What protein is, and what it does in the body
Protein is chains of amino acids that build muscle, enzymes, and hormones. What it does, why leucine and completeness matter, and the evidence-backed daily target.
What people ask us next.
- What is protein, in simple terms?
- A macronutrient made of amino acid chains. There are 20 amino acids; nine are essential and must come from food. The body breaks dietary protein into these building blocks and reassembles them to build muscle, enzymes, hormones, and antibodies. Protein quality depends on having all nine essential amino acids (completeness) and on leucine content, the trigger for muscle building.
- What does protein do for your body?
- It builds and repairs muscle and connective tissue, forms every enzyme that runs metabolism, makes many hormones and all antibodies, and transports molecules like oxygen. It is also the most satiating macronutrient per calorie. A chronic shortfall costs not just muscle but the body's functional machinery, which is why adequate intake is non-optional.
- How much protein do I need a day?
- For people who resistance train, the consensus is 1.6 g/kg/day, with diminishing returns above ~2.2 g/kg (Morton et al. 2018). For an 80 kg lifter that is ~128 g/day. The 0.8 g/kg RDA is a deficiency floor, not a performance target. Our how-much-protein-per-day page and the Macro Calculator convert your bodyweight and goal into a specific daily number.
- What makes a protein 'complete' or high quality?
- Completeness means it supplies all nine essential amino acids in usable ratios. Animal sources and soy are complete; most single plant sources are limited in one amino acid. Quality is scored by DIAAS (digestibility plus amino acid adequacy) and by leucine density, which drives muscle protein synthesis. Plant eaters reach completeness by combining sources like beans and rice.
- Why does leucine matter so much?
- Leucine is the amino acid that triggers muscle protein synthesis. A meal needs to clear roughly 2.5-3 g of leucine (about 0.4 g/kg of protein) to maximally switch on muscle building (Moore et al. 2015). This is why a source's leucine content, not just its total protein, determines how effective it is for muscle, and why our protein powder ranking prices products on $/gram of leucine.
- Can you eat too much protein?
- For healthy adults, no, within reason. Antonio et al. ran resistance-trained men at up to 4.4 g/kg/day with no renal, hepatic, or lipid harm. The renal caution applies only to pre-existing chronic kidney disease. For healthy adults, ~2.2 g/kg is the practical ceiling on performance returns, not a safety limit; above it you are paying for protein you will not use for muscle.
Every claim, cited.
- [01]Morton RW, Murphy KT, McKellar SR, et al. 2018. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength. Br J Sports Med 52(6):376-384. Consensus 1.6 g/kg/day target.
- [02]Moore DR, Churchward-Venne TA, Witard O, et al. 2015. Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men. J Gerontol A 70(1):57-62. Per-meal leucine threshold.
- [03]Phillips SM, Van Loon LJ. 2011. Dietary protein for athletes: from requirements to optimum adaptation. J Sports Sci 29 Suppl 1:S29-38. Muscle protein synthesis and recovery.
- [04]Jager R, Kerksick CM, Campbell BI, et al. 2017. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr 14:20. Protein quality, timing, and requirements.
- [05]FAO. 2013. Dietary protein quality evaluation in human nutrition (DIAAS). FAO Food and Nutrition Paper 92. Basis of the modern protein-quality score.
The Larderlab Team builds evidence-led frameworks for eating, lifting, and stocking a kitchen. We cite every claim. We publish the spreadsheet when possible. We buy what we review at retail price. When new data lands, we revise with a dated note.
Next in this hub.
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A Google Sheet that calculates your protein target, splits it across 3-5 meals, and ranks 20 protein sources by $/gram. Free. Copy-and-modify your own version.
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