Red Meat Consumption and Mortality, Results From 2 Prospective Cohort Studies, Archives of Internal Medicine, March 12, 2012
The participant pool was large: 37,698 men from the Health Professionals Follow-up Study, plus 83,644 women from the Nurses' Health Study. The follow-up was long: over 20 years.
It found the more red meat one consumed, the greater their risk of premature death from cancer, heart disease, and other causes. How much greater? After controlling for possible confounders,1 the risk of death was 13% greater for each serving of unprocessed red meat eaten in a day; 20% greater for processed meat.
Replacing meat with other sources of protein lowered risk:
"We estimated that substitutions of 1 serving per day of other foods (including fish, poultry, nuts, legumes, low-fat dairy, and whole grains) for 1 serving per day of red meat were associated with a 7% to 19% lower mortality risk.42 grams is about 1.5 ounces, or a couple big bites.
We also estimated that 9.3% of deaths in men and 7.6% in women in these cohorts could be prevented at the end of follow-up if all the individuals consumed fewer than 0.5 servings per day (approximately 42 g/d) of red meat."
Here's a chart from the study. Kind of small but you can see that swapping any of these foods (nuts, legumes, low-fat dairy, whole grains, poultry, fish) for meat lowered risk. Look at that protection from replacing processed meat with nuts! (The further the black dot is to the left, the more protective that food was when eaten in place of meat.)
These were epidemiological studies, not more telling clinical trials. It would be a challenge to design and conduct a randomized controlled trial to test this hypothesis. How could you realistically get tens of thousands of people to eat the exact foods except for varying quantities of red meat? For 2 decades! So, these observational studies are useful, but not without their weaknesses.
The increases in risk, I thought, were small. Still, the trend was apparent, and dose dependent. There could always be an unaccounted confounder, though. Maybe those who ate more red meat slept less, had less restful sleep, had poor mental health (depression, anxiety, unmanaged stress, and curiously narcissism all impact health). Even poor dental hygiene has been linked to heart disease.
But it's fun to speculate. What could be making red meat so risky? Dr. Gregor offered a list of possible causes:
"I think the most interesting finding in the new Harvard studies is that even after factoring out known contributors of disease, such as saturated fat and cholesterol, they still found increased mortality risk, raising the question: what exactly is in the meat that is so significantly increasing cancer death rates, heart disease, and shortening people’s lives? A few possibilities include heme iron, nitrosamines, biogenic amines, advanced glycation end products, arachidonic acid, steroids, toxic metals, drug residues, viruses, heterocyclic amines, PCBs, dioxins, and other industrial pollutants."