Protein is one of the three macronutrients your body cannot function without, and it does far more than build muscle. Every cell in your body contains protein. It forms the physical structure of your tissues, powers the chemical reactions that keep you alive, and produces the hormones and immune cells that protect you from disease. The recommended minimum intake is 0.8 grams per kilogram of body weight per day, which works out to about 60 grams for a 165-pound person.
Building and Repairing Tissue
The most familiar role of protein is structural. When you exercise, especially during resistance training, you create microscopic damage in muscle fibers. Your body repairs that damage by synthesizing new muscle protein, primarily the contractile filaments actin and myosin, which are the components that allow muscles to shorten and generate force. Over time, repeated cycles of damage and repair lead to muscle growth.
This process depends on essential amino acids, particularly leucine, which acts as the primary trigger for muscle protein synthesis. Your body cannot manufacture essential amino acids on its own, so they have to come from food. There are nine in total, and foods that contain all nine (meat, fish, eggs, dairy, soy, quinoa) are considered complete protein sources.
Beyond muscle, protein provides the raw material for skin, hair, nails, tendons, ligaments, and organs. Structural proteins like collagen (the most abundant protein in the body) and keratin (which forms the outer layer of skin, hair, and nails) are constantly being broken down and replaced. Without enough dietary protein, this turnover slows, and you may notice dry or brittle hair, peeling skin, and nails that break easily.
Running Your Body’s Chemical Reactions
Enzymes are proteins, and they carry out nearly all of the thousands of chemical reactions happening inside your cells at any given moment. Digesting food, converting nutrients into energy, reading the genetic instructions stored in your DNA: all of these processes rely on enzymes built from amino acids you consumed in your last meal or snack.
Many hormones are also proteins or peptides. Growth hormone, insulin, and the signaling molecules that coordinate activity between your brain, gut, and immune system are all synthesized from dietary protein. Without adequate intake, your body’s ability to produce these chemical messengers declines, disrupting everything from blood sugar regulation to growth and development.
Fueling Your Immune System
Antibodies, the molecules your immune system uses to identify and neutralize bacteria, viruses, and other threats, are proteins. Each antibody is built from four chains of amino acids: two heavy chains and two light chains. When your body encounters a new pathogen, specialized white blood cells called B cells divide rapidly and release millions of antibodies into your bloodstream. That entire defense system runs on protein.
One of the essential amino acids, histidine, plays a direct role in immune function by helping produce histamine, a chemical involved in inflammatory and allergic responses. Another, isoleucine, supports both immune cell activity and the production of hemoglobin, the protein in red blood cells that carries oxygen.
Controlling Hunger and Body Weight
Protein is the most satiating macronutrient. Eating it triggers the release of several gut hormones that signal fullness to your brain, including GLP-1, cholecystokinin, and peptide YY. At the same time, protein suppresses ghrelin, the hormone that drives hunger. These hormones also stimulate the vagus nerve, which connects your gut directly to your brain, reinforcing the feeling that you’ve had enough to eat.
This is why higher-protein meals tend to reduce snacking and overall calorie intake without requiring conscious restriction. It also explains why protein is consistently emphasized in weight management strategies: it helps you feel full on fewer total calories while preserving muscle mass that would otherwise be lost during a calorie deficit.
How Much You Actually Need
The official RDA of 0.8 grams per kilogram (0.36 grams per pound) represents the minimum to prevent deficiency in a sedentary adult. For a 165-pound person, that’s about 60 grams per day. Many nutrition researchers consider this a floor rather than a target, especially for people who are active, aging, or trying to change their body composition.
The International Olympic Committee recommends 1.6 grams per kilogram per day for athletes pursuing muscle growth and strength, roughly double the baseline RDA. Research on professional football players found that intakes of 1.6 to 2.2 grams per kilogram supported optimal performance and muscle gains. You don’t need to be a professional athlete to benefit from higher intake, but there is an upper limit: consuming more than about 0.9 grams per pound of body weight (around 150 grams per day for a 165-pound person) offers no additional benefit and can be harmful over time.
Older adults have a particular reason to pay attention. Age-related muscle loss, called sarcopenia, accelerates after 65 and contributes to falls, fractures, and loss of independence. Because aging muscles become less efficient at using dietary protein, many experts suggest that older adults aim above the standard RDA to maintain muscle mass and function.
What Happens When You Don’t Get Enough
Mild protein deficiency is uncommon in developed countries, but it does happen, particularly among older adults with poor appetites, people on very restrictive diets, and those recovering from illness or surgery. Early signs include fatigue, slow wound healing, frequent infections, and thinning hair.
Severe protein deficiency causes a condition called kwashiorkor, most often seen in children in regions with limited food access. Its hallmark symptoms include swelling in the ankles and feet from fluid retention, a bloated abdomen, dry and peeling skin, hair that loses its color and falls out, an enlarged liver, and depleted muscle mass. Children with kwashiorkor retain fat under the skin but lose the muscle underneath, which is why they can appear swollen rather than thin. Growth stunting, irritability, and complete loss of appetite follow as the condition worsens.
Practical Ways to Meet Your Needs
Protein doesn’t need to come from a single source or a single meal. Spacing intake across the day, with 20 to 40 grams per meal, gives your body a steady supply of amino acids for tissue repair and other functions. Foods that contain all nine essential amino acids (eggs, poultry, fish, dairy, soy, quinoa) are the most efficient choices, but combining complementary plant sources like beans and rice across the day works just as well.
For skin, hair, and nail health specifically, foods that support keratin production include eggs, salmon, sweet potatoes, broccoli, kale, and carrots. These provide both the amino acid building blocks and the vitamins and minerals that act as cofactors in protein synthesis. If you’re eating a varied diet with adequate total protein, you’re likely covering these needs without thinking about it.