Defining what represents a macronutritionally balanced diet remains an open question and a high priority in nutrition research. Health authorities recommend that Westerners reduce their intake of animal protein and increase their intake of plant protein. The assumption is that the recommended shift in protein sources will promote overall health. An analysis of prospective data from Harvard University provides support for this recommendation.
The analysis included 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. Of the 131,342 participants, 85,013 were women (64.7%) and 46,329 were men (35.3%). 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 (hazard ration, 1.08 per 10% energy increment; 95% confidence interval, 1.01-1.16; P for trend = .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 < .001). These associations were confined to participants with at least 1 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.
Replacing animal protein of various origins with plant protein was associated with lower mortality. In particular, the hazard ratios 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.
The authors of this study didn’t determine whether risk would have been further reduced had more than 3% plant protein been substituted for an equivalent amount of animal protein although it is reasonable to speculate the benefit would have been even greater. An important question is whether it is the protein per se that is responsible for the change in risk or the components in foods that are good sources of plant or animal protein.
Song, M., Fung, T.T., Hu, F.B., Willett, W.C., Longo, V.D., Chan, A.T., and Giovannucci, E.L. Association of Animal and Plant Protein Intake With All-Cause and Cause-Specific Mortality. JAMA Intern Med. 2016, 176, 1453-1463.