Association of Animal and Plant Protein Intake With Mortality
Is the choice of protein source a determinant of risk of death?
A prospective cohort diet study was conducted in US health care professionals including 131,342 participants from the Nurses’ Health Study (1980 to end of follow-up on June 1, 2012) and Health Professionals Follow-up Study (1986 to end of follow-up on January 31, 2012). Animal and plant protein intake was assessed by regularly updated validated food frequency questionnaires. Data were analyzed from June 20, 2014, to January 18, 2016. Cox proportional hazards regression models with age as the time scale was used to estimate the hazard ratio (HR) and 95% confidence interval (CI) for mortality associated with animal and plant protein intake.
Of the 131,342 participants, 85,013 were women (64.7%) and 46,329 were men (35.3%) (mean age, 49  years). The median protein intake, as assessed by percentage of energy, was 14% for animal protein (5th-95th percentile, 9%-22%) and 4% for plant protein (5th-95th percentile, 2%-6%). After adjusting for major lifestyle and dietary risk factors, animal protein intake was weakly associated with higher mortality, particularly cardiovascular mortality (HR, 1.08 per 10% energy increment; 95% CI, 1.01-1.16; p for trend = 0.04), whereas plant protein was associated with lower mortality (HR, 0.90 per 3% energy increment; 95% CI, 0.86-0.95; p for trend < 0.001). These associations were confined to participants with at least one unhealthy lifestyle factor. Replacing animal protein of various origins with plant protein was associated with lower mortality. In particular, the HRs for all-cause mortality were 0.66 (95% CI, 0.59-0.75) when 3% of energy from plant protein was substituted for an equivalent amount of protein from processed red meat, 0.88 (95% CI, 0.84-0.92) from unprocessed red meat, and 0.81 (95% CI, 0.75-0.88) from egg.
High animal protein intake was positively associated with mortality and high plant protein intake was inversely associated with mortality, especially among individuals with at least one lifestyle risk factor. Substitution of plant protein for animal protein, especially that from processed red meat, was associated with lower mortality, suggesting the importance of protein source.
As with other population studies there are confounders, but the results of the 32-year study of well-educated healthy health care professionals with repeated dietary assessments is about as good as it can get. The benefit of isocaloric incremental plant protein was restricted to participants with at least one of the unhealthy behaviors including smoking, heavy alcohol intake, overweight or obesity, and physical inactivity. The study does not address the possible separate benefit of plant sources compared to animal sources of protein by macro- and micronutrients and phytochemicals. But the required substitution of plant for animal protein (3% of total calories) to achieve the results seems doable with a modest increase in plant protein from bread, cereals, pasta, nuts, beans, and legumes and limitation of processed and unprocessed red meats. The findings also support the long held view that sodium, nitrates, and nitrites may have a deleterious effect on cardiovascular and overall health. Fish and poultry were not associated with mortality.
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