Friday, November 17, 2006

Where's the beef?

A study linking red meat intake and some forms of breast cancer in pre-menopausal women was reported around the world this week. Researchers at Harvard Medical School analysed data from the Nurses Health Study II following women aged between 26 and 46 years over a 12 year period. From over 90,000 women followed-up, there were 1,021 cases of breast cancer. Cases of breast cancer related to progesterone and oestrogen receptors were almost twice as common in those who ate 1.5 portions of red meat per day than in women who ate three or fewer portions per week.

Breast cancer is most commonly a post-menopausal condition. Even if the figures stated were an accurate reflection of an increased relative risk, the overall risk for any pre-menopausal woman is still small. For example, in this study, only around one per cent of the women involved developed breast cancer.

As Cancer Research UK noted in response to the report: 'According to this study, a woman would need to eat more than one-and-a-half portions of meat a day, every day, to significantly increase her risk of hormone-sensitive breast cancer before the menopause. But the overall risk of pre-menopausal breast cancer is low when compared to getting the disease after the menopause. So even at the highest rates of meat consumption this is overall still a relatively small increase.'

Indeed, as the Harvard researchers note: 'Previous epidemilogic data on red meat intake and breast cancer risk have been inconclusive.' Rather than accept that this link is unlikely to be of any practical significance, they appear to have sliced and diced the data further until finding something, then tacked a theory on afterwards.

However, even the results stated should come with a health warning. Relative risks of 2.0 (ie, a doubling of risk) or below need to be treated with extreme caution. There are plenty of reasons why the results might be inaccurate. This was a study based on food questionnaires completed in 1991, 1995 and 1999. Were these questionnaires filled in correctly? Our eating habits tend not to be fixed but vary over time - will these changes be captured accurately in four-yearly snapshots? How was this red meat consumed - did cooking methods make a difference? Did the women who ate more red meat tend not to eat other foods as a result which might be protective? There are so many possible questions with such a study that only relatively high relative risks should really be taken seriously.

Further, there is a striking difference in overall calorie intake for the different groups. The women who ate the least red meat consumed, on average, 1524 calories per day, while the most frequent red meat eaters consumed 2359 calories per day. Unsurprisingly, the latter group weighed, on average, well over a stone more than the former group. Are calorie intake or body weight factors?

There are so many uncertainties in this study that it is probably best to ignore it. Even if it is accurate, it can only make people feel guilty about eating what is, after all, nutritious and tasty food with almost certainly no effect on their life expectancy. What purpose was served by that?

Read on:

Red meat intake and risk of breast cancer among premenopausal women, Archives of Internal Medicine, November 2006