Lakhasly

Protein metabolism Amar B Elhussein BSc, MSc & PhD ?DIETARY PROTEIN: o Humans have no dietary requirement for protein, per se, but the protein in food does provide essential amino acids.o Thus, eating foods with different amino acids during the day can result in a dietary combination with a higher biologic value than either of the component proteins o [Note: Animal proteins can also complement the biologic value of plant proteins.] ?Nitrogen balance o Nitrogen balance occurs when the amount of nitrogen consumed equals that of the nitrogen excreted in the urine, sweat, and feces.o Protein consumed in excess of the body's needs is deaminated, and the resulting carbon skeletons are metabolized to provide energy or acetyl coenzyme A for fatty acid synthesis.o When excess protein is eliminated from the body as urinary nitrogen, it is often accompanied by increased urinary calcium, increasing the risk of nephrolithiasis and osteoporosis.Proteins from animal sources: o Proteins from animal sources (meat, poultry, milk, and fish) have a high quality because they contain all the essential amino acids in proportions similar to those required for synthesis of human tissue proteins.o When the intake of carbohydrates is low, amino acids are deaminated to provide carbon skeletons for the synthesis of glucose that is needed as a fuel by the central nervous system.o [Note: Gelatin prepared from animal collagen is an exception; it has a low biologic value as a result of deficiencies in several essential amino acids.] 2.o It is associated with inadequate dietary protein, lack of an essential amino acid, or during physiologic stresses, such as trauma, burns, illness, or surgery.o If carbohydrate intake is less than 130 g/day, substantial amounts of protein are metabolized to provide precursors for gluconeogenesis.The quality of a dietary protein is a measure of its ability to provide the essential amino acids required for tissue maintenance.?Most government agencies have adopted the Protein Digestibility- Corrected Amino Acid Score (PDCAAS) as the standard by which to evaluate protein quality.This amino acid score provides a method to balance intakes of poorer-quality proteins with high-quality dietary proteins.o For example, wheat (lysine-deficient but methionine-rich) may be combined with kidney beans (methionine-poor but lysine-rich) to produce an improved biologic value.o The RDA for protein is computed for proteins of mixed biologic value at 0.8 g/kg of body weight for adults, or about 56 g of protein for a 70-kg individual.?The protein-sparing effect of carbohydrate: o The dietary protein requirement is influenced by the carbohydrate content of the diet.o Nine of the 20 amino acids needed for the synthesis of body proteins are essential--that is, they cannot be synthesized in humans.PDCAAS is based on the profile of essential amino acids and the digestibility of the protein.o However, proteins from different plant sources may be combined in such a way that the result is equivalent in nutritional value to animal protein.o People who exercise strenuously on a regular basis may benefit from extra protein to maintain muscle mass; a daily intake of about 1 g/kg has been recommended for athletes.I. Positive nitrogen balance: o This occurs when nitrogen intake exceeds nitrogen excretion.?Requirement for protein in humans o The amount of dietary protein required in the diet varies with its biologic value.?Consumption of excess protein: o There is no physiologic advantage to the consumption of more protein than the RDA.o It is observed during situations in which tissue growth occurs, for example, in childhood, pregnancy, or during recovery from an emaciating illness.II. Negative nitrogen balance: o This occurs when nitrogen loss is greater than nitrogen intake.Proteins from plant sources: o Proteins from wheat, corn, rice, and beans have a lower quality than do animal proteins.o The greater the proportion of animal protein included in the diet, the less protein is required.o Women who are pregnant or lactating require up to 30 g/day in addition to their basal requirements.o To support growth, infants should consume 2 g/kg/day.?Quality of proteins ?The highest possible score under these guidelines is 1.00.o Most healthy adults are normally in nitrogen balance.???1.

Protein metabolism
Amar B Elhussein
BSc, MSc & PhD
➢DIETARY PROTEIN:
• Humans have no dietary requirement for protein, per se,
but the protein in food does provide essential amino
acids.
• Nine of the 20 amino acids needed for the synthesis of
body proteins are essential—that is, they cannot be
synthesized in humans.
❑Quality of proteins
✓ The quality of a dietary protein is a measure of its ability
to provide the essential amino acids required for tissue
maintenance.
✓Most government agencies have adopted the Protein
Digestibility- Corrected Amino Acid Score (PDCAAS) as
the standard by which to evaluate protein quality.
✓ PDCAAS is based on the profile of essential amino acids
and the digestibility of the protein.
✓ The highest possible score under these guidelines is 1.00.
✓ This amino acid score provides a method to balance
intakes of poorer-quality proteins with high-quality
dietary proteins.

1. Proteins from animal sources:
   • Proteins from animal sources (meat, poultry, milk, and
   fish) have a high quality because they contain all the
   essential amino acids in proportions similar to those
   required for synthesis of human tissue proteins.
   • [Note: Gelatin prepared from animal collagen is an
   exception; it has a low biologic value as a result of
   deficiencies in several essential amino acids.]


