How much protein you need, by body weight and age

The 0.8 g/kg in the guidelines is the intake that keeps a healthy adult out of deficit, not an optimum.¹ Measured with a newer tracer method, the average requirement for adults who do not train comes out 36% higher.² Above that floor, extra protein pays mainly alongside training or weight loss.

The floor was drawn with nitrogen

Every official protein figure traces back to nitrogen balance. Feed people known amounts of protein, collect what leaves the body, and find the intake at which losses stop exceeding intake. The largest reanalysis of that literature pooled 31 studies reporting nitrogen intake and excretion in healthy adults, keeping only those with at least three intake levels per person, for individual-level data on 395 people.³ The pooled mean nitrogen requirement came out at 104.2 mg of nitrogen per kilogram per day.³

That is an average requirement, not a recommendation. An allowance is deliberately set above the average so that it covers nearly everyone, which is why the number printed on labels sits above the figures the balance studies produce. The gap is a safety margin, and it is the reason the label number reads like a target when it is closer to a minimum.

The reanalysis also found something most readers will not expect. Nitrogen requirement did not differ significantly by sex, by age group above and below 60 years, by climate, or by whether the protein was animal, plant or mixed.³ Heterogeneity across the pooled studies ran above 90%, which the authors treat as a real limit on how firmly anything can be read off the dataset.³

A second method moves the answer up

Nitrogen balance is not the only way to ask. Indicator amino acid oxidation tracks how much of a labeled amino acid the body burns rather than builds into protein, and reads the requirement off the intake at which oxidation stops falling. An umbrella review put the two methods side by side: 43 nitrogen balance articles covering 777 participants against 17 oxidation articles covering 186.²

In people who were not athletes, the oxidation method gave requirements 36% higher, 0.88 g/kg/day against 0.64.² In athletes the gap was 27%, 1.61 g/kg/day against 1.27.² Oxidation values stayed higher across every age and sex subgroup examined, and overall the newer method landed about 30% above the older one.²

Which method is closer to the truth is unsettled, and the review does not settle it. What it establishes is that the requirement everyone treats as a fixed property of the human body shifts by roughly a third depending on how it is measured.

What eating more than the requirement buys, and when

The requirement is one question. What happens above it is another. The largest meta-analysis on that point pooled 74 randomized trials in healthy, non-obese adults who raised their daily protein intake.⁴ Where participants were also doing resistance exercise, more protein produced a small gain in lean body mass, a standardized mean difference of 0.22, rated moderate-quality evidence across 62 studies.⁴ Lower-body strength gains were slightly larger at intakes at or above 1.6 g/kg/day, a standardized mean difference of 0.40 from 19 studies, rated low-quality.⁴ Bench press strength moved by 0.18 in adults under 65, across 32 studies.⁴ Handgrip strength was unclear and physical function tests only marginal.⁴

Those thresholds are where by-body-weight advice comes from. The lean mass effect was significant in people aged 65 and over ingesting 1.2 to 1.59 g/kg/day, and in people under 65 ingesting 1.6 g/kg/day or more while doing resistance exercise.⁴

A second meta-analysis, 65 trials and 2,907 participants, put the same idea in kilograms.⁵ Protein supplementation improved lean body mass by 0.62 kg in adults and 0.46 kg in older adults.⁵ It did not improve handgrip strength, 0.26 kg with an interval from -0.51 to 1.04, or leg press strength, 5.80 kg from -0.33 to 11.93.⁵ Removing the trials without exercise training changed none of it, and subgroup analyses by when the protein was eaten found no advantage for any particular timing.⁵

The condition attached to all of it shows up most sharply in a meta-analysis that compared intakes above the allowance against intakes at it, in 18 randomized parallel trials of at least six weeks, giving 22 comparisons.¹ Pooled across all of them, eating more than the allowance was worth 0.32 kg of lean mass.¹

The subgroups are the point. Under energy restriction, extra protein preserved 0.36 kg across 14 comparisons.¹ With resistance training it added 0.77 kg, from three.¹ Under neither stressor it was worth 0.08 kg, with an interval from -0.59 to 0.75, which is to say nothing.¹ The authors conclude that the allowance is adequate to support lean mass in adults who are neither dieting nor training.¹

Age changes the response, not the requirement

The nitrogen data found no requirement difference between adults under 60 and adults 60 and over.³ What changes with age is not the intake needed to stay out of deficit but what protein can do once muscle is already going.

A meta-analysis of 28 trials in adults with a mean age of 50 or older tested that directly.⁶ Protein on its own moved nothing: total lean body mass 0.34 kg, appendicular lean mass 0.4 kg, handgrip 0.69 kg, knee extension 1.88 kg, every interval crossing zero.⁶ In the trials that added resistance exercise, appendicular lean mass rose by 0.54 kg and handgrip strength by 1.71 kg.⁶ The lean mass gain showed up only in the sarcopenic and frail participants, where it reached 0.88 kg.⁶ Meta-regression found no association between age, per-meal protein dose, trial duration or baseline protein intake and the change in any outcome.⁶

Among people who already meet a clinical definition of sarcopenia, the picture is more favorable. Ten trials with 1,154 participants gave whey protein with or without resistance training.⁷ Whey alone raised the appendicular skeletal muscle mass index by a standardized 0.47, appendicular muscle mass by 0.28 and gait speed by 1.13.⁷ Adding resistance training brought handgrip strength with it, at 0.67.⁷

Reading the numbers as thresholds

The by-body-weight figures are thresholds for a particular effect under particular conditions, not requirements. The allowance covers a sedentary adult who is not losing weight.¹ The 1.6 g/kg/day figure marks where the strength effect appeared in adults under 65 who lift, and the 1.2 to 1.59 g/kg/day band marks the same territory for adults 65 and over.⁴ For an older adult not training at all, the supplement did nothing measurable on its own.⁶

The size of the win is worth holding onto as well. Half a kilogram of lean mass over a few months is the typical result, which is a real effect and a small one.⁵

What remains unknown

The nitrogen balance dataset cannot be refreshed. The reanalysis notes that ethical and practical constraints have made new studies increasingly difficult to conduct, so the foundation under every official figure is a fixed and highly heterogeneous body of older work.³ The roughly 30% gap between measurement methods is unresolved, and no trial has been run to close it.²

The subgroup that produced the largest effect, resistance training, rested on three comparisons.¹ Most of the older-adult trials carried some risk of bias, 22 of the 28 in one review, and its meta-regression could not detect a dose-response at all: the per-kilogram numbers are thresholds observed across studies, not doses tested head to head.⁶ Nobody has followed adults for years on a weight-scaled prescription against a flat one and measured whether they can still rise from a chair unaided.