How much a healthy human requires protein per day?
The WHO protein figures translate into 56g of protein a day for a (75 kg) man, and 48g for a (64 kg) woman. The recommendations of the UK Department of Health and Social Security (DHSS) are slightly higher, at about 68g a day for sedentary or moderately active men, and 54g a day for women, but the quality is important. Protein quality here refers to the concentration of essential amino acids in a food relative to their concentrations in protein molecules being synthesised. Regardless of their source, amino acids that are not immediately incorporated into new protein are rapidly degraded; i.e., excess amino acids are not stored. Consumption of excess amino acids thus is wasteful, since this surplus is catabolised to form energy, a function that carbohydrates and lipids can serve at a lower cost.
Protein normally provides the body's requirement for amino acid nitrogen and amino acids themselves. All dietary protein is digested and enters the circulation as individual amino acids. The body requires 20 amino acids to synthesise specific proteins and other nitrogen - containing compounds such as purines, pyrimidines, and heme
The daily requirements for total protein and essential amino acids in humans are calculated on the basis of body weight, the extra growth needs of infants and children are clearly evident. Pregnancy, lactation, tissues repair after injury, recoveries from illness, and increased physical activity are other conditions requiring more dietary protein. For most situations, a diet in which 12% of the energy is supplied as protein is adequate.
The efficiency with which dietary protein is used determines the total quantity of protein required. This quantity is affected by three major factors: protein quality, energy intake, and physical activity.
The quality of protein is measured by comparing the proportions of essential amino acids in a food with the proportions required for good nutrition. The closer the two numbers are, the higher the protein quality. Egg and milk proteins are high - quality proteins that are efficiently used by the body and are used as reference standards against which other proteins can be compared. Meat protein is of high protein quality, whereas several proteins from plants used as major food sources are relatively deficient in certain essential amino acids, eg., tryptophan and lysine in maize (corn), lysine in wheat, and methionine in some beans. In a mixed diet, a deficiency of an amino acid in one protein is made up by its abundance in another; such proteins are described as complementary; eg., the protein of wheat and beans combined provides a satisfactory amino acid intake. Under such circumstances, a greater total amount of protein must he consumed to satisfy requirements.