Why protein needs rise as you age

Old muscle still works — it just stops answering small doses#

After about 65 the useful daily protein range moves up to 1.0–1.2 g per kg of body weight, higher if you exercise or are managing a chronic illness. But the daily number is the less interesting half of the answer. The change that actually happens with age is per meal: the dose of protein required to switch muscle protein synthesis on rises from roughly 0.24 g/kg in young men to roughly 0.40 g/kg in older men, and a serving below that threshold can produce no measurable response at all in someone who would have responded perfectly well at 25.

That phenomenon has a name — anabolic resistance — and it is unusually well pinned down for something in nutrition. It is also, on the best-controlled long trial we have, not something you can fix with protein alone. Both of those things are true and the tension between them is the useful part of this article. The general target-setting is in how much protein per day; this is about what changes when the person eating it is older.

The experiment that isolated the problem to one amino acid#

The cleanest demonstration is twenty years old and still the one worth knowing, because it changed exactly one variable.

Four groups — two elderly, two young — were studied before and after swallowing 6.7 g of essential amino acids. One version had the amino acid profile of whey protein, which is 26% leucine. The other was enriched to 41% leucine. Nothing else differed. Muscle fractional synthetic rate was measured by tracer infusion with vastus lateralis biopsies¹.

In the young men, both mixtures worked. The 26% version raised synthesis from 0.048 ± 0.005 to 0.063 ± 0.007 %/h; the 41% version from 0.036 ± 0.004 to 0.051 ± 0.007 %/h. Both significant.

In the elderly, the 26% mixture did nothing measurable — 0.044 ± 0.003 %/h at baseline, 0.049 ± 0.006 after, not significant. The same 6.7 g of amino acids, enriched to 41% leucine, produced 0.038 ± 0.007 rising to 0.056 ± 0.008 %/h (P < 0.05). Muscle net protein balance told the same story: it improved in every group except the elderly men given the whey-like blend.

The dose that failed in older muscle was the dose a protein shake actually delivers. Change one amino acid's share and the same gram total starts working again.

That is a mechanism, not a correlation, and it constrains what anabolic resistance can be. Old muscle is not simply worn out — the authors also note the 41% blend produced no extra stimulation in young subjects, meaning the young were already saturated at 26%. The deficit is a shifted threshold, not a broken machine. And basal synthesis rates are the same in old and young in the fasted state; the difference only appears when a meal arrives.

The dose that moves with you: 0.40 g/kg per meal, not 0.24#

Katsanos changed the composition. The other way to clear the threshold is simply to eat more at once, and somebody put a number on that too.

Pooling their dose-response data across ages, researchers found myofibrillar protein synthesis plateaued at 0.40 ± 0.19 g/kg of body mass in older men against 0.24 ± 0.06 g/kg in younger men. Scaled to lean body mass — the fairer comparison, since older bodies carry proportionally more fat — the gap is wider and the statistics firmer: 0.60 ± 0.29 versus 0.25 ± 0.13 g/kg lean mass, P < 0.01. On total body mass the difference sits at P = .055, which is worth stating plainly rather than rounding into significance². The standard deviations are large, so treat 0.40 as the centre of a wide band.

Translate that into food and something neat falls out. The table below is arithmetic on Moore's breakpoint, not a further measurement:

The per-meal threshold multiplied by three meals lands on 1.2 g/kg/day — the top of the headline recommendation for older adults, arrived at from completely independent evidence. That convergence is my arithmetic rather than either paper's claim, but it is a useful way to hold the number: the daily target for older adults is close to what you get if you simply insist every main meal clears the threshold. Which meal that fails is usually breakfast, and the case for restructuring the day rather than the total is in protein distribution across the day.

The recommendation, and who wrote it#

The formal guidance comes from an international study group convened by the European Union Geriatric Medicine Society, which reviewed the evidence and published a position paper. For people over 65 they recommend an average daily intake of at least 1.0 to 1.2 g/kg to maintain and regain lean mass and function; at least 1.2 g/kg for those who exercise; and 1.2 to 1.5 g/kg once acute or chronic illness enters the picture, which at this age it frequently does. The exception is explicit: older people with severe kidney disease (eGFR below 30) who are not on dialysis may need to limit protein instead³.

The paper's stated rationale is the same physiology as above — it names "high splanchnic extraction and declining anabolic responses to ingested protein" as the reasons older adults need more than younger ones. It is a consensus document rather than a trial, which is a real limitation: expert panels are how a field summarizes evidence and also how a field's assumptions get laundered into numbers. This one is candid about its own gaps, noting that protein quality and timing "may be relevant, but evidence is not yet sufficient to support specific recommendations."

The observational layer points the same way. Among 2,066 community-dwelling adults aged 70–79 followed for three years, those in the highest quintile of energy-adjusted protein intake lost about 40% less lean mass than those in the lowest — 0.501 ± 0.106 kg against 0.883 ± 0.104 kg⁵. Hold that one loosely. It is a food-frequency questionnaire in a cohort, and people who eat more protein tend to eat better and move more in ways no adjustment fully removes. It is consistent with the mechanism; it does not test it.

Six months of extra protein, and nothing to show for it#

Here the literature genuinely splits, and it is not a rhetorical concession — it is the best-designed trial in this article disagreeing with the rest of it.

