This article is about protein as a nutrient. For protein as protein ice cream recipe class of molecule, see Protein. For other uses, see Bodybuilding supplement.

Amino acids are the building blocks of protein. Present in every cell, they are also precursors to nucleic acids, co-enzymes, hormones, immune response, repair and other molecules essential for life. Proteins are essential nutrients for the human body. They are one of the building blocks of body tissue and can also serve as a fuel source.

Proteins are polymer chains made of amino acids linked together by peptide bonds. Protein is a nutrient needed by the human body for growth and maintenance. Aside from water, proteins are the most abundant kind of molecules in the body. Protein can be found in all cells of the body and is the major structural component of all cells in the body, especially muscle. This also includes body organs, hair and skin. Proteins are also used in membranes, such as glycoproteins. Protein occurs in a wide range of food.

Meat, dairy, eggs, soy, fish, whole grains, and cereals are sources of protein. Vegan sources of proteins include legumes, nuts, seeds and fruits. Brazil nuts, cashews, pecans, walnuts, cotton seeds, pumpkin seeds, hemp seeds, sesame seeds, and sunflower seeds. Photovoltaic-driven microbial protein production uses electricity from solar panels and carbon dioxide from the air to create fuel for microbes, which are grown in bioreactor vats and then processed into dry protein powders.

The process makes highly efficient use of land, water and fertiliser. People eating a balanced diet do not need protein supplements. The table below presents food groups as protein sources. Protein source with highest density of respective amino acid. Protein source with lowest density of respective amino acid. The classic assays for protein concentration in food are the Kjeldahl method and the Dumas method. These tests determine the total nitrogen in a sample.

Accidental contamination and intentional adulteration of protein meals with non-protein nitrogen sources that inflate crude protein content measurements have been known to occur in the food industry for decades. True protein is a measure of only the proteins in milk, whereas crude protein is a measure of all sources of nitrogen and includes nonprotein nitrogen, such as urea, which has no food value to humans. The testing method for protein in beef cattle feed has grown into a science over the post-war years. The standard text in the United States, Nutrient Requirements of Beef Cattle, has been through eight editions over at least seventy years. The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.

There are multiple systems which rate proteins by their usefulness to an organism based on their relative percentage of amino acids and, in some systems, the digestibility of the protein source. Most proteins are decomposed to single amino acids by digestion in the gastro-intestinal tract. Digestion typically begins in the stomach when pepsinogen is converted to pepsin by the action of hydrochloric acid, and continued by trypsin and chymotrypsin in the small intestine. Absorption of the amino acids and their derivatives into which dietary protein is degraded is done by the gastrointestinal tract. Newborns of mammals are exceptional in protein digestion and assimilation in that they can absorb intact proteins at the small intestine. An education campaign launched by the United States Department of Agriculture about 100 years ago, on cottage cheese as a lower-cost protein substitute for meat.

Considerable debate has taken place regarding issues surrounding protein intake requirements. 70 need to consume 56 grams of protein per day to minimize risk of deficiency. In addition, some have suggested that athletes using restricted-calorie diets for weight loss should further increase their protein consumption, possibly to 1. Endurance athletes differ from strength-building athletes in that endurance athletes do not build as much muscle mass from training as strength-building athletes do. Research suggests that individuals performing endurance activity require more protein intake than sedentary individuals so that muscles broken down during endurance workouts can be repaired. Research also indicates that individuals performing strength training activity require more protein than sedentary individuals. Strength-training athletes may increase their daily protein intake to a maximum of 1.

8 g per kg body weight to enhance muscle protein synthesis, or to make up for the loss of amino acid oxidation during exercise. A food allergy is an abnormal immune response to proteins in food. The signs and symptoms may range from mild to severe. They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressure. These symptoms typically occurs within minutes to one hour after exposure. While there is no conclusive evidence that a high protein diet can cause chronic kidney disease, there is a consensus that people with this disease should decrease consumption of protein. According to one 2009 review updated in 2018, people with chronic kidney disease who reduce protein consumption have less likelihood of progressing to end stage kidney disease.