Amino acids: what actually happens to protein after you eat it

Some physiology, briefly, because it explains several of the practical points elsewhere on this site.

Digestion

Protein is broken down mechanically and chemically. Stomach acid denatures it and pepsin begins cleaving it. In the small intestine, pancreatic enzymes continue the process, breaking proteins into short chains and individual amino acids.

These are absorbed across the intestinal wall by transporters and enter the bloodstream.

What arrives in your blood is amino acids, not the protein you ate. The body does not distinguish between an amino acid that came from a lentil and one that came from a steak.

This is the fact that dissolves most of the protein quality argument at the margin.

The amino acid pool

Absorbed amino acids join a circulating pool, replenished from two sources: dietary protein, and the continuous breakdown of the body's own proteins.

Body protein turnover is substantial. A considerable quantity of protein is broken down and resynthesised daily, and the amino acids released are largely recycled.

Which means the pool does not empty between meals. Amino acids from breakfast are still available at lunch.

This is why complementing proteins at each meal is unnecessary, and why the retracted advice was wrong.

What limits synthesis

Protein synthesis requires all necessary amino acids simultaneously. If one is absent, synthesis of that protein stops.

Which is the basis of the limiting amino acid concept — the essential amino acid present in the lowest amount relative to requirements determines how much of the dietary protein can be used for synthesis.

Over a day, drawing from the pool, a varied intake covers this. Over a single meal in isolation, it might not.

Since the pool persists, the day is the relevant unit.

What happens to the excess

Amino acids cannot be stored in the way fat and glycogen can.

Excess beyond what is needed for synthesis is deaminated — the nitrogen removed, converted to urea and excreted by the kidneys — and the carbon skeleton is used for energy or converted to fat.

Which means very high protein intakes beyond requirements are not useful, and the excess is oxidised.

It also explains the urea excretion that underlies concerns about protein and kidney function. In people with healthy kidneys, high protein intakes have not been shown to cause damage. In people with existing kidney disease, protein restriction is sometimes advised and should be managed medically.

The synthesis response

Eating protein stimulates muscle protein synthesis, which rises, peaks and returns to baseline over a period of hours.

Two things determine the size of the response: total amino acid availability, and leucine, which appears to act as a trigger.

Above a certain dose, the synthesis response does not increase further — the response saturates. This is the basis for recommendations about per-meal doses and for spreading intake across the day rather than concentrating it.

The saturating dose is higher in older adults, which is anabolic resistance.

Plant proteins generally contain less leucine per gram, which means a slightly larger dose achieves the same trigger. This is the whole practical implication of the leucine point.

Digestibility, precisely

Not all protein eaten is absorbed.

Plant proteins are somewhat less digestible than animal proteins, principally because fibre and antinutrients interfere with enzyme access and because some protein is bound within cell walls.

Processing changes this substantially. Cooking, soaking, sprouting, fermenting, milling and isolating all improve digestibility, in some cases considerably.

Which means digestibility figures quoted for raw plant foods overstate the problem for foods as actually eaten.

What follows practically

Total daily intake dominates. Everything else is at the margin.

Variety across the day handles amino acid profile. Not variety within a meal.

Distribution helps modestly. Three to five feedings with a meaningful dose each, rather than one large one.

Preparation matters. Cook your legumes properly, use fermented and sprouted forms where convenient.

A modest upward adjustment to total intake accounts for digestibility and profile.

Adequate energy matters, because protein eaten in an energy deficit is partly used for fuel rather than for synthesis.

That last point is worth emphasising. Someone eating adequate protein but insufficient energy is not getting the benefit of the protein.

The thing not to worry about

The idea that plant protein is fundamentally inferior does not survive contact with the physiology.

The amino acids are identical. The differences are in proportion and in absorption efficiency, both of which are addressed by eating varied, well-prepared food in adequate quantity.

Trials comparing outcomes when total protein is matched find comparable results, which is what the mechanism predicts.

General information rather than individual advice. Anyone with kidney disease should discuss protein intake with their doctor or a renal dietitian.