PROTEIN ACROSS THE LIFESPAN: WHAT CLINICIANS NEED TO KNOW

By David Church, PhD

Protein is essential for the maintenance, repair, and growth of skeletal muscle, yet questions remain about optimal intake and whether the current Recommended Dietary Allowance (RDA) adequately serves all populations. This article examines protein needs across five domains: aging, exercise, the RDA framework, protein distribution, and the role of protein source in supporting muscle health.

Protein Needs in Older Adults

Aging is accompanied by a progressive loss of skeletal muscle mass and strength. Multiple lines of evidence converge on the conclusion that protein requirements for adults over 65 exceed the general RDA of 0.8 g/kg/day.¹ Research suggests higher protein intakes may better support muscle health and function across populations, and are associated with better lean mass, strength, and reduced frailty risk.² Acute tracer studies have demonstrated that older adults exhibit anabolic resistance.³ This refers to a blunted muscle protein synthetic response to protein ingestion. To overcome this, Moore and colleagues indicate older men require approximately 0.40 g/kg per meal to maximally stimulate myofibrillar protein synthesis versus ~0.24 g/kg in younger adults.⁴

However, the framing of anabolic resistance as an inevitable consequence of chronological aging deserves scrutiny. A growing body of evidence suggests that physical inactivity, not aging per se, is the primary driver of anabolic resistance. For example, reducing daily step count for just one week in younger men blunts muscle protein synthesis by approximately 27% with concomitant increase in markers of muscle protein breakdown.⁵ Thus, the phenotype of inactivity-induced anabolic resistance in young people is similar to that associated with aging.

Further, healthy older adults who underwent two weeks of step reduction displayed reduced muscle protein synthesis and insulin sensitivity with increases in insulin resistance and inflammatory markers.⁶ Conversely, when older men performed resistance exercise prior to protein ingestion, the anabolic response approached levels seen in younger men.⁷ As Paulussen et al. have argued, anabolic resistance exists on a dimmer switch wherein the response is modulated up or down by habitual physical activity, body composition, and inflammatory status.⁸

Reframing the role of anabolic resistance as something that can be corrected, rather than a consequence of age, has pragmatic implications. Maintaining or increasing physical activity while increasing protein intake is crucial for preserving muscle health in older adults; a notion supported by randomized controlled trials. Increased protein intake can produce beneficial effects on muscle strength and physical performance in older adults^.9,10 However, these effects are enhanced with resistance exercise. Both the PROT-AGE Study Group and ESPEN recommend 1.0–1.2g/kg/day for healthy older adults, with exercise emphasized as a critical complement.^1,9 In short, that postprandial walk is a good idea.

Protein for Active Individuals

For individuals engaged in regular resistance training, the evidence for protein intakes above the RDA to support training adaptations is robust. Observational data confirm that habitual intakes of 1.2–2.0 g/kg/day are common,and acute metabolic studies demonstrate a dose-response relationship between post-exercise protein ingestion and muscle protein synthesis.^4,11 Previous work confirms that resistance exercise itself sensitizes muscle to dietary amino acids for up to 48 hours after the exercise bout.¹² While resistance exercise is often associated with increased protein intake, it is important to note that those engaged in forms of aerobic/endurance exercise often have similar protein requirements.^13,14

The strongest evidence comes from a meta-analysis which pooled 49 studies encompassing 1,863 participants and identified 1.6 g/kg/day as the intake beyond which additional protein conferred no further benefit for lean mass gains.¹¹ These results were reaffirmed by Nunes and colleagues.¹⁵ It’s also worth noting that the 1.6 g/kg/day ceiling should not be treated as a hard rule. It represents the point of diminishing returns for one outcome. In this case, fat-free mass increases during resistance training in mostly young, healthy populations. For individuals in energy deficit, requirements may be considerably higher. In that regard, previous evidence both acutely and chronically supports the notion that lean body mass and therefore body composition improvements are enhanced when protein intake is 2.4 g/kg/day during a 40% energy restriction in conjunction with resistance training.¹⁶

Is the Protein RDA Too Low?

The general RDA for protein (0.8 g/kg/day) is derived from nitrogen balance studies.¹⁷ This methodology has well-documented technical limitations: it tends to overestimate nitrogen intake and underestimate losses. This results in biased findings for lower intakes.^17,18

Deeper conceptual limitations also exist. The Adaptive Metabolic Demand model shows the body adjusts amino acid oxidation rates to match habitual protein intake, and this adaptation occurs slowly over days to weeks.¹⁹ When subjects are fed less protein, they gradually reduce amino acid oxidation, become more“efficient,” and eventually achieve nitrogen equilibrium at that lower intake. This adapted equilibrium point can be confused for a “requirement.” But nitrogen balance achieved through metabolic accommodation is not the same as the intake needed for optimal function or health. Rather, it tells us the body can survive on less by downregulating its own protein turnover. In that regard, the wide range of reported minimum requirements (0.39–1.09 g/kg/day) is best explained by this adaptive phenomenon.¹⁹

Interestingly, positive nitrogen balance observed at higher intakes have historically been dismissed as technical artifacts.¹⁷ This dismissal assumes adults cannot accrue body protein – an assumption that clearly does not hold for individuals engaged in exercise, going through pregnancy, or recovering from catabolic episodes.The RDA thus defines a minimum to prevent a negative nitrogen balance, not an intake to optimize a functional or disease outcome.^18,20

