cancer’ will be published by the World Cancer Research Fund on Tuesday 23 May 2017.

Declared interests

Prof Sir David Spiegelhalter: “No conflict of interest.”

Prof. Kevin McConway: “I have no relevant interests to declare.”