Updated April 2026 · Reviewed by the Online Nutrition Planet editorial team

Protein gets most of its attention for building muscle — but that's just one of its many roles. Protein is genuinely one of the most versatile nutrients in the human body, involved in nearly every physiological process. This guide walks through the six main functions of proteins and what they mean for your daily eating.

The six main functions of proteins

1. Building and repairing tissues

The most well-known function. Protein provides the amino acid building blocks for muscles, skin, bone matrix, connective tissue, organs, hair, and nails. Every cell in your body contains protein. Tissues are constantly being broken down and rebuilt — adequate dietary protein is what makes the rebuild possible. Without enough protein, wound healing slows, muscle mass declines, and tissue maintenance suffers.

2. Making enzymes

Enzymes are proteins that catalyze chemical reactions. Thousands of different enzymes regulate digestion (breaking down food), metabolism (converting nutrients into energy), cellular function, and essentially every biochemical process. Without adequate protein to synthesize enzymes, critical metabolic reactions slow down or fail.

3. Making hormones

Many hormones are proteins or protein-derived molecules. Insulin (regulating blood sugar), growth hormone (growth and repair), glucagon (glucose mobilization), and many others are built from amino acids. Protein deficiency can affect hormone production and disrupt normal physiological regulation.

4. Supporting immune function

Antibodies — the proteins your immune system uses to recognize and neutralize pathogens — are made from amino acids. Adequate protein intake is essential for mounting effective immune responses. Severe protein malnutrition dramatically increases infection risk, which is why adequate protein intake during illness and recovery matters.

5. Transporting substances

Many proteins act as carriers, moving essential molecules through the body. Hemoglobin (a protein) transports oxygen from lungs to tissues. Albumin transports hormones, medications, and nutrients. Lipoproteins carry cholesterol and triglycerides through the bloodstream. Without adequate protein, these transport systems weaken.

6. Providing structural support

Structural proteins like collagen, keratin, and elastin give your body its physical form. Collagen is the most abundant protein in the body, providing strength to skin, bones, tendons, ligaments, cartilage, and blood vessels. Keratin gives structure to hair and nails. Elastin allows tissues to stretch and recoil (important in skin, lungs, and blood vessels).

How much protein do you need?

The RDA (Recommended Dietary Allowance) is 0.8 grams per kilogram of body weight for sedentary adults — but this is a minimum, not an optimum. Current research supports higher intakes for specific populations:

  • Older adults: 1.0–1.2 grams per kilogram to counteract age-related muscle loss
  • Athletes and active people: 1.4–2.0 grams per kilogram for muscle building and recovery
  • People recovering from illness or injury: Higher intakes support tissue repair
  • Pregnant women: Roughly 25 additional grams per day in second and third trimesters

For most active adults, 0.7–1.0 grams per pound of body weight is a reasonable target. A 150-pound adult would aim for roughly 100–150 grams of protein daily.

Best protein sources

Animal sources (complete proteins)

  • Chicken, turkey, beef, pork, fish, seafood
  • Eggs
  • Dairy products (milk, yogurt, cheese, cottage cheese)

Plant sources

  • Legumes (lentils, chickpeas, black beans, kidney beans)
  • Tofu, tempeh, edamame, seitan
  • Whole grains (quinoa is a complete protein; others combine with legumes for completeness)
  • Nuts and seeds
  • Pea protein powder and other plant protein supplements

Plant proteins have different amino acid profiles than animal proteins. A varied plant-based diet over the course of a day provides all essential amino acids — you don't need to "combine" proteins at every meal as older advice suggested. See our plant-based diet benefits guide for more on plant protein specifically.

Protein distribution matters

Research increasingly suggests that spreading protein intake across meals — 25–30 grams per meal, three meals per day — optimizes muscle protein synthesis better than loading most protein into a single meal. Most Americans eat a small-protein breakfast, modest lunch, and large dinner — improving breakfast protein alone often helps with satiety, energy, and muscle maintenance.

Signs of inadequate protein intake

  • Slow wound healing
  • Loss of muscle mass, especially in older adults
  • Brittle hair and nails
  • Frequent illnesses or slow recovery
  • Persistent fatigue
  • Swelling (edema) in severe cases
  • Reduced appetite regulation and stronger cravings

Most Americans eating varied diets get enough protein. Under-intake is more common in older adults, people on restrictive diets without planning, and people with certain medical conditions affecting appetite or absorption.

The bottom line

Protein is far more than a muscle-building nutrient. It supports nearly every physiological process — enzymes, hormones, immunity, tissue repair, transport, and structural integrity. Adequate intake from a variety of sources, distributed across your daily meals, supports long-term health for most adults.

For personalized protein targets based on your specific goals, age, and activity level, a registered dietitian can help. See our online nutrition coach reviews.

What to read next:

Written by the Online Nutrition Planet editorial team. Questions? Contact us.

Online Nutrition Planet tracks 687 accredited nutrition programs. Not sure which credential fits? Take the 60-second Match Me Quiz.