A study published in JAMA Internal Medicine analysing data from 131,342 participants in the Nurses' Health Study and Health Professionals Follow-up Study found that replacing 3 percent of calories from animal protein with plant protein was associated with a 10 percent lower all-cause mortality and a 12 percent lower cardiovascular mortality. The benefit was strongest for replacing red meat and egg protein with plant sources. However, replacing fish and poultry protein with plant sources showed no mortality benefit, suggesting the relationship is more nuanced than simply favouring plants over animals.
Why the Source of Protein Matters Beyond Amino Acids
Protein foods deliver more than amino acids. A portion of lentils delivers protein, fibre, folate, iron, and polyphenols. A portion of processed red meat delivers protein, saturated fat, haem iron, nitrates from preservatives, and AGEs from processing. A portion of salmon delivers protein, EPA and DHA, vitamin D, and selenium. The foods that serve as protein delivery vehicles bring their entire nutritional matrix, and it is that matrix, not the amino acids alone, that determines the health outcome.
The Practical Approach for Adults Over 40
Adults over 35 need sufficient leucine-containing complete protein to counter anabolic resistance and maintain muscle mass. Plant proteins are generally lower in leucine than animal proteins, which is a relevant consideration. The practical approach supported by the evidence is not elimination of animal protein but increasing the proportion of plant protein in the diet: targeting at least 50 percent of daily protein from legumes, whole grains, nuts, seeds, and tofu while retaining fish, eggs, and some quality red meat for leucine density and micronutrients including B12, zinc, and haem iron.