Protein gets a lot of attention in fitness and nutrition circles, but its role in the body goes far beyond building muscle. Here’s an accurate look at the nine essential functions protein performs, how much you actually need, and where to get it.
9 Important Functions of Protein
1. Building and Repairing Tissue
Protein provides the amino acid building blocks used to build and repair virtually every tissue in the body — muscle, skin, organs, and more. This is why adequate protein matters not just for athletes building muscle, but for anyone recovering from an injury, surgery, or illness.
2. Enzyme Production
Enzymes — the proteins that catalyze virtually every biochemical reaction in the body, from digestion to energy production — are built from protein. Without adequate protein intake, the body’s ability to produce these essential enzymes can be compromised.
3. Hormone Production
Many hormones are proteins or protein-derived, including insulin (which regulates blood sugar) and growth hormone. Protein intake supports the body’s ability to produce and regulate these critical chemical messengers.
4. Immune Function
Antibodies — the proteins your immune system produces to fight off infections — require adequate protein intake to be produced effectively. Chronic protein deficiency is associated with impaired immune function and increased susceptibility to infection.
5. Transport and Storage
Certain proteins transport essential substances throughout the body — hemoglobin, for example, is a protein that carries oxygen in red blood cells, while other proteins help store and transport nutrients like iron.
6. Structural Support
Structural proteins like collagen and keratin provide the framework for skin, hair, nails, tendons, and connective tissue throughout the body, contributing to their strength and elasticity.
7. Fluid and pH Balance
Proteins like albumin help maintain proper fluid balance between blood vessels and surrounding tissues, and proteins throughout the body act as buffers that help maintain stable blood pH within a narrow, healthy range.
8. A Backup Energy Source
While carbohydrates and fats are the body’s preferred energy sources, protein can be broken down and used for energy when those sources are insufficient — though relying on protein for energy on a regular basis isn’t ideal, since it diverts protein away from its other essential functions.
9. Cell Signaling and Communication
Many proteins act as signaling molecules or receptors, playing essential roles in how cells communicate with each other — a function that underlies everything from nerve transmission to the coordination of the immune response.
How Much Protein Do You Actually Need?
The general Recommended Dietary Allowance (RDA) for protein is about 0.8 grams per kilogram of body weight per day for a sedentary adult — this is the minimum to prevent deficiency, not necessarily an optimal target for everyone. Actual needs vary based on several factors:
- Active individuals and athletes: Often benefit from higher intake, commonly cited in the range of 1.2 to 2.0 grams per kilogram of body weight, depending on training intensity and goals.
- Pregnant and breastfeeding women: Have increased protein needs to support fetal growth and milk production.
- Older adults: May benefit from higher intake (often cited around 1.0 to 1.2 grams per kilogram) to help counter the natural muscle loss that occurs with aging.
- People recovering from illness, surgery, or injury: Often have increased protein needs to support tissue repair.
These are general guidelines — a registered dietitian can help determine the right target for your specific health status, activity level, and goals.
Good Sources of Protein
Protein is available from both animal and plant sources:
- Animal sources: Meat, poultry, fish, eggs, and dairy products are considered “complete” proteins, meaning they contain all nine essential amino acids your body can’t produce on its own.
- Plant sources: Legumes, beans, lentils, tofu, tempeh, nuts, seeds, and whole grains all provide protein. Most individual plant sources are “incomplete” (missing one or more essential amino acids in sufficient amounts), but eating a variety of plant proteins across the day — not necessarily in the same meal — provides all the essential amino acids your body needs.
Signs of Inadequate Protein Intake
True protein deficiency is uncommon in populations with reliable access to varied food, but inadequate intake can contribute to:
- Muscle loss or weakness
- Slower wound healing
- More frequent infections, related to impaired immune function
- Hair, skin, and nail changes
- Fatigue
If you’re concerned about your protein intake, a registered dietitian or your doctor can help assess whether you’re meeting your specific needs, rather than guessing based on general guidelines alone.
Protein’s role in the body extends well beyond muscle — it’s essential for tissue repair, immune function, hormone and enzyme production, structural support, and more. Getting adequate protein from a varied diet supports all of these functions, and individual needs vary based on activity level, life stage, and health status. (For more on how protein supports fitness goals, see our guides on how weightlifting builds strength and confidence and whether you’re really resting properly after exercise.)
Frequently Asked Questions
What are the main functions of protein in the body?
Protein builds and repairs tissue, produces enzymes and hormones, supports immune function, transports substances like oxygen, provides structural support (collagen, keratin), helps maintain fluid and pH balance, serves as a backup energy source, and supports cell signaling.
How much protein do I need per day?
The general RDA is about 0.8 grams per kilogram of body weight for sedentary adults, though active individuals, older adults, and pregnant or breastfeeding women often benefit from higher intake, commonly in the range of 1.0 to 2.0 grams per kilogram depending on individual factors.
What’s the difference between complete and incomplete proteins?
Complete proteins (mostly animal sources) contain all nine essential amino acids your body can't produce on its own. Incomplete proteins (most individual plant sources) are missing one or more in sufficient amounts, but eating a variety of plant proteins across the day provides all essential amino acids.
What happens if I don’t get enough protein?
Inadequate protein intake can contribute to muscle loss, slower wound healing, more frequent infections, changes in hair, skin, and nails, and fatigue. True deficiency is uncommon with access to varied food, but insufficient intake is possible even without full deficiency.
Can plant-based diets provide enough protein?
Yes, with attention to variety. Combining different plant protein sources (legumes, grains, nuts, seeds) across the day provides all essential amino acids, even though most individual plant sources are incomplete on their own.
References
- National Institutes of Health, MedlinePlus — protein in diet (medlineplus.gov).
- Academy of Nutrition and Dietetics — protein needs across the lifespan (eatright.org).
- Harvard T.H. Chan School of Public Health — The Nutrition Source: Protein (hsph.harvard.edu).