Proteins are involved in nearly every process in your body, from building tissue and fighting infections to carrying oxygen through your blood and regulating your metabolism. They are large, complex molecules made of smaller building blocks called amino acids, and your body uses them for far more than just muscle. Here’s a breakdown of what proteins actually do.
Building and Maintaining Body Structure
The most visible job of proteins is structural. Collagen, the most abundant protein in your body, forms the framework of your skin, bones, tendons, and ligaments. Keratin makes up your hair, nails, and the outer layer of your skin. Elastin, which accounts for roughly 2% to 4% of the dry matter in adult skin, gives tissues the ability to stretch and snap back into shape. Without it, your skin, lungs, and blood vessels would lose their flexibility.
These structural proteins aren’t static. Your body constantly breaks them down and rebuilds them, which is why a steady supply of dietary protein matters for wound healing, tissue repair, and maintaining the strength of connective tissues as you age.
Speeding Up Chemical Reactions
Thousands of chemical reactions happen in your body every second, and nearly all of them depend on proteins called enzymes. Enzymes act as catalysts: they dramatically speed up reactions that would otherwise take too long to sustain life. Digestive enzymes, for example, break the peptide bonds in food proteins so your gut can absorb the individual amino acids. Other enzymes manage energy production by converting molecules like ADP into ATP, the cell’s primary fuel currency.
Without enzymes, processes like digesting a meal, replicating DNA, or converting stored sugar into usable energy would essentially stall. Each enzyme is highly specific, shaped to interact with only one type of molecule, which is why your body produces so many different kinds.
Defending Against Infection
Your immune system relies on specialized proteins called antibodies (also known as immunoglobulins) to identify and neutralize threats. When a virus, bacterium, or toxin enters your body, antibodies latch onto molecules on its surface and flag it for destruction by white blood cells.
There are five main classes of antibodies, each stationed in different parts of the body. IgG is the most common, making up about 70% to 75% of all immunoglobulins, and circulates in your blood and tissue fluids to fight bacterial and viral infections. IgA lives in saliva, tears, breast milk, and the lining of your gut, protecting against pathogens you inhale or swallow. IgM acts as the first responder during a new infection. IgE, concentrated in your skin, lungs, and mucous membranes, triggers the release of histamine during allergic reactions. IgD sits on the surface of certain immune cells and helps activate them.
Regulating Hormones and Metabolism
Many hormones are proteins or short chains of amino acids. Insulin and glucagon are classic examples. Both are produced in the pancreas, and they work as a pair to keep blood sugar stable. Insulin signals cells to absorb glucose from the blood after a meal. Glucagon does the opposite: when blood sugar drops, it triggers the liver to break down stored glycogen and release glucose, while also promoting the creation of new glucose from non-sugar sources.
Glucagon’s reach extends beyond blood sugar. It also promotes the breakdown of stored fat for energy, increases the feeling of fullness after eating, and helps the liver process amino acids. Growth hormone, another protein-based hormone, drives cell growth and repair throughout your life. These signaling proteins allow distant organs to coordinate with each other in real time.
Transporting and Storing Nutrients
Hemoglobin, the protein inside red blood cells, picks up oxygen in your lungs and delivers it to every tissue in your body. It also carries a portion of carbon dioxide back to the lungs for you to exhale. Without hemoglobin, your cells would starve for oxygen within minutes.
Other transport proteins move vitamins, minerals, and fats through your bloodstream. Ferritin, for instance, stores iron in the liver. A single ferritin molecule can hold up to 4,500 iron atoms, making it the body’s primary iron reserve. When your cells need iron (to make new red blood cells, for example), ferritin releases it in a controlled way. This storage system prevents both iron deficiency and the toxic buildup of free iron in tissues.
Maintaining Fluid and pH Balance
Albumin, the most abundant protein in blood plasma, plays a critical but often overlooked role: it keeps fluid from leaking out of your blood vessels. Proteins in the blood create what’s called oncotic pressure, a force that pulls water back into capillaries and prevents it from accumulating in surrounding tissues. Albumin alone accounts for about 80% of this pressure, with other blood proteins (globulins) contributing the remaining 20%.
This is why people with severe protein deficiency or liver disease, where albumin production drops, often develop swelling in the abdomen and limbs. The blood simply can’t hold onto its fluid. Blood proteins also act as buffers, helping maintain the narrow pH range (around 7.35 to 7.45) that your cells need to function properly.
Providing Energy as a Last Resort
Protein is not your body’s preferred fuel source. Carbohydrates and fats handle that job far more efficiently. But when glycogen stores run low and fat isn’t sufficient, your body will break down amino acids and convert them into glucose through a process called gluconeogenesis, which takes place mainly in the liver.
This typically happens during prolonged fasting, very low-carbohydrate diets, or exhaustive exercise that depletes glycogen. In one study, healthy men placed on a high-protein, zero-carbohydrate diet after a glycogen-depleting exercise session relied heavily on this conversion pathway to maintain blood sugar. Protein provides about 4 calories per gram, the same as carbohydrates, but the conversion process itself requires extra energy, which is one reason high-protein diets slightly boost calorie burning.
Building and Repairing Muscle
After resistance training, your muscles enter a state where they actively rebuild and grow, a process called muscle protein synthesis. The rate and duration of this rebuilding depends heavily on how much protein you eat and when.
Research on young adults shows that roughly 20 grams of high-quality protein (about 0.25 grams per kilogram of body weight) consumed after exercise is enough to maximize muscle rebuilding in most people. Going higher, to around 0.31 grams per kilogram, accounts for individual variation and serves as a more conservative target. Eating beyond that in a single sitting doesn’t further boost muscle growth. Instead, the excess amino acids get oxidized for energy or excreted.
Spacing matters too. Consuming 20 grams of protein every three hours over the course of a day has been shown to support the highest rates of muscle protein synthesis compared to larger, less frequent doses. This is why many athletes and active people spread their protein intake across meals rather than loading it all into dinner.
How Much Protein You Need
The current Recommended Dietary Allowance for protein is 0.8 grams per kilogram of body weight per day for sedentary adults. For a 70-kilogram (154-pound) person, that comes out to about 56 grams daily. This is the minimum to prevent deficiency, not necessarily the amount for optimal health or performance.
People who exercise regularly, are recovering from injury, or are older than 65 generally benefit from more. Many sports nutrition guidelines suggest 1.2 to 2.0 grams per kilogram for active individuals, depending on training intensity. The U.S. Dietary Guidelines note that a formal re-examination of protein requirements is being planned, but no updated timeline has been set. In the meantime, most people can meet their needs through a mix of animal and plant sources like meat, fish, eggs, dairy, beans, lentils, and soy.