On March 5, news reports started to surface about a meta-analysis showing that processed meat, but not red meat, was linked to heart disease and diabetes. It was the first systematic review or meta-analysis of its kind, and the paper was released this month.
The authors broke meat down into three categories:
- Processed – might have included some poultry
- Total – the above two categories combined
Averaged across studies, consumption levels in the lowest versus highest category of intake were (in servings per week):
Red – 1.1 vs. 8.3
Processed – 0.4 vs. 5.7
Total – 1.8 vs. 10.5
The results were:
Coronary Heart Disease
Per serving per day:
Red: 1.00 (0.81 – 1.23)
Processed: 1.42 (1.07 – 1.89)
Total: 1.27 (0.94 – 1.72)
Per serving per day:
Red: 1.16 (0.92 – 1.46)
Processed: 1.19 (1.11 – 1.27)
Total: 1.12 (1.05 – 1.19)
Per serving per day:
Total: 1.24 (1.08 – 1.43)
No other statistically significant findings.
In other words, more servings of red meat did not increase the risk of heart disease, diabetes, or stroke, while more servings of processed meat increased the risk of heart disease and diabetes. Only more servings of total meat increased the risk of stroke, but there was not nearly as much data for stroke.
This raised the question, “Could it be that non-processed red meat – and, by proxy, saturated fat – is NOT linked to heart disease?”
Before I go and do something that really annoys me, I want to point out that it really annoys me when people nickel and dime a study to death because it disagrees with their pre-drawn conclusions. That said, I do not think you can rule out the possibility that if the researchers had included white meat in their analysis, unprocessed meat could have been linked to heart disease and diabetes. There is some evidence that people who eat no red meat or poultry have lower rates of heart disease.
Additionally, how the variables were adjusted is not clear. The authors’ state:
If multivariable models were reported with and without additional adjustment for variables that could be either confounders or intermediates (eg, high cholesterol), the multivariable model without such variables was selected. If the only multivariable model included such variables, this was selected in reference to crude or minimally adjusted models.
What this means is that if a study adjusted their results for body mass, caloric intake, or cholesterol levels, those results might have been included in the meta-analysis. This would possibly make red meat look better than it actually is because, for example, if red meat increases cholesterol levels, but you then adjust for cholesterol levels, you will lose the effect of red meat. That said, these adjustments would also have affected the processed meats category and it was not enough to ameliorate those results.
The authors note the differences between processed and unprocessed red meats:
Per 50-g serving, processed meats contained modestly higher calories and percent energy from fat and lower percent energy from protein compared with 50 g of red meats. Consistent with lower protein content, processed meats also contained less iron. Processed meats contained relatively similar saturated fat and slightly lower cholesterol, the latter perhaps related to some processed meats being derived from pork and/or lower-cholesterol deli meats. Relatively small differences were present in contents of monounsaturated fat, polyunsaturated fat, or potassium. Largest differences were seen in levels of sodium, with processed meats containing 4-fold higher levels (622 versus 155 mg per serving), as well as 50% higher nonsalt preservatives including nitrates, nitrites, and nitrosamines… Nitrates and their byproducts (eg, peroxynitrite) experimentally promote atherosclerosis and vascular dysfunction, reduce insulin secretion, and impair glucose tolerance, and streptozotocin, a nitrosamine-related compound, is a known diabetogenic compound.
The funding sources for the study were the Bill & Melinda Gates Foundation/World Health Organization Global Burden of Diseases, Risk Factors, and Injuries Study; the National Heart, Lung, and Blood Foundation, National Institutes of Health; and the Searle Scholars Program. The authors listed no conflicts of interest.
In conclusion, processed meats increase the risk of heart disease and diabetes. Unprocessed red meat may not be harmful at the levels measured in this analysis, but there are still some unanswered questions before that should be considered definitive. Total red meat may increase the risk of diabetes and stroke.