Ninety-two functionally limited men aged 65 or over, all of whom habitually ate at or below the RDA, were randomized to six months of controlled diets — food provided, not advice given — at either 0.8 or 1.3 g/kg/day, crossed with testosterone or placebo. The primary outcome was change in lean body mass. The difference between the protein groups was 0.31 kg (95% CI, −0.46 to 1.08; P = .43). Appendicular lean mass: 0.04 kg (95% CI, −0.48 to 0.55). Muscle strength, power, walking speed, stair-climbing power, quality of life, fatigue and well-being all failed to separate as well⁴.

"The RDA for protein is sufficient to maintain LBM." — Bhasin et al., JAMA Internal Medicine, 2018

That is a direct contradiction of the position paper, from a controlled-feeding RCT, and it should not be waved away. So what separates it from everything above?

Three things, all specific. First, the outcome is not the same outcome: Katsanos and Moore measured the response to a meal, and Bhasin measured lean mass over six months of a daily total. A per-meal threshold effect can be perfectly real and still be invisible in a trial whose lower arm was already eating three meals a day. Second, and most important, nobody lifted anything. There was no resistance-training intervention — and the anabolic-resistance literature is consistent that loading the muscle is what restores its sensitivity to protein. Third, these were men already living with functional limitation, which is a harder population to move than healthy 70-year-olds, and their baseline intake was capped at 0.83 g/kg by design.

One detail cuts the other way and is worth keeping: the higher-protein group did lose more fat mass (−1.12 kg; 95% CI, −2.04 to −0.21; P = .02). Not the primary outcome, and one secondary finding out of many, so it is a hint rather than a result.

The defensible reading is narrower than either side's headline. Protein above the RDA, by itself, in sedentary older men, did not build or protect lean mass over six months. That is not the same claim as "older adults don't need more protein" — and it is a strong argument that the protein advice is worthless without the other half.

What that leaves you doing#

Treat the per-meal amount as the target, not the daily one. Roughly 0.40 g/kg at each main meal — 30 g for a 75 kg person — is the number with a mechanism behind it. Most older adults' shortfall is concentrated at breakfast, where toast and cereal rarely clear 10 g.

Lift something, or the protein is theoretical. This is the lesson of the null trial rather than a general exhortation. Resistance training is what makes muscle answer a protein meal again, and the training itself is covered in calories burned strength training.

Favor sources that are dense in leucine and easy to chew. The threshold argument means quality matters more with age than it does at 25, which inverts the usual advice — the source ranking is a tiebreaker for young lifters and closer to a lever for older adults. The amino acid doing the switching has its own explainer.

Know the one hard exception. Severe kidney disease without dialysis is where this advice reverses, and that is a clinical decision rather than a dietary one.

Worth separating from a related worry: the resting metabolic decline people expect with age is smaller and later than the folklore suggests, and losing muscle is not the same problem as a slowing metabolism. Protein is not defending your calorie burn here. It is defending your ability to get out of a chair.

FAQ#

What is anabolic resistance?#

It is the reduced ability of aging muscle to respond to a normal-sized dose of dietary protein. Fasted, resting synthesis rates are similar in older and younger adults — the difference only shows up after a meal. Given 6.7 g of essential amino acids at 26% leucine, young men raised muscle protein synthesis significantly while elderly men showed no measurable increase; enriching the same dose to 41% leucine restored the response¹.

How much protein does a 70-year-old need per day?#

At least 1.0–1.2 g per kg of body weight — about 70–84 g for a 70 kg person — rising to at least 1.2 g/kg for those exercising and 1.2–1.5 g/kg for most older adults with acute or chronic illness³. Older adults with severe kidney disease who are not on dialysis are the exception and may need less.

Does eating more protein on its own prevent muscle loss with age?#

On the best evidence, no. Six months of controlled feeding at 1.3 versus 0.8 g/kg/day in functionally limited older men produced no difference in lean body mass, strength, power, walking speed or well-being⁴. Crucially, that trial included no resistance training. Protein appears to be the permissive half of the equation; loading the muscle is the half that does the work.

Sources#

1. Katsanos CS, Kobayashi H, Sheffield-Moore M, Aarsland A, Wolfe RR. A high proportion of leucine is required for optimal stimulation of the rate of muscle protein synthesis by essential amino acids in the elderly. Am J Physiol Endocrinol Metab. 2006;291(2):E381-E387.
2. Moore DR, Churchward-Venne TA, Witard O, et al. Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men. J Gerontol A Biol Sci Med Sci. 2015;70(1):57-62.
3. Bauer J, Biolo G, Cederholm T, et al. Evidence-based recommendations for optimal dietary protein intake in older people: a position paper from the PROT-AGE Study Group. J Am Med Dir Assoc. 2013;14(8):542-559.
4. Bhasin S, Apovian CM, Travison TG, et al. Effect of Protein Intake on Lean Body Mass in Functionally Limited Older Men: A Randomized Clinical Trial. JAMA Intern Med. 2018;178(4):530-541.
5. Houston DK, Nicklas BJ, Ding J, et al. Dietary protein intake is associated with lean mass change in older, community-dwelling adults: the Health, Aging, and Body Composition (Health ABC) Study. Am J Clin Nutr. 2008;87(1):150-155.