Protein is one of three major nutrients your body needs in large amounts, alongside carbohydrates and fat. At its most basic, protein is a large molecule made of smaller building blocks called amino acids, linked together in long chains. Your body uses these chains to build and repair tissue, speed up chemical reactions, send signals between organs, and perform thousands of other jobs that keep you alive. Every cell in your body contains protein.
How Proteins Are Built
Amino acids are the raw materials. Each one is a small organic compound made of carbon, hydrogen, oxygen, and nitrogen. Your body strings amino acids together using chemical links called peptide bonds, forming chains that can be dozens or thousands of units long. The specific order and length of the chain determines what the protein does. A protein that helps you digest food looks completely different from one that contracts a muscle fiber, even though both are made from the same set of amino acids.
There are 20 amino acids your body uses. Nine of them are considered essential, meaning your body cannot manufacture them on its own. These nine (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine) must come from food. The remaining 11 are nonessential, which doesn’t mean unimportant. It simply means your body can produce them internally when needed.
What Protein Does in Your Body
Protein plays so many roles that listing them all would fill a textbook. The major categories give you a good picture.
Enzymes. Roughly 80% of the chemical reactions in your body would not happen without enzyme proteins acting as catalysts. These proteins speed reactions up by a factor of a million or more. Digestion, energy production, DNA copying: enzymes drive all of it.
Structure. Proteins form the physical scaffolding of your cells and tissues. In your muscles, a protein called actin provides the framework that allows muscle fibers to contract and generate force. Other structural proteins give shape to your skin, hair, nails, and connective tissue.
Hormones and signaling. Many hormones are proteins. They travel through your bloodstream and bind to receptors on target cells, triggering a chain reaction that changes what the cell does. Insulin, for example, is a protein hormone that tells cells to absorb sugar from the blood.
Transport. Proteins carry molecules where they need to go. Hemoglobin, the protein in red blood cells, picks up oxygen in your lungs and delivers it to tissues throughout your body.
How Your Body Digests Protein
When you eat a piece of chicken or a bowl of lentils, your body has to break those large protein molecules back down into individual amino acids before it can use them. The process starts in your stomach, where cells in the stomach lining release an inactive enzyme called pepsinogen. Stomach acid (with a pH around 1.5 to 2) activates it into its working form, pepsin, which begins slicing proteins into shorter fragments.
Those fragments then move into the small intestine, where a second wave of enzymes from the pancreas finishes the job. By the end, the protein you ate has been reduced to individual amino acids and very short chains, small enough to pass through the intestinal wall into your bloodstream. From there, your body reassembles them into whatever proteins it needs at the moment.
How Much Protein You Need
The baseline recommendation for a healthy adult with minimal physical activity is 0.8 grams of protein per kilogram of body weight per day. For a 150-pound (68 kg) person, that works out to about 54 grams daily. But that number is a minimum to prevent deficiency, not necessarily an optimal target.
If you’re physically active, the numbers go up. Research supports about 1.0 g per kg for light activity, 1.3 g per kg for moderate activity, and 1.6 g per kg for intense exercise like regular strength training. For that same 150-pound person doing intense workouts, the target would be closer to 109 grams per day. These higher amounts support muscle repair and growth.
Complete and Incomplete Sources
A food counts as a “complete protein” when it contains adequate amounts of all nine essential amino acids. Most animal foods fall into this category: fish, poultry, eggs, beef, pork, and dairy all provide the full set. Soy-based foods like tofu, edamame, and tempeh are among the few plant sources that also qualify as complete.
Most other plant foods, including beans, lentils, nuts, seeds, and whole grains, are incomplete proteins. They contain all or most of the essential amino acids, but in lower amounts for one or more. This doesn’t mean plant-based eaters are at a disadvantage. You don’t need to get all nine essential amino acids in a single meal. Eating a variety of plant proteins over the course of a day fills in the gaps naturally. A classic example: beans and rice each lack what the other provides, so eating both gives you the complete set.
What Happens When You Get Too Little
True protein deficiency is uncommon in developed countries but serious when it occurs. Without enough amino acids coming in, your body starts breaking down its own muscle tissue to harvest what it needs. Over time, this leads to muscle wasting, a weakened immune system, slower wound healing, thinning hair, and fatigue. In severe cases, particularly in malnourished children, protein deficiency causes swelling in the abdomen and extremities because blood proteins that regulate fluid balance drop too low.
Milder shortfalls are more common than outright deficiency, especially in older adults who tend to eat less and in people on very restrictive diets. The signs are subtler: losing muscle mass gradually, feeling weak, getting sick more often, or recovering slowly from injuries.
Risks of Chronically High Intake
More protein is not always better. Consistently eating well above your needs, particularly from animal sources high in saturated fat, can create problems over time. The potential concerns include increased calcium loss through urine (which may weaken bones over the long term), added strain on the kidneys, and a higher risk of kidney stones. Low fluid intake combined with excessive protein is a particularly strong risk factor for stones.
Research has also linked very high animal protein intake to increased risk of coronary heart disease, likely driven by the saturated fat and cholesterol that come along with it, and to changes in liver function. For people who already have mild kidney problems, a chronically high protein diet can accelerate the decline in kidney function. These risks are associated with long-term excess, not with occasionally eating a high-protein meal. For most healthy people, staying in the range of 0.8 to 1.6 g per kg of body weight provides the benefits without the downsides.