Plant Versus Animal Protein: Is it Composition or Total Protein that Makes the Difference

There has been a generalized global trend towards diets that are plant centric as opposed to omnivorous or animal centric. The major justifications for this trend are the attainment of a healthy lifestyle and the protection of the planet.

Much of the justification for the health benefits of plant-based diets states that plant protein is healthier than animal protein. For example, blood lipid levels (particularly LDL), which are biomarkers for cardiovascular disease, have been shown to be lowered by the consumption of plant protein when compared to animal protein. In addition, plant-based protein consumption has been associated with better weight management and a reduction in the level of obesity and the associated metabolic syndrome, and also in the potential for reduction in the risk for type 2 diabetes.  

However, it is important to understand that whatever ones diet, the consumption of various plant species and parts of plants and the consumption of parts of animals means that one is consuming not just protein but also carbohydrates (sugars) and lipids (fats). In addition, different plant sources (e.g. cereals, legumes, oils seeds) and different animal sources (e.g. chicken, pork, beef) have different percentages of protein, carbohydrate, and lipid as well as types of carbohydrates, lipids, and proteins.

Nonetheless, when considering plant versus animal protein, is it just differences in the composition of the amino acids of these two sources of protein, or is it the total amount of protein on an isocaloric basis?

A recent paper in the prestigious journal, Cell Metabolism, examines this question by looking at health benefits, in laboratory mice, of animal (omnivorous) versus plant protein as well as addressing the question of composition or amount:

Total protein, not amino acid composition, differs in plant-based versus omnivorous dietary patterns and determines metabolic health effects in mice

The study took into consideration the contributions of total protein amount and amino acid composition in comparison of plant-based versus animal-based protein. Although the analysis of total protein and amino acid composition revealed differences between individual plant and animal diet items there were few differences between the amino acid profiles of

plant versus animal (omnivorous) diets.

Effects on markers of cardiac and metabolic health were found to be associated with protein quantity, but not quality. The results showed relatively little difference in protein quality between plant-based and animal-based diets. The major difference is that there is reduced total protein intake in typical plant-based diets, when compared to omnivorous diets, and this may be the significant contributor to the benefits of plant-based diets.

Animal-based food items contain more total protein by weight as well as by percentage of energy composition and more amino acids by weight. When plant-based diet items were compared to animal-based diet items were compared by normalizing each of the 20 amino acids found in proteins to total protein this study found that isoleucine, lysine, methionine, alanine, glycine, and tyrosine were proportionally higher in the animal protein whereas, aspartic acid and glutamic acid were proportionally higher in plant protein.

When the investigators compared non-essential versus essential amino acids they found that plant-based proteins had significantly less of both groups on a weight basis, but on a ratio basis, plant-based diet items contained proportionally more non-essential amino acids and less essential amino acids when compared to animal protein. However, it is important to note that neither essential nor non-essential amino acid levels were statistically different plant-based and animal-based food items.

This manuscript is dense with experimental data and a heavy read if the subject material is not one's area. Nonetheless, the principal outcome from this study shows that there is a much larger effect of protein quantity over protein quality, specifically at low levels of total protein intake, on both metabolic and cardiovascular health

I would not go so far as to say that if one were to consume an animal-based diet that was equal in calorie content and total protein content to a plant-based diet that there would be cardiometabolic benefits. For one thing plant-based diets are rich in antioxidant phytochemicals (of the polyphenol class) that are known to exert significant health benefits. In addition, animal-based diets, of equal calorie content, are much higher in lipids than plant-based diets, and many of these lipids contribute to cardiovascular disease.