The whey protein guide covers a product. This one covers the question underneath it: how much protein a day, from any source, does what. The reference intake most countries publish is set to prevent deficiency, and the argument in sports and aging nutrition is whether more than that improves muscle, function or lifespan.

The trial evidence and the cohort evidence point in slightly different directions, which is not a contradiction once the outcomes are separated. More protein with training builds a little more muscle. Where the protein comes from may matter for how long you live.

Dietary protein supplies amino acids, and leucine in particular signals muscle to synthesize protein, an effect amplified by resistance exercise. Muscle protein synthesis responds to a meal up to a ceiling, and across a day the total intake sets how much building material is available. Older muscle responds less to the same dose, which is the basis for the suggestion that older adults need more per kilogram than younger ones.

For mortality, the proposed mechanisms run through what comes with the protein: saturated fat and heme iron with meat, fiber and polyphenols with legumes and nuts. Protein as a nutrient may matter less than the food it arrives in.

Muscle and strength: more helps, with training, up to a point. A 2022 meta-analysis of 74 randomized trials in healthy adults found increasing daily protein produced small additional gains in lean mass when combined with resistance training, with moderate-level evidence, and no clear effect without it. The lean mass effect was significant in people 65 and over eating 1.2 to 1.59 g per kg a day, and in younger people eating 1.6 g per kg or more. Lower-body strength gains were slightly larger at 1.6 g per kg or more; handgrip and physical function barely moved.

Older adults. A 2022 meta-analysis of 22 cross-sectional studies in 11,332 older people found intake above the reference amount associated with better physical performance, walking speed, strength and balance. Nine longitudinal studies in 12,424 people found the same higher intake did not slow decline in grip strength or walking speed over time. A 2019 meta-analysis of 14 trials of dairy protein in 1,424 adults aged 61 to 81 found a small gain in appendicular muscle mass and no effect on handgrip or leg press.

Mortality. In the Rotterdam cohort of 7,786 people and a meta-analysis of eleven cohorts with 64,306 deaths, higher total protein intake was associated with slightly higher all-cause mortality, driven by animal protein and cardiovascular death, while higher plant protein from legumes, nuts, vegetables and fruit was associated with lower all-cause and cardiovascular mortality.

In people with normal kidney function, higher protein intake has not been shown to cause kidney damage, and the trials of high intake in healthy adults report no harm. In established chronic kidney disease, protein restriction is a standard part of management, so intake is a clinical decision. The cohort signal on animal protein and cardiovascular mortality is an association that may reflect the rest of a meat-heavy diet; it is a reason to think about sources rather than a demonstrated harm of protein itself.

Not a product. Nutrition authorities publish a reference intake for protein, set to prevent deficiency rather than optimize muscle, and this site links to those. Food labels carry protein content under food law. Claims about muscle on protein foods are regulated as food health claims, with specific permitted wording in the European Union.