What protein is, and what it does in the body

1. 01

   Built from amino acids, nine of them essential

   Protein is a chain of amino acids, of which there are 20. Nine are essential: the body cannot make them, so they must come from food (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine). The body breaks dietary protein down to these building blocks and reassembles them into whatever it needs. This is why amino acid profile, not just grams, determines a protein's value.

2. 02

   Builds and repairs muscle and tissue

   The role most people associate with protein is structural: it builds and repairs skeletal muscle, and it forms collagen in skin, tendon, and bone. After resistance training, muscle protein synthesis rises and stays elevated for ~24-48 hours, and dietary protein supplies the amino acids that turn that signal into actual tissue (Phillips & Van Loon 2011). Without adequate intake, training breaks tissue down faster than it rebuilds.

3. 03

   Runs metabolism as enzymes and hormones

   Beyond muscle, nearly every enzyme is a protein, and enzymes catalyze essentially all metabolic reactions. Many hormones (insulin, growth hormone) and all antibodies are proteins, and proteins transport molecules, hemoglobin carries oxygen, albumin carries fats and drugs. This is why protein is non-optional: a chronic shortfall does not just cost muscle, it degrades the body's functional machinery.

4. 04

   Leucine triggers muscle protein synthesis

   Not all amino acids are equal for building muscle. Leucine acts as the molecular trigger for muscle protein synthesis, and a per-meal dose needs to clear ~2.5-3 g of leucine (roughly 0.4 g/kg of protein) to maximally switch it on (Moore et al. 2015). This is why protein quality is measured partly by leucine density, and why our protein powder ranking prices products on $/gram of leucine rather than $/gram of total protein.

5. 05

   Completeness separates the sources

   A 'complete' protein supplies all nine essential amino acids in usable proportions. Animal sources (meat, eggs, dairy, fish) and soy are complete; most single plant sources are limited in one amino acid (grains in lysine, legumes in methionine). The modern standard for ranking this is DIAAS, which scores digestibility and amino acid adequacy. Plant eaters solve it by combining complementary sources, beans plus rice being the classic pair.

6. 06

   How much you need, and how it is satiating

   The consensus target for people who resistance train 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. Protein is also the most satiating macronutrient per calorie, which is why higher-protein diets help with appetite control on a cut. Our how-much-protein-per-day breakdown and the Macro Calculator turn the per-kg target into your specific daily grams.

What people ask us next.

Q01

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.

Q02

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.

Q03

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.

Q04

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.

Q05

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.

Q06

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.