Red meat consumption has been linked to diabetes, cardiovascular disease, and several types of cancer (breast, colorectal, stomach, bladder, prostate, and lymphoma). There are plausible mechanisms: meat is a source of carcinogens, iron that may increase oxidative damage, and saturated fat. But correlation and plausibility are not enough to establish causation. Is red meat really dangerous? If so, how great is the risk? A couple of recent studies have tried to shed light on these questions, but they have raised more questions than they have answered.
A Systematic Review and Meta-Analysis
A new study in Circulation, “Red and Processed Meat Consumption and Risk of Incident Coronary Heart Disease, Stroke, and Diabetes Mellitus. A Systematic Review and Meta-Analysis,” by Micha, Wallace and Mozaffarian, is a systematic review of the literature. It analyzed 17 prospective cohort studies and 3 case-control studies, with a total of 1.2 million subjects. As far as I can judge, it appears to be a well-done systematic review with excellent methodology and multiple precautions. They even looked for things like publication bias (which they did not find).
They found that the consumption of processed meats, but not red meats, is associated with a higher incidence of coronary heart disease and diabetes. (Processed meats include bacon, sausage, ham, hot dogs, salami, luncheon meat and other cured meats.) The increased risk per 50 gram serving of processed meats per day was 42% for heart disease and 19% for diabetes. Unprocessed red meats were not associated with CHD and were associated with a nonsignificant trend towards higher risk of diabetes. They found no association with stroke, but this was based only on 3 studies.
They commented that
“…each of these individual studies has potential limitations, and our findings should be interpreted in that context. On the other hand, this represents the most complete worldwide evidence to date of the potential effects of red and processed meat consumption on incidence of CHD, stroke, and diabetes mellitus.”
A Large Study of Meat and Mortality
A 2009 study in the Archives of Internal Medicine, “Meat intake and mortality: a prospective study of over half a million people,” by Sinha et al., was more comprehensive in that it looked at many different conditions like cancer and cardiovascular disease, and it measured various causes of mortality as well as all-cause mortality.
The half a million subjects were 51-70 years old and were from various geographic locations in the US. They filled out a questionnaire that asked about their usual consumption of foods and drinks and portion sizes over the previous twelve months. Their diets were classified as high, medium or low risk meat diets based on the amount of red meat and white meat adjusted for energy, and they were split into two groups using median consumption as cutpoints. The study was prospective: it assessed diet first and then followed subjects for 10 years and recorded deaths and causes of death.
It concluded that red and processed meat intakes were associated with modest increases in total mortality, cancer mortality and CVD mortality.
In general, those in the highest quintile of red meat intake tended to consume a slightly lower amount of white meat, but a higher amount of processed meat than those in the lowest quintile. Subjects who consumed more red meat tended to be married, more likely to be of non-Hispanic white ethnicity, more likely to be a current smoker, have a higher body mass index, and a higher daily intake of energy, total fat and saturated fat; whereas they tended to have a lower education level, were less physically active and consumed less fruits, vegetables, fiber and vitamin supplements.
They found an increased risk associated with accidental deaths with higher consumption of red meat in men but not in women. It’s hard to know how to interpret that. They found an inverse association for white meat intake: it appeared protective against total mortality, but there was a small increase in risk for CVD mortality in men.
The overall hazard ratios for men ranged from 1.06 to 1.31 for red meat (increasing steadily by quintile of meat intake), .90 to.92 for white meat, and 1.01 to 1.16 for processed meats. The effect of red meat was greater than the effect of processed meats, which was opposite to the findings of the review in Circulation.
They tried to correct for confounders. In the process, they found an increased risk with white meat consumption among never-smokers and commented that the reason was not readily apparent. I suspect that the reason was that if you look at a large enough number of subgroups you can always find an occasional chance correlation that is meaningless.
Their data also showed that increased red meat consumption was correlated to smoking, lack of exercise, higher total calorie intake, higher body weight, higher total fat and saturated fat intake, lower intake of fruits, vegetables and fiber, and lower use of vitamin supplements. Could it be this constellation of factors, rather than red meat itself, that leads to higher mortality?
They estimated that
For overall mortality … 11% of deaths in men and 16% of deaths in women could be prevented if people decreased their red meat consumption to the level of intake in the first quintile.
I don’t think this can be determined from the data. They haven’t reliably ruled out all possible confounding factors and they don’t have any direct evidence that taking people with a high red meat intake and reducing their intake improves their longevity.
What about Vegetarians?
…in comparison with regular meat eaters, mortality from ischemic heart disease was 20% lower in occasional meat eaters, 34% lower in people who ate fish but not meat, 34% lower in lactoovovegetarians, and 26% lower in vegans. There were no significant differences between vegetarians and nonvegetarians in mortality from cerebrovascular disease, stomach cancer, colorectal cancer, lung cancer, breast cancer, prostate cancer, or all other causes combined.
Meta-analysis of several prospective studies showed no significant differences in the mortality caused by colorectal, stomach, lung, prostate or breast cancers and stroke between vegetarians and “health-conscious” nonvegetarians.
In vegetarians, a decrease of ischemic heart disease mortality was observed probably due to lower total serum cholesterol levels, lower prevalence of obesity and higher consumption of antioxidants. Very probably, an ample consumption of fruits and vegetables and not the exclusion of meat make vegetarians healthful.
Epidemiologic studies based on self-reporting and recall are not the most reliable form of evidence. What are we to make of all the confusing data? The evidence is far from conclusive, but it suggests that it would be wise to limit our consumption of red meat. The evidence is not strong enough to support recommendations that we give up red meat entirely or become vegetarians.
This article was originally published in the Science-Based Medicine Blog.