Protein is used for nearly every structural and chemical process in your body, from building muscle and repairing tissue to making hormones, enzymes, and immune cells. It’s one of three macronutrients you eat daily, but unlike fat and carbohydrates, your body doesn’t store protein in reserve. That means you need a steady supply to keep these systems running.
Building and Repairing Muscle
The most well-known job of protein is maintaining your muscles. When you exercise, lift something heavy, or even just go about your day, small amounts of muscle tissue break down. Protein provides the amino acids your body uses to repair that damage and, over time, build new muscle fiber.
This process works through a cellular signaling system that detects amino acids arriving from digested protein. When enough amino acids are available, especially alongside physical activity, your cells ramp up production of new muscle protein. Without that raw material, recovery slows and muscle mass gradually declines. This is particularly relevant as you age: older adults need more protein per meal to trigger the same repair response. Research suggests that roughly 3 grams of the amino acid leucine per meal is the threshold needed to maximally stimulate muscle rebuilding in older adults, which translates to about 30 grams of high-quality protein.
Enzymes That Run Your Metabolism
Thousands of chemical reactions happen in your body every second, and nearly all of them depend on enzymes, which are proteins. Digestive enzymes, for example, break food into absorbable pieces. Your pancreas produces enzymes like trypsin, chymotrypsin, and elastase, each designed to cut proteins at different points along the chain so your gut can absorb the individual amino acids.
Beyond digestion, protein-based enzymes manage your energy supply. Some shuttle energy between different parts of a cell, acting as metabolic monitors that tell other systems to speed up or slow down based on how much fuel is available. Without functioning enzymes, you couldn’t extract energy from food, clear waste products, or synthesize the molecules you need to survive.
Hormones That Regulate Blood Sugar and Growth
Several of your body’s most important hormones are made from protein. Insulin and glucagon, both produced by your pancreas, work as a paired system to keep blood sugar stable. Insulin lowers blood sugar by helping cells absorb glucose. Glucagon does the opposite: it signals your liver to convert stored glycogen back into glucose and release it into your bloodstream when levels drop too low.
During prolonged fasting, glucagon goes further. It triggers your body to manufacture glucose from non-carbohydrate sources, including amino acids and fats. Growth hormone, another protein-based hormone, regulates cell reproduction, tissue repair, and metabolism throughout your life. Without adequate protein intake, your body can’t produce enough of these signaling molecules to keep hormonal systems balanced.
Immune Defense
Antibodies, the molecules your immune system produces to fight viruses and bacteria, are proteins called immunoglobulins. When your body encounters a pathogen, immune cells build antibodies specifically shaped to latch onto that invader and mark it for destruction.
This is one reason protein deficiency hits the immune system hard. Conditions that reduce the body’s protein supply, including malnutrition, severe burns, and certain kidney diseases, are associated with low immunoglobulin levels. If you’re not eating enough protein, your body has fewer building blocks to mount an effective immune response, leaving you more vulnerable to infections.
Structural Support for Skin, Bones, and Joints
Collagen is the single most abundant protein in your body, making up about 90% of your connective tissue. It provides the structural framework for your skin, bones, tendons, ligaments, and even the protective covering around your organs. In your skin specifically, collagen helps new cells grow in the middle layer (the dermis), replaces dead skin cells, and gives skin its elasticity. Type II collagen lines your cartilage and cushions your joints.
Other structural proteins handle different jobs. Keratin forms the tough outer layer of your hair, nails, and skin surface. Elastin, as the name suggests, gives tissues like your lungs and blood vessels the ability to stretch and snap back. Your body is constantly breaking down and rebuilding these structural proteins, which is why a steady protein intake matters even if you’re not trying to build muscle.
Transporting and Storing Essential Nutrients
Proteins also work as delivery vehicles and storage units. Hemoglobin, the protein inside red blood cells, carries oxygen from your lungs to every tissue in your body. Without enough hemoglobin, cells can’t produce energy efficiently, which is why anemia causes fatigue.
Ferritin is a storage protein built from 24 interlocking subunits that form a hollow sphere. Inside that sphere, your body can store up to 4,500 atoms of iron, keeping it safely locked away until needed. Channels in the protein shell allow iron to enter and exit as your body’s demands change. Other transport proteins move vitamins, minerals, and fats through your bloodstream to wherever they’re needed.
Weight Management and Satiety
Protein is the most filling macronutrient, which is a major reason high-protein diets are effective for weight management. Part of this comes down to energy cost: your body burns 15 to 30% of protein’s calories just digesting and processing it. Compare that to 5 to 10% for carbohydrates and 0 to 3% for fats. So a 200-calorie serving of chicken requires significantly more energy to process than 200 calories of bread or butter.
The satiety effect of protein is well established, though interestingly, research from a study published in the Journal of Nutrition found that the feeling of fullness after a high-protein meal doesn’t appear to be driven by changes in hunger-related hormones like ghrelin or peptide YY. The exact mechanism is still being worked out, but the practical result is consistent: people who eat more protein at meals tend to feel satisfied longer and consume fewer total calories.
How Much Protein You Actually Need
The most recent Dietary Guidelines for Americans (2025-2030) recommend that adults consume 1.2 to 1.6 grams of protein per kilogram of body weight per day. That’s 50 to 100% higher than the old minimum recommendation, reflecting newer evidence about optimal health. For a 150-pound (68 kg) person, this works out to roughly 82 to 109 grams of protein daily.
People who exercise regularly, are recovering from injury, or are over 65 generally benefit from the higher end of that range. Spreading protein intake across meals rather than loading it into one sitting helps maintain a steady supply of amino acids for muscle repair and other functions throughout the day. Most people in Western countries get enough total protein, but many eat the bulk of it at dinner, leaving breakfast and lunch protein-light.
What Happens When You Don’t Get Enough
Because protein serves so many roles simultaneously, a shortfall shows up in multiple systems. Early signs include slow wound healing, thinning hair, brittle nails, and frequent infections. Muscle loss is another hallmark, especially in older adults who may already be losing muscle mass naturally. Over time, inadequate protein weakens bones, impairs immune function, and disrupts hormone balance.
Severe protein deficiency is rare in developed countries, but marginal intake is common among older adults, people on very restrictive diets, and those recovering from illness or surgery. If you’re eating a varied diet that includes meat, fish, eggs, dairy, legumes, or soy products at most meals, you’re likely meeting your needs. Plant-based eaters can get enough protein by combining sources like beans with grains, nuts, and seeds throughout the day.