2. Proteins from plant sources:
   • Proteins from wheat, corn, rice, and beans have a lower
   quality than do animal proteins.
   • However, proteins from different plant sources may be
   combined in such a way that the result is equivalent in
   nutritional value to animal protein.
   • For example, wheat (lysine-deficient but methionine-rich)
   may be combined with kidney beans (methionine-poor but
   lysine-rich) to produce an improved biologic value.
   • Thus, eating foods with different amino acids during the
   day can result in a dietary combination with a higher
   biologic value than either of the component proteins
   • [Note: Animal proteins can also complement the biologic
   value of plant proteins.]
   ❑Nitrogen balance
   • Nitrogen balance occurs when the amount of nitrogen
   consumed equals that of the nitrogen excreted in the urine,
   sweat, and feces.
   • Most healthy adults are normally in nitrogen balance.
   I. Positive nitrogen balance:
   • This occurs when nitrogen intake exceeds nitrogen excretion.
   • It is observed during situations in which tissue growth occurs,
   for example, in childhood, pregnancy, or during recovery from
   an emaciating illness.
   II. Negative nitrogen balance:
   • This occurs when nitrogen loss is greater than nitrogen
   intake.
   • It is associated with inadequate dietary protein, lack of an
   essential amino acid, or during physiologic stresses, such
   as trauma, burns, illness, or surgery.
   ❑Requirement for protein in humans
   • The amount of dietary protein required in the diet varies
   with its biologic value.
   • The greater the proportion of animal protein included in
   the diet, the less protein is required.
   • The RDA for protein is computed for proteins of mixed
   biologic value at 0.8 g/kg of body weight for adults, or
   about 56 g of protein for a 70-kg individual.
   • People who exercise strenuously on a regular basis may
   benefit from extra protein to maintain muscle mass; a
   daily intake of about 1 g/kg has been recommended for
   athletes.
   • Women who are pregnant or lactating require up to 30
   g/day in addition to their basal requirements.
   • To support growth, infants should consume 2 g/kg/day.
   ❑Consumption of excess protein:
   • There is no physiologic advantage to the consumption of more
   protein than the RDA.
   • Protein consumed in excess of the body’s needs is deaminated,
   and the resulting carbon skeletons are metabolized to provide
   energy or acetyl coenzyme A for fatty acid synthesis.
   
   • When excess protein is eliminated from the body as urinary
   nitrogen, it is often accompanied by increased urinary calcium,
   increasing the risk of nephrolithiasis and osteoporosis.
   ❑The protein-sparing effect of carbohydrate:
   • The dietary protein requirement is influenced by the
   carbohydrate content of the diet.
   • When the intake of carbohydrates is low, amino acids are
   deaminated to provide carbon skeletons for the synthesis
   of glucose that is needed as a fuel by the central nervous
   system.
   • If carbohydrate intake is less than 130 g/day, substantial
   amounts of protein are metabolized to provide precursors
   for gluconeogenesis.
   • Therefore, carbohydrate is considered to be “protein-
   sparing,” because it allows amino acids to be used for
   repair and maintenance of tissue protein rather than for
   gluconeogenesis.
   ❑Protein energy (calorie) malnutrition (PEM)
   • In developed countries, PEM is most frequently seen in
   patients with medical conditions that decrease appetite or
   alter how nutrients are digested or absorbed, or in
   hospitalized patients with major trauma or infections.
   • [Note: Such highly catabolic patients frequently require
   intravenous (parenteral) or tube-based (enteral)
   administration of nutrients.]
   • PEM may also be seen in children or the elderly who are
   malnourished.
   • In developing countries, an inadequate intake of protein and/or
   energy is the primary cause of PEM.
   • Affected individuals show a variety of symptoms, including a
   depressed immune system with a reduced ability to resist
   infection.
   • Death from secondary infection is common.
   • Two extreme forms of PEM are kwashiorkor and marasmus
   ❑Kwashiorkor:
   • Kwashiorkor occurs when protein deprivation is
   relatively greater than the reduction in total calories.
   • Protein deprivation is associated with severely
   decreased synthesis of visceral protein.
   • Kwashiorkor is frequently seen in children after weaning
   at about one year of age, when their diet consists
   predominantly of carbohydrates.
   ❖Typical symptoms include:
   • Stunted growth, edema, skin lesions, depigmented
   hair, anorexia, enlarged fatty liver, and decreased
   plasma albumin concentration.
   • Edema results from the lack of adequate plasma proteins
   to maintain the distribution of water between blood and
   tissues.
   • Edema may mask muscle loss.
   • Marasmus:
   • Marasmus occurs when calorie deprivation is relatively
   greater than the reduction in protein.
   • It usually occurs in children younger than 1 year of age
   when breast milk is supplemented with watery gruels of
   native cereals that are usually deficient in protein and
   calories.
   ❑Typical symptoms include:
   • Arrested growth, extreme muscle wasting
   (emaciation), weakness, and anemia.
   • Victims of marasmus do not show the edema or changes
   in plasma proteins observed in kwashiorkor.
   A. Listless child with kwashiorkor.
   B. Child suffering with marasmus.