What are proteins?
Similar to carbohydrates and lipids, proteins are made of the elements carbon, hydrogen, and oxygen. However, all proteins also contain the element nitrogen and several proteins contain the mineral sulfur. The nitrogen in proteins is in a special form that can be readily used by our bodies for vital functions. Together, these elements form various amino acids, which serve as the building blocks for protein synthesis.
Amino Acids
Amino acids, the building blocks of proteins, contain a usable form of nitrogen for humans. Of the 20 amino acids needed by the body, 9 must be provided in the diet (essential). The other 11 can be synthesized by the body (nonessential) from amino acids in the body’s amino acid pool. Important body components – such as muscles, connective tissue, transport proteins, enzymes, hormones, and antibodies are made from proteins. Proteins also provide carbon skeleton, which can be used to synthesize glucose when necessary.
High-quality, also called complete, proteins contain ample amounts of all 9 essential amino acids. Foods derived from animal sources provide high-quality, or complete protein. Lower-quality, or incomplete, proteins lack sufficient amounts of 1 or more essential amino acids. This is typical of plant food. However, different types of plant foods eaten together often complement each other’s amino acid deficits, thereby providing high-quality protein in the diet.
When 2 or more plant proteins are combined to compensate for deficiencies in essential amino acid content in each protein, these proteins are called complementary proteins.
Cells require a pool of essential amino acids for the synthesis of body proteins. Thus, a single plant protein, such as wheat (which is low in the amino acid lysine), cannot support the synthesis of body protein if it is the sole source of dietary protein.
Protein Functions
- Providing vital body structures
- Maintaining fluid balance
- Contributing to acid-base balance
- Forming hormones, enzymes, and neurotransmitters
- Contributing to immune system
- Transporting nutrients
Food sources of Protein
In the North American diets, about 70% of dietary protein is supplies by meat, poultry, fish, milk and some milk products. However, plants are the major source of protein in many areas of the world. Plants can provide ample amounts of dietary protein in addition to providing fiber and a variety of vitamins, minerals, and phytochemicals.
Plant proteins also contain no cholesterol and little saturated fat, unless added during processing. Great sources of dietary protein in plants are nuts and seeds,nut butter, edamame (green soybeans), quinoa, beans, chickpeas, tempeh, tofu, and chia seeds.
Protein digestion
Protein digestion begins in the stomach, where proteins are broken down into shorter polypeptide chains of amino acids. In the small intestine, these polypeptide chains are digested into dipeptides and amino acids, which are absorbed by the small intestine, where any remaining peptides are broken down into amino acids. Absorbed amino acids then travel via the portal vein to the liver.
Recommended Intakes of Protein
Healthy individuals who are not in periods of growth or recovering from illness or injury need to consume protein in an amount that replaces the protein lost in urine, sweat, skin cells, feces, hair, and nails. When protein intake equals the amount in the losses, protein balance is maintained as long as energy intake is adequate to prevent the use of protein for energy.
The adult RDA for protein is 0.8 g per kg of healthy body weight. For a typical 154-lb (70-kg) person, this corresponds to 56 g of protein daily. For a 125-lb (58-kg) person, this corresponds to 46 g/ daily.
Very high intake of protein in the diet
In addition to recommending adequate protein consumption, the Food and nutrition Board also suggests that protein intake should not exceed 35% of energy intake. Diets containing an excessive or disproportionate amount of protein do not provide additional health benefits. Instead, high protein intakes may increase health and disease risks. One area of concern is the effect of excess protein on the kidneys. Kidneys are responsible for excreting excess nitrogen as urea. Therefore, high-protein diets may overburden the kidney’s capacity to excrete nitrogen wastes. Additionally, because water is needed to dilute and excrete urea, inadequate fluid intake can increase the risk of dehydration as the kidneys use body water to dispose of the urea. These concerns are greatest for people who already have impaired kidney functions. A lower-protein diet with adequate fluid intake is recommended for these individuals to help preserve kidney health.
When excess protein is primarily from a high intake of animal products, the overall diet is likely to be low in plant-based food and consequently low in fiber, some vitamins (vitamin C, E and folate), minerals (magnesium and potassium), and beneficial phytochemicals. Animal proteins are often rich in saturated fat and cholesterol. As a result, these diets can increase the risk of cardiovascular disease.
Learn about carbohydrates.
Learn about fats.
