Are you getting enough protein in your diet? Part 2: Protein and muscle health

Part 1 covered why the standard RDA of 0.8g/kg is a floor, not a target, for most adults. This part gets specific about muscle. When it comes to building or maintaining lean mass, how you eat your protein matters almost as much as how much.

It's not just about the daily total‍ ‍

Hitting your daily protein target with a small breakfast, a light lunch, and a large protein-heavy dinner isn't as effective as it sounds. Muscle protein synthesis (MPS) - the process your body uses to repair and build muscle tissue - responds to protein in a dose-dependent way within each meal, not just as a daily sum.

This is the basis of the leucine threshold: each meal needs to deliver enough of the amino acid leucine (roughly 2.5–3g) to fully switch on MPS. Below that threshold, the response is muted, regardless of how much protein you eat later in the day. In practical terms, that's roughly 25–40g of high-quality protein per meal: a palm-sized portion of meat, fish, or tofu, two to three eggs plus Greek yoghurt, or a well-formulated plant protein blend.

Dose-response research from Moore et al. and Witard et al. established this per-meal effect. Schoenfeld and Aragon's 2018 review used it to recommend spreading protein across three to four meals rather than concentrating it in one or two, so each meal triggers its own MPS response.

Newer trial data (Trommelen et al., 2023) suggests the old "25g per meal and no more" ceiling was too rigid; larger doses still contribute to MPS, just with diminishing efficiency. The practical guidance hasn't changed much: even distribution across the day beats front- or back-loading. ‍

Resistance training changes the equation

Protein alone doesn't build muscle. It provides the raw material; the stimulus for growth comes from mechanical loading. The two work together - resistance exercise sensitises muscle tissue to protein for roughly 24–48 hours afterward, so the same meal produces a larger MPS response post-workout than it would on a rest day.

This is one reason strength-trained individuals are typically advised toward the higher end of the protein range from Part 1, around 1.6–2.2g/kg/day, with particular attention to intake in the hours around training sessions.

Carbohydrates matter here too

Muscle-focused advice can get so protein-centric that carbohydrates get sidelined, but they're doing real work. Carbohydrate is the primary fuel for resistance training, stored in muscle as glycogen. Glycogen availability affects how much volume and intensity you can push in a session, and under-fuelled sessions produce a weaker growth stimulus regardless of protein intake afterward. Carbohydrate also supports recovery post-workout: co-ingesting it with protein helps replenish glycogen stores, and the insulin response it triggers may modestly assist repair, though protein remains the primary driver of MPS itself. If you're training seriously for muscle, adequate carbohydrate around sessions isn't optional extra fuel - it's part of what makes the protein work.

Plant protein can keep up

There's a persistent idea that plant proteins can't adequately support muscle building. Recent head-to-head trials complicate that. A 2024 study by van der Heijden et al. found a well-formulated plant protein blend stimulated post-exercise MPS equivalently to whey in resistance-trained adults, provided the blend was dosed and composed to deliver enough leucine and a complete amino acid profile. The key word is blend - single plant sources often fall short on one or more essential amino acids, but combining sources (legumes with grains, for example) closes the gap.

Digestibility is worth being upfront about too, since it changes how much plant protein you actually need to eat. Whole plant proteins are generally digested and absorbed less completely than animal proteins. Intact animal protein sources sit close to 90–100% digestibility; whole plant sources (legumes, grains, nuts) commonly land in the 70–90% range, partly due to naturally occurring anti-nutrients and fibre that partially block absorption. DIAAS (Digestible Indispensable Amino Acid Score) - a more accurate quality measure than the older PDCAAS - captures this combined effect of amino acid completeness and digestibility, and most individual plant proteins score meaningfully lower than animal proteins or soy isolate. In practice, the grams of plant protein on a nutrition label overstate what your body has available to use. People eating primarily plant-based often need somewhat more total protein - commonly cited estimates range from 10% to 30% more - to land on the same usable amino acid intake as someone eating animal protein. Part 3 covers DIAAS and digestibility in full.

The practical takeaway

• Aim for 25–40g of protein per meal, spread across 3–4 meals rather than 1–2.

• Prioritise protein intake around resistance training sessions.

• Don't under-fuel training sessions with inadequate carbohydrate.

• If eating plant-based, favour blended protein sources over single ingredients, and account for lower digestibility in your totals.

• Total daily intake still matters most - per-meal distribution is a refinement, not a replacement, for hitting your overall target.

Part 3 covers digestibility and bioavailability in more depth - why the protein on the label isn't always the protein your body absorbs.

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References

• Schoenfeld, B.J., Aragon, A.A. (2018). How much protein can the body use in a single meal for muscle-building? Implications for daily protein distribution. Journal of the International Society of Sports Nutrition, 15, 10.

• Moore, D.R. et al. (2009). Ingested protein dose response of muscle and albumin protein synthesis after resistance exercise in young men. American Journal of Clinical Nutrition, 89(1), 161-168.

• Witard, O.C. et al. (2014). Myofibrillar muscle protein synthesis rates subsequent to a meal in response to increasing doses of whey protein at rest and after resistance exercise. American Journal of Clinical Nutrition, 99(1), 86-95.

• Trommelen, J. et al. (2023). Resistance exercise augments postprandial overnight muscle protein synthesis rates. Medicine & Science in Sports & Exercise.

• Haff, G.G. et al. (2003). Carbohydrate supplementation and resistance training. Journal of Strength and Conditioning Research, 17(1), 187-196.

• Alghannam, A.F. et al. (2021). Effect of consuming carbohydrate with and without protein on the rate of muscle glycogen re-synthesis during short-term post-exercise recovery: a systematic review and meta-analysis. Sports Medicine - Open, 7, 9.

• van der Heijden, I., Monteyne, A.J., West, S. et al. (2024). Plant Protein Blend Ingestion Stimulates Postexercise Myofibrillar Protein Synthesis Rates Equivalently to Whey in Resistance-Trained Adults. Medicine and Science in Sports and Exercise, 56(8), 1467-1479.

• Berrazaga, I., Micard, V., Gueugneau, M., Walrand, S. (2019). The Role of the Anabolic Properties of Plant- versus Animal-Based Protein Sources in Supporting Muscle Mass Maintenance: A Critical Review. Nutrients, 11(8), 1825.

• Herreman, L., Nommensen, P., Pennings, B., Laus, M.C. (2020). Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science & Nutrition, 8(10), 5379-5391.