What is Protein?

What is Protein?

Protein is one of the three macronutrients, made up of chains of amino acids that the body breaks down and reassembles to build and repair tissue — most importantly for lifters, skeletal muscle. Of the 20 amino acids used to build human protein, 9 are "essential," meaning the body can't produce them and must get them from food. Protein sources are described as "complete" when they contain all 9 essential amino acids in sufficient amounts (meat, eggs, dairy, soy) and "incomplete" when they are low in one or more (most single plant sources, though combining several plant proteins across a day covers the gap).

How it works

After you eat protein, digestion breaks it down into individual amino acids, which are absorbed into the bloodstream and used to build new proteins throughout the body — muscle fibers, enzymes, hormones, and immune cells among them. Resistance training damages muscle fibers at a microscopic level and signals the body to repair them larger and stronger than before, a process called muscle protein synthesis, which requires an adequate supply of amino acids (especially the essential amino acid leucine) to occur. This is why protein intake and resistance training work together: training creates the stimulus for muscle growth, and dietary protein supplies the material the body needs to act on it.

Types

Worked example

Say you weigh 80 kg and train with weights 4 days a week. Here's how a typical daily protein target compares to the baseline RDA.

Most trained lifters land in the 1.6-2.2 g/kg range — well above the sedentary RDA, which is designed only to prevent deficiency, not to optimize muscle growth.

Protein vs carbohydrates as a macro

By goal

• Building muscle: Aim for 1.6-2.2 g/kg bodyweight per day, spread across 3-5 meals to keep muscle protein synthesis elevated.
• Cutting/fat loss: Push toward the higher end of that range (2.0-2.4 g/kg) since protein needs rise in a calorie deficit to protect muscle.

Common misconceptions

• "Your body can only absorb 20-30 g of protein per meal." The body can digest and use larger amounts of protein per meal; the "20-30 g limit" refers to the amount that maximally stimulates muscle protein synthesis in a single sitting, not an absorption ceiling.
• "More protein is always better for building muscle." Benefits plateau once you're consistently hitting roughly 1.6-2.2 g/kg bodyweight per day; extra protein beyond that mostly just gets used for energy.
• "Plant-based diets can't provide enough protein." Plant sources can meet all protein needs, including all essential amino acids, when a variety of sources (legumes, grains, soy) are combined across the day.

Related terms

Protein FAQ

How much protein do I need per day?

Sedentary adults need at least 0.8 g/kg bodyweight per day (the RDA), but people who strength train benefit from roughly 1.6-2.2 g/kg per day to maximize muscle growth and repair.

What are complete vs incomplete proteins?

Complete proteins contain all 9 essential amino acids in adequate amounts (meat, eggs, dairy, soy), while incomplete proteins are low in one or more (most single plant foods) — combining plant sources across the day covers the gap.

Is protein powder necessary to build muscle?

No — protein powder is a convenient way to hit a daily protein target but isn't required if you can get enough protein from whole foods like meat, eggs, dairy, or legumes.

How many calories are in a gram of protein?

Protein provides 4 calories per gram, the same as carbohydrates and less than half of fat's 9 calories per gram.

Can you eat too much protein?

For most healthy people, higher protein intakes (even above 2.2 g/kg) are safe, though intake beyond what your body can use for repair and growth is simply used for energy rather than building extra muscle.

References

1. Protein Harvard T.H. Chan School of Public Health, The Nutrition Source
2. Dietary Protein and Muscle Mass: Translating Science to Application and Health Benefit Wu G, et al. / PMC, 2019
3. Protein and muscle health during aging PMC, NIH
4. International Society of Sports Nutrition Position Stand: protein and exercise Jäger R, et al. J Int Soc Sports Nutr, 2017