Protein

"We have discussed what proteins are, how they are made, how they are digested and absorbed, the many functions of proteins in the body, and the consequences of having too little or too much protein in the diet. This section will provide you with information on how to determine the recommended amount of protein for you, and your many choices in designing an optimal diet with high-quality protein sources.\n\nThe recommendations set by the IOM for the Recommended Daily Allowance (RDA) and AMDR for protein for different age groups are listed in Table 6.2 “Dietary Reference Intakes for Protein”. A Tolerable Upper Intake Limit for protein has not been set, but it is recommended that you do not exceed the upper end of the AMDR. Infants ages 0–6 months have an RDA of 9.1 grams per day, denoted as Adequate Intake, and infants ages 7–12 months have an RDA of 11.0 grams per day; an AMDR is not determined for either group. Children ages 1–3 have an RDA of 13.0 grams per day and an AMDR of 5–20 percent of calories. Children ages 4–8 have an RDA of 19.0 grams per day and an AMDR of 10–30 percent. Children ages 9–13 have an RDA of 34.0 grams per day and an AMDR of 10–30 percent. Males ages 14–18 have an RDA of 52.0 grams per day, females ages 14–18 have an RDA of 46.0 grams per day, and both have an AMDR of 10–30 percent. Adult males ages 19 and older have an RDA of 56.0 grams per day, and adult females ages 19 and older have an RDA of 46.0 grams per day; both have an AMDR of 10–35 percent.\n\nThe appropriate amount of protein in a person’s diet is that which maintains a balance between what is taken in and what is used. The RDAs for protein were determined by assessing nitrogen balance. Nitrogen is one of the four basic elements contained in all amino acids. When proteins are broken down and amino acids are catabolized, nitrogen is released. Remember that when the liver breaks down amino acids, it produces ammonia, which is rapidly converted to nontoxic, nitrogen-containing urea, which is then transported to the kidneys for excretion. Most nitrogen is lost as urea in the urine, but urea is also excreted in the feces. Proteins are also lost in sweat and as hair and nails grow. The RDA, therefore, is the amount of protein a person should consume in their diet to balance the amount of protein used up and lost from the body. For healthy adults, this amount of protein was determined to be 0.8 grams of protein per kilogram of body weight. You can calculate your exact recommended protein intake per day based on your weight by using the following equation: (Weight in lbs. ÷ 2.2 lb/kg) × 0.8 g/kg. Note that if a person is overweight, the amount of dietary protein recommended can be overestimated.\n\nThe IOM used data from multiple studies that determined nitrogen balance in people of different age groups to calculate the RDA for protein. A person is said to be in nitrogen balance when the nitrogen input equals the amount of nitrogen used and excreted. A person is in negative nitrogen balance when the amount of excreted nitrogen is greater than that consumed, meaning that the body is breaking down more protein to meet its demands. This state of imbalance can occur in people who have certain diseases, such as cancer or muscular dystrophy. Someone who has a low-protein diet may also be in negative nitrogen balance as they are taking in less protein than what they actually need. Positive nitrogen balance occurs when a person excretes less nitrogen than what is taken in by the diet, such as during child growth or pregnancy. At these times the body requires more protein to build new tissues, so more of what gets consumed gets used up and less nitrogen is excreted. A person healing from a severe wound may also be in positive nitrogen balance because protein is being used up to repair tissues."