Good and bad protein habits

A study recently published in the International Journal of Epidemiology by researchers at Loma Linda University in California, and the Université Paris-Saclay, in Paris, reports noteworthy health benefit from plant protein, and significant harms from animal protein.

The study examined protein intake patterns among over 80,000 people in the Adventist Health Study-2, a large observational cohort study in which people are observed and assessed over time, but not randomized to any given intervention. The highest quintile of meat protein intake, as compared to the lowest, was associated with a 60 percent relative increase in cardiovascular risk, while the highest compared to lowest intake of protein from nuts and seeds was associated with a 40 percent reduction.

The first consideration is that other dietary factors might account for some, or even all, of the effects attributed here to protein per se. There was careful adjustment for such factors in the analysis, but such adjustment can never be complete.

There were obviously systematic diet pattern differences between those eating a lot of meat and those eating little meat but lots of nuts and seeds, and possibly other important lifestyle differences. Some or all of these, especially those not included in the analysis, might account for some or all of the effect attributed to protein.

The long prevailing position has been that animal sources of protein are complete, and plant sources are not — with complete referring to the complement of essential amino acids. This position is, in fact, false. All plant foods contain all essential amino acids, albeit some at low concentrations. An exclusively plant-based diet provides all essential proteins for human health, and the practice of food combining is not required to meet the relevant threshold quantities of each.

Some have argued that an important difference is more sulfur-containing amino acids in animal foods, leading to the production of acidic metabolites that require buffering. The complete amino acid profile of animal foods, and the relatively high concentration of those amino acids, may also foster brisk anabolic responses (i.e., rapid tissue growth) which may be detrimental — other than for body builders.

Other potential explanations for differential health effects of plant and animal protein include different amino acid ratios, and differential effects on the microbiome — notably the production of TMAO. TMAO is a metabolite of several nutrients, notably choline, carnitine, and lecithin, that accompany protein in animal foods, and is strongly linked to cardiovascular risk. There are, however, compounds in plant foods that can also induce the production of TMAO, and the topic is complicated.

An extensive recent review of protein sources rightly notes the near impossibility of isolating the effects of animal protein, per se, from differences in overall dietary pattern. The same paper concludes that few of

the proposed mechanisms of harm from animal protein are well substantiated, and some are not even entirely plausible. Far more likely, these authors suggest, is that differences in protein are one among many factors, and of uncertain independent significance.

Of clearer importance is differences in overall nutrient patterns from given sources, and the differences in overall dietary patterns and lifestyle between those with significant meat intake versus those with plant predominant diets.

Studies have shown repeatedly that a predominance of plant-food protein sources in the diet is associated with reductions in chronic disease and premature death as compared to a predominance of animal food sources. Environmental

arguments for a shift to plant protein sources — notably beans and legumes — may be even stronger. Arguments about meat intake extend well beyond considerations of protein content or even just health effects.

We will almost certainly never indict the “right” nutrient or canonize the right one, because it is the pattern of our diets and the foods we eat that most meaningfully feeds our medical destinies. If we direct our focus there, at last, we can reliably get all of the nutrients right. But we may have to renounce the satisfaction that comes with having a perpetrator to blame.