Cut red meat? We already have
Eating one less portion of red meat a day can cut your risk of dying by 7-19 per cent, according to a widely-publicised paper (1) published this week.
For many of us this is an impossible recommendation, as we already average less than one portion of red meat a day. So, indeed, did the participants in the study: by 2006, the last time that their diets were recorded, the mean daily intake of red meat in men was 0.63 servings a day, and for women 0.55 servings.
The data came from Harvard School of Public Health, which for at least a decade has been pouring out data on nutrition and health from two long-running cohort studies: the Health Professionals Follow-up Study (nearly 40,000 dentists, vets, optometrists and osteopaths) and the Nurses Health Study (84,000 female nurses). The HPFS started in 1986, the NHS in 1980.
Over the lengthy run of these two studies, the diets consumed by the participants have changed a lot, especially among women. Their average intake of red meat halved between 1980 and 2006, the authors report. The decline among men is less marked – from 0.73 to 0.63 servings a day between 1986 and 2006.
To provide a measure of meat intake over the whole period, the authors averaged consumption from baseline to death. This means that the diets against which the risks are being assessed are not the same as those we are eating today. We have anticipated the conclusions by pre-empting the advice.
So any additional gains from cutting meat consumption are more limited than the paper in Archives of Internal Medicine implies. For example, the lowest quintile of red meat consumption in women – which is the one with the lowest risk of death – is 0.51 portions a day (95 per cent CI 0.37 to 0.61). That is now almost exactly the same as the average woman in the study consumes.
For men, today’s average consumption of 0.63 portions a day is very close to the second quintile (0.61, CI 0.53 to 0.70). At this level of consumption, there is no statistically significant increase in risk of death compared with the lowest quintile. So the average person in the study, it seems, is already at or very close to the minimum level of risk.
But the puzzles with this study do not end there. If the answers given to the dietary questionnaires were accurate, we are expected to believe that women eat more red meat than men, which common experience suggests is unlikely. At every quintile level, the nurses’ taste for red meat exceeded that of the dentists, optometrists, vets and osteopaths (though their average consumption is reported as being less). This could be a consequence of the nurses’ study starting six years earlier, when everybody ate more meat; or it could be caused by uncertainties in accurately ascertaining diet.
There are also very big differences between meat-lovers and meat-avoiders that may confound the findings. For example, the men who ate most meat were also three times as likely to smoke. They also drank considerably more, and were much less likely to take multivitamins, which probably do little good but serve as a marker for the health-conscious.
Among the nurses, the same was true, though to a less marked degree. The final results correct for smoking, drinking, body mass index and family history of various diseases, among other things, but there may be residual confounding that is not corrected for.
Finally, the study does not tell us how many deaths were premature. The men were aged 40-75 at baseline, so by 2008 when the deaths were counted would have been between 62 and 97; the nurses were aged 30-55 at baseline so would have been 62-87 by 2008. By all normal expectations many would have been expected to die. In fact, 8,926 men died (out of 37,698) and 15,000 women (out of 83,644). But how many died before their time is unknown, or unreported.
In both men and women, there were more deaths in the lowest quintile of red meat consumption than in any other but the highest – so the raw data suggest that red meat consumption, except at the highest levels, protects against dying. When adjusted for age, however, the trend is reversed, which tells us that the lowest quintile must have been, on average, older. It is possible they ate less meat because they ate less of everything.
In nutrition science, there are no certainties. This is a good study with a methodology better than most, but it certainly does not prove that we could all benefit from the advice it offers. Only a minority of people eat enough red meat to be able to cut out a portion a day without becoming vegetarians. The rest of us have little to gain by eating less.
- Red Meat Consumption and Mortality, by An Pan et al Arch Intern Med, online March 12 2012.