Alternative methodologies have challenged the RDA. The indicator amino acid oxidation (IAAO) technique has consistently estimated requirements 30–50% higher than nitrogen balance-derived values.²¹ Humayun et al. reported a population-safe intake of 1.2 g/kg/day in healthy young men: 50% above the nitrogen balance-based RDA.22 The IAAO methodology uses a breakpoint to discern the requirements, whereas the RDA was derived from linear regression. When using this technique on the same nitrogen balance data used to derive 0.8 g/kg/d value, requirements increase by 25%.²¹ Subsequent IAAO studies across several populations have also suggested higher requirements.²³

Two Agency for Healthcare Research and Quality (AHRQ) systematic reviews highlight major evidence gaps in protein research.^24,25 Of 11,408 studies screened in each review, only 68 met inclusion criteria, with 45 being low to moderate risk of bias. Of these, two studies were available for adults 19-50 and four for 51->70. The second review focused on protein and risk of bone disease, kidney disease, and sarcopenia. It screened 11,015 studies; only five for bone disease, one for kidney disease, and nine for sarcopenia were included. Both concluded “insufficient and inconclusive" evidence.^24,25 These findings expose a structural problem in protein nutrition research. The field has been dominated by epidemiology, which is hypothesis-generating research but limited in establishing causal relationships. There is a lack of well-powered, longer-duration randomized trials with clinically meaningful endpoints.Emerging methods, such as deuterated water to track biomarkers over time, represent an ideal way to advance research under this framework.

Protein Distribution

In many Western dietary patterns, protein intake is skewed toward the evening, with minimal intake at breakfast.²⁶ Classically, muscle protein synthesis was thought to exhibit a saturable dose-response (“muscle full”effect), becoming refractory after ~two to three hours, regardless of dose. This led to the hypothesis that evenly distributing protein across meals optimizes synthesis.²⁷

Early support came from Mamerow et al., who found ~25% higher 24-hour muscle protein synthesis with evenly distributed protein (~30 g/meal) versus a skewed pattern (~10/16/63 g) at the same total intake.²⁸ Interestingly, all follow-up studies have found no difference in the pattern of intake between even and skewed distribution.^26,29-32

Recent evidence challenges the idea of a fixed “per-meal ceiling.” Post-exercise, larger doses (e.g., 100 g vs 25 g of milk protein concentrate) can sustain greater muscle protein synthesis over time, suggesting no practical upper limit for the acute stimulation.³³ While this does not mean we should advise 100 g/meal, these findings indicate that lower-protein meals can be offset later in the day. Overall, total daily protein intake remains the primary priority for practitioners.

Protein and Muscle Health

Animal proteins have been long considered superior for muscle health due to higher digestibility and more complete essential amino acid profiles.³⁴ Acute metabolic studies support this, showing greater postprandial muscle protein synthesis rates following an omnivorous versus a vegan meal in older adults.^35,36

Hevia-Larraín et al. compared vegan and omnivorous men consuming 1.6 g/kg/day (soy vs whey) during 12 weeks of supervised resistance training and found no significant differences in outcomes, although strength gains were approximately 20 kg higher in the omnivorous group. It is important to note that a recent meta-analysis indicates exercise trials display larger effect size for animal versus plant as opposed to non-exercise trials.^37,38 The reasons for this are not clear but could simply be that there is a greater demand for essential amino acids when exercising. If true, increasing the amount of plant protein ingested to make up the 15 grams of essential amino acids is feasible.

Meta-analyses generally show no statistically significant differences between protein sources for lean mass or strength.^39,40 However, a recent meta-analysis performed by Reid-McCann and colleagues displays more nuanced data.³⁸ In a subgroup analysis of whole-diet interventions and non-soy plant protein supplements, animal proteins produced small but significant advantages in muscle mass outcomes.

However, in a subgroup analysis focused on milk versus soy proteins, there were no significant differences in indices of muscle mass.³⁷ This is in line with the meta-analysis by Messina et al. that found no difference between soy and animal protein supplements for hypertrophy and strength gains. These pooled analyses are encouraging but most relied on isolated supplements under expert supervision. In the real world, individuals who adopt plant-based diets tend to consume less total protein than omnivorous counterparts.⁴⁰ This reduction is not inevitable; many intervention studies demonstrate adequate protein intakes can be achieved with plant-based diets. Emphasizing total protein, EAA content, and complementary sources is essential. Notably, factors beyond source, such as food form and fat content, can also influence anabolic response.^41-43 Regardless, the simple fact that soy is consistently used as the comparator in “animal versus plant” studies is indicative of its gold-standard status as the plant protein of choice.

Soy is a high-quality, well-studied plant protein and consistent evidence supporting cardiovascular benefits.^44,45 This, along with its ability to stimulate positive effects on protein status, positions soy as one of the few protein sources that can reliably support multiple body functions in individuals who want to increase their plant protein intake. Concerns about hormonal effects have been refuted, with meta-analyses showing no impact on testosterone or estrogen levels in men.^46,47 For practitioners seeking a high-quality plant protein, soy remains the strongest option.

Conclusion

Protein needs likely exceed the RDA for older adults and physically active individuals, with recommendations generally falling between 1.0–1.6 g/kg/day depending on the population. Protein distribution and source can play a role; total daily intake remains the most important factor. Ultimately, focusing on protein in isolation, with particular emphasis on structured exercise and physical activity, risks overlooking the broader lifestyle that supports overall health.

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