The average human body contains about 11 kilograms (24 pounds) of protein in men and 9 kilograms (20 pounds) in women. That works out to roughly 14% to 16% of total body weight, making protein the second most abundant substance in your body after water.
Where All That Protein Lives
Your body’s protein isn’t sitting in one place. It’s distributed across every tissue, organ, and fluid you have, but it concentrates heavily in two areas: your muscles and your connective tissues.
Skeletal muscle is the single largest protein reservoir. About 40% of your total body protein sits in your muscles, which is why protein intake and muscle mass are so closely linked in nutrition conversations. When your body runs low on amino acids (the building blocks of protein), muscle tissue is the first place it pulls from.
Connective tissue is the other major store. Collagen, the structural protein in your skin, bones, tendons, ligaments, and cartilage, accounts for about 30% of all the protein in your body. That makes collagen the single most abundant protein you carry. After collagen, actin (a protein critical for muscle contraction and cell movement) makes up another 5% to 10% of your total protein content.
The rest is spread across your organs, blood, immune cells, enzymes, and hormones. Your liver, kidneys, and gut lining are especially protein-dense tissues that turn over rapidly, constantly breaking down and rebuilding their protein content.
How Sex and Body Size Affect the Total
The roughly 2-kilogram gap between men and women comes down mostly to muscle mass. Men typically carry more skeletal muscle, which directly translates to more total body protein. A larger person of either sex will have more protein in absolute terms, though the 14% to 16% range holds fairly steady as a percentage of body weight across healthy adults.
Body composition matters more than body weight alone. Two people who weigh the same can have meaningfully different protein totals if one carries more muscle and the other carries more fat. Adipose tissue (body fat) contains very little protein, so a higher body fat percentage dilutes the protein-to-weight ratio.
How Protein Mass Changes With Age
Your body’s protein stores don’t stay constant over a lifetime. Muscle mass peaks somewhere between your 30s and 40s, then begins a slow decline. In women, research on large population samples shows muscle mass peaks between ages 40 and 49, then drops at an average rate of about 5.7% per decade from the 60s onward. In men, the decline tends to begin in the 50s, and some studies suggest men lose muscle at a faster absolute rate than women in later decades.
This age-related muscle loss, known as sarcopenia, doesn’t just mean you lose strength. It means your body literally contains less protein than it used to. That shrinking protein pool has downstream effects on mobility, metabolic health, bone density, and even immune function, since many immune proteins depend on adequate amino acid availability. It’s one reason protein needs per kilogram of body weight actually increase as you get older, not decrease.
How Scientists Measure Body Protein
You can’t directly weigh all the protein in a living person, so researchers use an indirect method: they measure total body nitrogen. Protein is the only major nutrient that contains nitrogen, and protein averages about 16% nitrogen by weight. Multiplying total body nitrogen by 6.25 (the inverse of 0.16) gives a reliable estimate of total protein mass. This conversion factor, established by the Food and Agriculture Organization, has been the standard in nutrition science for decades.
In clinical and research settings, total body nitrogen is measured using a technique called neutron activation analysis, where the body is briefly exposed to a neutron beam that causes nitrogen atoms to emit detectable energy. More commonly, body composition scans like DEXA estimate lean mass, which serves as a practical proxy for protein stores since lean tissue is where nearly all your protein resides.
Protein Turnover: A Constant Cycle
Your body doesn’t just hold protein passively. It breaks down and rebuilds roughly 250 to 300 grams of protein every day, a process called protein turnover. Most of the amino acids released during breakdown get recycled into new proteins, but some are lost through urine, skin, hair, and digestive secretions. That daily loss is what makes dietary protein essential: you need a steady intake to replace what your body can’t recycle.
Different tissues turn over at wildly different rates. The cells lining your gut replace themselves every three to five days, demanding a constant supply of fresh protein. Muscle protein turns over much more slowly, on the order of weeks to months. Collagen in tendons and cartilage is among the slowest, taking months to years to fully remodel. This variation explains why gut and immune health can deteriorate quickly during protein deficiency, while muscle wasting takes longer to become visible.