Protein (nutrient)

Proteins are essential nutrients that serve as fundamental building blocks for life. After water, proteins account for more mass in an organism than any other type of molecule. They are present in every single cell and act as structural components for tissues like skin, hair, blood, and bone. Proteins are especially abundant in muscle tissue. Beyond structure, they serve as a vital fuel source for the body. They provide an energy density of 17 kJ (4 kcal) per gram, which is the same as carbohydrates.

To understand protein, one must look at its molecular structure. Proteins are polymer chains composed of individual units called amino acids. These amino acids are linked together by chemical connections known as peptide bonds. The specific sequence and types of amino acids determine the unique three-dimensional structure of each protein. This shape is critical because it dictates the protein's specific function. Proteins also act as cellular messengers, such as hormones, and as transport molecules like enzymes and antibodies.

Digestion is the process of breaking these complex chains back down into usable parts. This process begins in the stomach, where hydrochloric acid and the enzyme protease work together. They break proteins into smaller polypeptide chains. As the food moves into the small intestine, other enzymes like trypsin and chymotrypsin continue the work. Most proteins are eventually reduced to single amino acids or very small peptides. Before they can be absorbed by the intestinal cells, most peptides longer than four amino acids must be broken down into single amino acids.

Amino acids are categorized based on whether the body can produce them. There are five amino acids that the human body can synthesize on its own: alanine, aspartic acid, asparagine, glutamic acid, and serine. There are also six conditionally essential amino acids. These are arginine, cysteine, glycine, glutamine, proline, and tyrosine. Their synthesis may be limited during specific medical conditions, such as prematurity in infants or severe catabolic distress.

Most importantly, there are nine essential amino acids that humans must obtain through their diet. These include phenylalanine, valine, threonine, tryptophan, methionine, leucine, isoleucine, lysine, and histidine. While there has been debate about whether there are eight or nine, the consensus leans toward nine because adults do not synthesize histidine. A lack of these amino acids can lead to protein-energy malnutrition, which can be fatal.

Dietary sources of protein vary greatly across the globe. Plant-based foods contribute over 60% of the per capita protein supply worldwide. In North America, however, animal-derived foods contribute about 70% of protein sources. In some parts of Africa, insects provide up to 50% of dietary protein, and more than 2 billion people eat insects daily.

Many grains and legumes are excellent sources. Staples like buckwheat, oats, rye, millet, maize, rice, wheat, sorghum, amaranth, and quinoa all contain protein concentrations greater than 7%.

Scientists use specific methods to measure the protein content in food. A common technique is measuring nitrogen content, as nitrogen is a fundamental component of amino acids. This is often called "crude protein" content. Using the Kjeldahl method or the Dumas method, scientists multiply the nitrogen level by a factor to estimate total protein. On food labels, this is often calculated by multiplying nitrogen by 6.25. However, this method cannot distinguish between true protein and non-protein nitrogen (NPN). To ensure quality, some industries now use "true protein" measurements, which specifically measure peptide bonds to ensure the nitrogen comes from actual protein.