A study recently published online by the journal Circulation provides some rather meaty data to chew on. Red meat may not increase the risk of heart disease. Processed meat, in contrast, apparently does.
But before the carnivores start licking their chops and stoking their coals, the food for thought served up in this paper requires considerable slicing and dicing. So out with the steak knives, and let's get to it.
The new study is a meta-analysis examining the effects of red meat and processed meat on heart disease, stroke and diabetes risk. Meta-analyses can be very powerful, but they are intrinsically limited to the quality of the research from which they are pooling data.
In this case, that is an important limitation. Data in the new report are all derived from trials in which consumption of red meat and processed meat were compared. There are relatively few such studies that exclude poultry and fish; the studies in question control variably for other health behaviors that might confound the findings; and most importantly, all of the studies were observational.
That means participants simply reported what they ate, rather than being assigned. While intervention studies are designed to establish cause and effect, observational studies can generally only suggest associations. It may be, for instance, that people who eat beef, but avoid processed meat, are generally more health conscious than those who eat both.
Still, the meta-analysis assessed over a million people. So its findings are worthy of consideration, even if they come encumbered by caveats.
The study suggests that when isolated from processed meat, pure red meat has no meaningful association with heart disease risk. Total meat intake was, the authors state, "associated with a trend toward higher [heart disease] risk."
Each daily serving of processed meat raised the apparent risk of heart disease by a relative 40 percent. Each serving of total meat per day was linked to a 12 percent rise in the apparent relative risk of diabetes.
Some of the findings came down to statistical subtleties. For example, a 19 percent increase in diabetes risk associated with processed meat intake was significant, whereas a 16 percent increase in such risk with red meat consumption was not. That three percent relative risk difference is decisively trivial. The review lacked statistical power for stroke, but there were positive associations between red meat, processed meat and total meat with stroke risk.
Research findings are more reliable when there are mechanisms to account for them, and in this case, there are. In general, processed meats are higher in saturated fat and lower in protein than pure red meats. More importantly, processed meats are much higher in sodium, and contain compounds such as nitrates and nitrites -- both linked to vascular injury and atherosclerosis -- in relatively high concentrations.
Of course, red meat does contain saturated fat and cholesterol, which is what makes an apparent lack of association between its intake and heart disease noteworthy. As for saturated fat, it is not all created equal. We have already learned to distinguish saturated fat from unsaturated varieties, and most people know that some sub-categories of fat, such as omega-3, have unique health effects. Our next collective step forward will be to refer to the health effects of specific fatty acids within a given class. About a third of the saturated fat in red meat is stearic acid, which appears to be free of the harmful effects of its classmates.
Dietary cholesterol is very weakly associated with heart disease risk, and may be all but irrelevant. This is unsurprising -- cholesterol has been a normal part of the human diet since the Stone Age, when it came from meat and eggs.
Our Stone Age proclivities do not, however, directly support a modern carnivorous bent. Anthropologists suggest that antelope flesh is fairly representative of the meat our ancestors ate. While the flesh of beef cattle is roughly 35 percent fat by calories, most of it saturated, the flesh of antelope is as low as five percent of calories from fat, all of it unsaturated, and some of it omega-3. Not all ungulates are created equal.
The new study cannot distinguish among varieties of red meat. Some is leaner, some is fattier. Just as we are what we eat, so, too, is what we eat. The flesh of grass fed cattle, for example, is more nutritious than that of grain fed cattle.
The study looked at heart disease, stroke and diabetes only. Many studies have linked higher intake of red meat with increased cancer risk, colon cancer risk in particular. This study was blind to that issue.
Also ignored was the fact that eating more meat probably means eating less of other foods. Other foods -- namely vegetables, fruits, whole grains, nuts, seeds and fish -- have been shown to reduce the risk of heart disease, and of premature death from any cause. What we eat matters both because of what it puts into our mouths, and what it bumps out. A switch to more meat-based eating could very well confer net harm in part because of what it is taking out of your diet.
As pointed out by, among others, T. Colin Campbell in 'The China Study,' prevailing protein intake in the U.S. tends to be much in excess of need, and is likely associated with adverse effects on everything from bone density to cancer risk. Eating more meat would compound such concerns.
Raising feed animals comes at a very high environmental cost. As Michael Jacobson and colleagues point out in "Six Arguments for a Greener Diet", it takes roughly seven lbs of corn to grow one pound of beef; five times as much water to grow feed grains for cattle as to grow fruits and vegetables for ourselves; and roughly ten times the acreage to raise cattle for food as to raise comparable plant food calories for direct human consumption.
Processed meats -- sausage, bacon, and the like -- are almost certainly harmful in ways that simple, unprocessed red meats are not. But however you choose to digest the news about meat, chew on this: Red meats are, at best, less harmful; there is nothing to suggest they actually promote health. Plant foods do -- for people and planet alike.
However you dice the new data, in other words, Michael Pollan's advice still stands: eat food, not too much ... mostly plants!
Dr. David L. Katz; www.davidkatzmd.com
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