nutritional disorders
Overview of Nutrition
Proteins
Proteins are complex organic molecules that contain carbon, hydrogen, oxygen, and nitrogen. Proteins are required for tissue maintenance, replacement, function, and growth. Specific proteins act as enzymes, make up certain hormones, and play an important role in maintaining fluid balance. If the body is not getting enough calories from dietary sources or tissue stores (particularly of fat), protein may be used for energy. Dietary proteins are broken down into peptides and amino acids. As the body uses dietary protein for tissue production, there is a net gain of protein (positive nitrogen balance). During catabolic states (eg, starvation, infections, burns), more protein may be used (because body tissues are broken down) than is absorbed, resulting in a net loss of protein (negative nitrogen balance). Nitrogen balance is best determined by subtracting the amount of nitrogen excreted in urine and feces from the amount of nitrogen consumed. Of the 20 amino acids, 9 are essential amino acids (EAAs); they cannot be synthesized and must be obtained from the diet. All people require 8 EAAs; infants also require histidine. The weight-adjusted requirement for dietary protein correlates with growth rate, which decreases from infancy until adulthood. The daily dietary protein requirement decreases from 2.2 g/kg in 3-month-old infants to 1.2 g/kg in 5-year-old children and to 0.8 g/kg in adults. Protein requirements correspond to EAA requirements. Also, protein requirements are higher among certain patients. For example, protein needs increase dramatically during pregnancy and lactation and during periods of rapid growth and recovery from illness. Protein requirements also increase with aging ( Macronutrients references , The amino acid composition of protein varies widely. Biological value (BV) reflects the similarity in amino acid composition of protein to that of animal tissues; thus, BV indicates what percentage of a dietary protein provides EAAs for the body: A perfect match is egg protein, with a value of 100. Animal proteins in milk and meat have a high BV (~90). Proteins in cereal and vegetables have a lower BV (~40) Some derived proteins (eg, gelatin) have a BV of 0. The extent to which dietary proteins supply each other's missing amino acids (complementarity) determines the overall BV of the diet. The recommended daily allowances (RDA) for protein assumes that the average mixed diet has a BV of 70. 