Your body needs protein to build and repair tissue, make hormones and enzymes, regulate fluid balance, and support your immune system. It is one of three macronutrients (alongside carbohydrates and fat), and every cell in your body contains it. Unlike fat, protein isn’t stored in large reserves, so your body depends on a steady dietary supply to keep these processes running.
Building and Repairing Muscle
Protein’s most well-known job is constructing and maintaining skeletal muscle. Your muscles are in a constant state of turnover: old or damaged proteins are broken down while new ones are assembled from amino acids, the building blocks you get from food. This assembly process is called muscle protein synthesis. Whether you gain, maintain, or lose muscle depends on the balance between synthesis and breakdown. When synthesis outpaces breakdown, muscle grows. When the reverse happens, muscle shrinks.
This balance matters beyond the gym. During the early phase of an exercise program, the increased demand for protein is driven less by new muscle growth and more by repairing and remodeling existing proteins that sustained minor damage during activity. Even if you don’t exercise intensely, your body still cycles through this repair process every day just to maintain the muscle you already have.
Providing Structure for Skin, Hair, and Nails
Structural proteins act as scaffolding throughout your body. Keratin, for example, forms the tough outer layer of your skin, strengthens your hair shafts, and hardens your nails. It also helps protect cells from internal mechanical stress, essentially cushioning them against physical forces. Collagen, another structural protein, reinforces tendons, ligaments, bones, and the deeper layers of skin. Without adequate protein intake, these tissues lose resilience. Hair becomes brittle and breaks easily, skin dries out and looks pale, and wounds heal more slowly.
Running Your Body’s Chemical Reactions
Nearly every chemical reaction inside your cells is powered by enzymes, and enzymes are proteins. They break down food during digestion, copy genetic information when cells divide, detoxify harmful substances in the liver, and convert nutrients into usable energy. Without the right enzymes, these reactions would happen too slowly to sustain life. Your body constantly produces new enzymes to replace old ones, which is one reason a steady protein supply matters even when you’re not growing or exercising.
Sending Hormonal Signals
Some hormones are built from amino acids and function as chemical messengers, coordinating activity between organs. Growth hormone, which regulates tissue repair and metabolism, is one example. Insulin, which controls blood sugar, is another. These protein-based hormones travel through the bloodstream and tell distant tissues what to do, when to grow, or when to release stored energy. If your body can’t produce enough of them because raw materials are short, the signaling breaks down.
Keeping Fluids in Balance
A less obvious role of protein is preventing fluid from pooling in your tissues. Albumin, a protein made by the liver, circulates in your blood and generates what’s called oncotic pressure, a pulling force that keeps water inside your blood vessels rather than leaking out into surrounding tissue. Albumin is responsible for roughly 80% of this pressure. When protein intake drops low enough that albumin levels fall, fluid escapes into the spaces between cells, causing visible swelling (edema), particularly in the hands and legs. This is one of the hallmark signs of severe protein deficiency.
Supporting Immune Defense
Your immune system relies on protein to manufacture antibodies, the Y-shaped molecules that latch onto bacteria, viruses, fungi, and toxins so the body can neutralize them. If you’re getting sick more often than usual and notice other signs like thinning hair or slow wound healing, inadequate protein intake may be a contributing factor. Immune cells themselves also turn over rapidly, and building replacements requires a steady amino acid supply.
Controlling Appetite and Weight
Protein is the most satiating macronutrient, meaning it keeps you feeling full longer than the same number of calories from carbohydrates or fat. The mechanism is hormonal. Eating protein triggers the gut to release several appetite-suppressing hormones, including GLP-1, CCK, and peptide YY, while simultaneously lowering levels of ghrelin, the hormone that makes you feel hungry. Clinical trials comparing higher-protein diets to standard-protein diets consistently show that participants on the higher-protein plan report greater fullness and less hunger between meals. This is one reason higher protein intake is frequently recommended during weight loss: it helps reduce overall calorie intake without relying purely on willpower.
Serving as a Backup Energy Source
Your body prefers carbohydrates and fat for fuel, but it can convert protein into glucose when those are in short supply. This happens during prolonged fasting or on very low-carbohydrate diets. Amino acids are stripped of their nitrogen and their carbon skeletons are either burned directly for energy or transformed into glucose. Under normal eating conditions, though, this pathway is minor. In one study, participants who ate a high-protein, low-carbohydrate meal converted only about 4 grams of dietary amino acids into glucose over eight hours, accounting for just 8% of total glucose production. The body treats protein as a last-resort fuel, preferring to save it for building and repair.
How Much You Actually Need
The recommended dietary allowance for a sedentary adult is 0.8 grams of protein per kilogram of body weight per day. For a 165-pound (75 kg) person, that works out to about 60 grams. This number represents the minimum to prevent deficiency, not necessarily the amount for optimal health.
If you exercise regularly, your needs go up. People who do moderate exercise generally need 1.1 to 1.5 grams per kilogram, while those who lift weights or train for endurance events need 1.2 to 1.7 grams per kilogram. For that same 165-pound person, the upper end comes to roughly 128 grams per day.
Older adults deserve special attention. Age-related muscle loss begins as early as your 30s and accelerates after 65. Research shows that combining higher protein intake with resistance exercise produces the best results for preserving muscle mass and strength in older adults. Spreading protein across all three meals, rather than loading it into dinner, helps the body use it more efficiently. Experts generally caution against exceeding about 0.9 grams per pound of body weight (around 150 grams per day for a 165-pound person), as very high intakes can strain the kidneys over time.
What Happens When You Don’t Get Enough
Mild protein shortfalls may show up subtly: slower recovery from workouts, thinning hair, dry skin, or a nagging sense of hunger that snacking doesn’t resolve. More severe deficiency leads to a condition called kwashiorkor, a form of protein-energy malnutrition marked by pronounced edema on both sides of the body, a weakened immune system, rapid hair loss (known as telogen effluvium), and poor wound healing. In wealthy countries, kwashiorkor is rare, but borderline protein intake is more common than many people realize, particularly among older adults, people on highly restrictive diets, and those recovering from surgery or illness.