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. 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 and as precursors to nucleic acid, co-enzymes, hormones, immune response, cellular repair, and other molecules essential for life. Additionally, protein is needed to form blood cells.
The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition. Proteins are polymer chains made of amino acids linked together by peptide bonds. During human digestion, proteins are broken down in the stomach to smaller polypeptide chains via hydrochloric acid and protease actions. This is crucial for the absorption of the essential amino acids that cannot be biosynthesized by the body.
Protein can be found in a wide range of food. The best combination of protein sources depends on the region of the world, access, cost, amino acid types and nutrition balance, as well as acquired tastes. On a worldwide basis, plant protein foods contribute over 60 percent of the per capita supply of protein, on average. Meat, products from milk, eggs, soy, and fish are sources of complete protein. Vegetarian sources of proteins include legumes, nuts, seeds and fruits. Legumes, some of which are called pulses in certain parts of the world, have higher concentrations of amino acids and are more complete sources of protein than whole grains and cereals.
The amount of protein required in a person's diet is determined in large part by overall energy intake, the body's need for nitrogen and essential amino acids, body weight and composition, rate of growth in the individual, physical activity level, the individual's energy and carbohydrate intake, and the presence of illness or injury.
Protein powders – such as casein, whey, egg, rice and soy – are processed and manufactured sources of protein. These protein powders may provide an additional source of protein for bodybuilders. Several studies have concluded that active people and athletes may require elevated protein intake due to increase in muscle mass and sweat losses, as well as need for body repair and energy source.
Protein deficiency and malnutrition (PEM) can lead to variety of ailments including mental retardation and kwashiorkor. Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs. PEM is fairly common worldwide in both children and adults and accounts for 6 million deaths annually. In the industrialized world, PEM is predominantly seen in hospitals, is associated with disease, or is often found in the elderly.
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