Protein as nutrient
The most important aspect and defining characteristic of protein from a nutritional standpoint is its amino acid composition.
During human digestion, proteins are broken down in the stomach to smaller polypeptide chains via hydrochloric acid and protease actions.
[3] There are nine essential amino acids which humans must obtain from their diet in order to prevent protein-energy malnutrition and resulting death.
[2] Dietary sources of protein include grains, legumes,[7] nuts,[8] seeds,[7] meats, dairy products, fish, eggs, edible insects, and seaweeds.
[11] Examples of food staples and cereal sources of protein, each with a concentration greater than 7%, are (in no particular order) buckwheat, oats, rye, millet, maize (corn), rice, wheat, sorghum, amaranth, and quinoa.
[15] Plant sources of proteins include legumes, nuts, seeds, grains, and some vegetables and fruits.
The use of correct conversion factors is heavily debated, specifically with the introduction of more plant-derived protein products.
"[23] Measuring peptide bonds in grains has also been put into practice in several countries including Canada, the UK, Australia, Russia and Argentina where near-infrared reflectance (NIR) technology, a type of infrared spectroscopy is used.
[24] The Food and Agriculture Organization of the United Nations (FAO) recommends that only amino acid analysis be used to determine protein in, inter alia, foods used as the sole source of nourishment, such as infant formula, but also provides: "When data on amino acids analyses are not available, determination of protein based on total N content by Kjeldahl (AOAC, 2000) or similar method ... is considered acceptable.
"[25] 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.
[31] 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.
[36][37] 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.
Requirements are also greater during childhood for growth and development, during pregnancy, or when breastfeeding in order to nourish a baby or when the body needs to recover from malnutrition or trauma or after an operation.
According to results of the National Health and Nutrition Examination Survey (NHANES 2013–2014), average protein consumption for women ages 20 and older was 69.8 grams and for men 98.3 grams/day.
Several studies have concluded that active people and athletes may require elevated protein intake (compared to 0.8 g/kg) due to increase in muscle mass and sweat losses, as well as need for body repair and energy source.
[44] Endurance athletes who exercise over a long period (2–5 hours per training session) use protein as a source of 5–10% of their total energy expended.
One review concluded that endurance athletes may increase daily protein intake to a maximum of 1.2–1.4 g per kg body weight.
They may include itchiness, swelling of the tongue, vomiting, diarrhea, hives, trouble breathing, or low blood pressure.
The following eight foods are responsible for about 90% of allergic reactions: cow's milk, eggs, wheat, shellfish, fish, peanuts, tree nuts and soy.
[48] Individuals with phenylketonuria (PKU) must keep their intake of phenylalanine – an essential amino acid – extremely low to prevent a mental disability and other metabolic complications.
Phenylalanine is a component of the artificial sweetener aspartame, so people with PKU need to avoid low calorie beverages and foods with this ingredient.
[2] Excessive protein intake may increase calcium excretion in urine, occurring to compensate for the pH imbalance from oxidation of sulfur amino acids.
The recommendation is not to limit intake of meats or protein, but rather to monitor and keep within daily limits the sodium (< 2300 mg), saturated fats (less than 10% of total calories per day), and added sugars (less than 10% of total calories per day) that may be increased as a result of consumption of certain meats and proteins.
[57] Protein deficiency and malnutrition (PEM) can lead to variety of ailments including Intellectual disability and kwashiorkor.
[59] Symptoms of kwashiorkor include apathy, diarrhea, inactivity, failure to grow, flaky skin, fatty liver, and edema of the belly and legs.
This edema is explained by the action of lipoxygenase on arachidonic acid to form leukotrienes and the normal functioning of proteins in fluid balance and lipoprotein transport.
