AMDR for protein: 10% - 35%

• provides the essential amino acids

Protein Quality

Protein Quality: a measure of a dietary proteins’ ability to provide the essential amino acids (EAA) required for tissue maintenance

Protein Digestibility-corrected amino acid score (PDCAAS): the standard adopted to evaluate protein quality, based upon the profile of essential amino acids and digestibility of protein.

• highest score: 1.00

• provides a method to balances intakes of poorer-quality proteins with high-quality proteins

Proteins from animal sources:

• meat poultry, milk and fish

• high quality (contain all the EAA and are more readily digested)

  • Gelatin is an exception, it has low biological value

Proteins from plant sources:

• lower quality than animal proteins

• proteins from different plant sources may be combined to form high biological value

  • e.g. wheat (lysine deficient but methionine rich) may be combined with kidney beans (methionine poor but lysine rich)

Nitrogen Balance

Nitrogen balance: amount of nitrogen consumed equals that of the nitrogen excreted.

• most normal healthy adults are normally in nitrogen balance

Positive Nitrogen Balance

Positive nitrogen balance: nitrogen intake exceeds nitrogen excretion

Occurs in situations in which tissue growth is observed:

• childhood

• pregnancy

• convalescing

Negative Nitrogen Balance

Negative nitrogen balance: nitrogen loss is greater than nitrogen intake.

Associated with:

• inadequate dietary protein

• lack of an essential amino acid

• during physiological stress

  • trauma

  • burn

  • illness

  • surgery

Protein Requirements

• amount of dietary protein required varies with its biological value

• disease states influence protein needs

  • protein restriction may be needed in kidney disease

  • burns require increased protein intake

• recommended intake: 0.8g/kg/day

• people who exercise strenuously on a regular basis may benefit from extra protein to maintain muscle mass

  • daily intake of: ~1g/kg/day

• pregnant or lactating women require up to 30g/kg in addition to their basal requirements

• infants should consume 2 g/kg/day

Consumption of excess protein:

• no physiological advantage to the consumption of more protein than the RDA

• protein consumed in excess of the body’s needs is deaminated

  • the resulting carbon skeletons are metabolized to provide energy or acetyl CoA for fatty acid synthesis

• when excess is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, increasing the risk of nephrolithiasis (kidney stones) and osteoporosis

The protein-sparing effect of carbohydrates:

• dietary protein requirement is influenced by the carbohydrate content of the diet

  • if carbohydrate intake is low amino acids are deaminated to provide carbon skeletons for the synthesis of glucose that is needed as fuel for the central nervous system

  • if carbohydrate intake is less than 130 g/day sustainable amounts of protein are metabolized to provide precursors for gluconeogenesis

• carbohydrate allows amino acids to be used for repair and maintenance of tissue protein rather than for gluconeogenesis

Protein-energy (calorie) malnutrition (PEM)

• also known as protein-energy undernutrition (PEU)

• in developed countriess it is mostly seen in patients with medical conditions that:

  • decrease appetite

  • alter how nutrients are digested or absorbed

  • in hospitalized patients with major trauma or infections

    • often require intravenous or tube-based administration of nutrients

• may be seen in children or elderly who are malnourished

• inadequate intake of protein and/or energy is the primary cause of PEM in developing countries

• symptoms include depressed immune system, reduced ability to resist infection

• secondary infections can lead to death

Two extreme forms of PEM:

• Kwashiorkor

• Marasmus

Kwashiorkor

• protein deprivation is relatively greater than the reduction in total calories

  • associated with severely decreased synthesis of visceral protein

• commonly seen in developing countries in children after weaning at about the age of 1 year

Typical symptoms:

• stunted growth

• skin lesions

• depigmented hair

• anorexia

• edema (results from the lack of adequate blood proteins to maintain the distribution of water between blood and tissues)

• fatty liver

• decreased serum albumin concentration

Marasmus

• calorie deprivation is relatively greater than the reduction in protein

• usually occurs in children younger than 1 year of age

  • when the mother’s breast milk is supplemented with thin watery gruels of native cereals, which are usually deficient in protein and calories

Typical symptoms:

• arrested growth

• extreme muscle wasting and depletion of subcutaneous fat (emaciatation)

• weakness

• anemia