Plants and Animals: Protein’s Link to Mortality and Cardiovascular Death Depends on Its Source

People who get their protein from plant rather than animal sources have a lower risk of dying and conversely, those who eat more meat have an increased mortality risk. The added harm of animal-based protein diets was seen in subjects with at least one unhealthy lifestyle habit—things like smoking or high alcohol consumption—according to a prospective cohort study of more than 130,000 healthcare professionals.

“The current data are mainly focused on the amount of protein intake especially relative to the intake of carbohydrates or fats. But there is very limited data on the food sources of protein in relation to long-term health,” Mingyang Song, MD (Massachusetts General Hospital, Boston, MA), the study’s lead investigator, told TCTMD. “It’s definitely important for the overall health, so that’s the motivation for this study. And we basically want to fill that knowledge gap in this area.”

Song et al published their results earlier this week in JAMA Internal Medicine.

The Many Sources of Protein

The researchers analyzed data on 121,700 individuals from the Nurses’ Health Study (collected from 1980 to 2012) and 51,529 from the Health Professionals Follow-up Study (collected from 1986 to 2012). In each study, individuals answered questionnaires about their lifestyle, medical history, and eating habits. Questions about diet assessed many sources of protein including processed and unprocessed red meat, poultry, fish, egg, plants, and dairy.

During follow-up, 36,115 deaths were reported, including 8,851 deaths from CVD and 13,159 deaths from cancer.

Protein consumption was analyzed as a percent of energy intake, or calories eaten per day. Median protein intake was 14% for animal protein and 4% for plant protein.

After accounting for major lifestyle and dietary risk factors, CVD mortality increased by 8% per 10% increment of animal protein consumed (P for trend = 0.04). Plant protein, meanwhile, was linked to 12% lower CVD mortality per 3% increment of daily energy consumed (P for trend < 0.007). All-cause mortality followed similar patterns, though the differences were slightly less pronounced.

Importantly, “these associations were confined to participants with at least one unhealthy lifestyle factor based on smoking, heavy alcohol intake, overweight or obesity, and physical inactivity, but not evident among those without any of these risk factors,” Song et al say.

For cancer-related mortality, the researchers found no significant relationship with protein source. 

Subbing in various plant-based sources for animal protein reduced the risk of all-cause mortality. When 3% of energy intake was replaced with plant protein, the likelihood of death was 44% lower than with the same amount of protein from processed red meat, 12% lower than with protein from unprocessed red meat, and 19% lower than with protein from eggs.

“The substitution associations were generally stronger for death due to CVD and other causes than those due to cancer, except substitution of egg, for which substitution of 3% energy plant protein was associated with 17% lower [risk of cancer-related morality],” the paper reports.

Not All Protein Is Created Equal

The study gives more support to the idea that plant-based diets are healthy for people in both the short term and the long term, according to Kim Williams, MD (Rush University Medical Center, Chicago, IL), a vocal advocate for veganism. He told TCTMD that there is increasing evidence that eating animal protein, especially processed red meat, carries the same added risk of mortality as smoking cigarettes.

“You have a risk factor. People enjoy smoking, but it’s hazardous to their health and it can actually in a very quantitative way reduce their life expectancy,” he said. “So the people who choose to not smoke have eliminated the ability of cigarettes to decrease their life expectancy. And I would say, of course, the same thing is true of diet.”

Interestingly, Williams stressed, eggs came in just behind processed red meat in terms of all-cause mortality risk but were not significantly linked to CVD mortality. The explanation, he suggested, could lie in the strong association between egg consumption and cancer-related mortality: in short, people who eat a lot of eggs are dying from cancer before they have a chance to die from MI.

While this may be possible, Song said, more research will be required to understand how egg consumption plays a role in all-cause mortality. “We are also puzzled by the quite strong association between egg consumption and cancer mortality,” he said.

Going so far as to say that the current study should inspire restaurants to change their menu options, Williams concluded: “The most important finding here, [which] is consistent with everything else that's been published, is that vegetarian protein improves life and animal protein consumption decreases life.”

Michael H. Wilson is the 2016 recipient of the Jason Kahn Fellowship in Medical Journalism.

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