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Macronutrients **************** Proteins Protein Nutrition and Metabolism In the U. S. and other industrialized nations average adult consumes ~100 g protein/day This accounts for about 12% of daily caloric need This is about 2x the RDA set by the U. S. and other countries and agencies

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slide1

Macronutrients

****************

Proteins

protein nutrition and metabolism
Protein Nutrition and Metabolism
  • In the U. S. and other industrialized nations average adult consumes ~100 g protein/day
  • This accounts for about 12% of daily caloric need
  • This is about 2x the RDA set by the U. S. and other countries and agencies
  • Intake of protein in U. S. has remained rather constant since 1900, when it was ~10% of consumed calories
  • However, proportion of animal protein has more than doubled in the intervening period
slide3
Protein malnutrition, also called Kwashiorkor, is a common problem in less developed countries where meat, fish and other good sources of protein are scarce
slide4
In addition to ingested protein, another ~70g/day of protein enters digestive system via gastric and intestinal juices, digestive enzymes, and cells sloughed from lining of gastrointestinal tract
  • Note: life span of the gastrointestinal mucosal cell is about 3-4 days; this means that 1/4 to 1/3 of these cells are sloughed daily
  • Of this daily total of ~170 g of protein entering digestive tract, about 1.6 g total N (=10 g protein) is excreted in the feces
  • Remaining 160 g of protein enzymatically hydrolyzed to amino acids and small peptides
slide6

Indispensible Amino Acids

also called

Essential Amino Acids

slide7

Indispensible Amino Acids

Branch chain AAs

Aromatic AAs

Val

Phe

Leu

Trp

Ile

Other AAs

Basic AAs

Thr

Lys

His

Met

slide8
Much, but not all, of the methionine requirement can be replaced by dietary cysteine, since there is a pathway for conversion of MET to CYS
  • Much, but not all, of the phenylalanine requirement can be replaced by dietary tyrosine, since there is a pathway for conversion of PHE to TYR
  • In this way CYS and TYR serve to “spare” requirements for MET and PHE, respectively
arginine is synthesized by humans but not at a rate to meet needs during times of rapid growth
Arginine is synthesized by humans, but not at a rate to meet needs during times of rapid growth
  • infancy and childhood
  • pregnancy
dietary protein requirement
Dietary Protein Requirement
  • In 1985, WHO/FAO/UNO set daily protein requirement for adults at 0.75g/kg body wt
  • This has been accepted by U. S. and Canadian governments
  • Current (2002) RDA is 0.80 g/kg “ideal body weight” per day for adults
  • This is 56 g/day for adult males and 46 g/day for adult females in U. S.
slide11
Note:
  • There are significant differences in protein RDA as a function of age and during pregnancy and lactation
  • For example, infants from birth to 6 months of age have a protein AI of 9.1 g/day (1.52 g/d/kg body weight)
  • See “Protein DRI 2002” table on p. 3, Macronutrient-III handout
sources of protein and protein quality
Sources of Protein and Protein Quality
  • Numerous studies carried out to determine normal human requirements for individual essential amino acids
  • This has led to the formulation of a so-called “ideal” protein
  • cf., Table 4.2, Macronutrient-III handout, p. 5
slide14
Proportion of essential amino acids in “ideal” protein similar to that found in eggs and milk proteins
  • In general, proteins from animals, including fish and fowl, have good proportions of essential amino acids
  • Except for soybean protein, most plant proteins do not meet the ideal and usually are short of ideal in one or two of the essential amino acids
slide15
Grains and nuts tend to be low in lysine and, sometimes, tryptophan
  • Legumes tend to be deficient in sulfur amino acids, although they are important as concentrated protein foods
  • As a consequence, care must be taken to combine vegetable proteins to insure combinations will supply adequate amounts of essential amino acids
slide16
For example, black beans are deficient in sulfur amino acids, while corn meal is deficient in lysine and tryptophan
  • However, in appropriate combination, black beans and corn meal constitute a complete “ideal” protein
slide18
This is of special importance to pure vegetarians (vegans) who have no milk or egg protein in their diets.

However, this seems not to be a problem in the U. S. where vegans eat considerably more protein than they require, thus making up for deficiencies in any specific essential amino acid

nitrogen balance
Nitrogen Balance

Some important relationships to remember:

  • Protein = 16% N

Therefore: 0.16 x g protein = g N or

6.25 x g N = g protein

slide21

To do a completely accurate N balance study on an individual would require measuring all sources of N loss from the body.

This is very difficult even in research setting and, pragmatically, is not possible in clinical setting

slide22

What is used are estimates based on estimating protein intake per day from standard tables of nutrient content for various foods and comparing that to the total N excreted in feces and urine

or, more commonly

comparing the N in a 24-hour urine sample and estimating the non-urinary N losses from literature values

in clinical setting the procedure involves use of an empirically derived formula
In clinical setting, the procedure involves use of an empirically derived formula

N balance = (Protein intake/6.25) – [(1.25 x urinary urea N) + 4] (grams) (grams)

NOTE:

  • 1.25 corrects for the fact that not all urinary N is in form of urea
  • 4 grams added are estimate of N loss by non-urinary routes
nitrogen balance n in n out is positive for
Nitrogen balance (Nin - Nout) is positive for:
  • growing infants and children
  • pregnant or lactating women or body-building adult
  • when there is tissue growth or replenishment such as recovering from metabolic stress or nutritional deficiency
  • Adults receiving a minimally adequate or greater amount of protein will be at zero balance, where input = output
slide25

(a) Positive N balance

growth, lactation,

recovery from metabolic stress

Adapted from Devlin, 5/e (2002) fig. 26.1

negative n balance occurs
Negative N balance occurs:
  • in fasting or starvation when there is no or inadequate protein intake
  • in pathological conditions (burns, traumatic injury, fevers) and in severe psychological stress
slide27
These are all conditions in which body function is diverted or activity reduced relative to the normal (bed confinement causes muscle atrophy) and/or
  • conditions when there is abnormally high secretion of glucocortico-steroids (which causes catabolism of muscle protein)
slide28

(b) Negative N balance

metabolic stress

Adapted from Devlin, 5/e (2002) fig. 26.1

slide29

(c) Negative N balance

inadequate dietary protein

Adapted from Devlin, 5/e (2002) fig. 26.1

slide30
It should also be noted that no matter how much protein is ingested, if there is an essential amino acid deficiency, there will be a negative protein balance
  • This is because the other amino acids absorbed cannot be used for protein synthesis to replace those proteins lost during normal daily protein turnover.
slide31

(d) Negative N balance

lack of an essential amino acid

Adapted from Devlin, 5/e (2002) fig. 26.1

slide32
The daily requirement for dietary protein may more than double, both acutely and long term, for patients with burns or injuries to support tissue healing.
  • Requirements also may be increased in terminal cancer and total parenteral nutrition (TPN, formerly called hyperalimentation) for such patients is often carried out (no evidence that it prolongs life).
slide33

On the other hand, restricted (decreased) protein intake is indicated in the treatment of persons with liver, kidney, or intestinal diseases, since these organs are highly involved in the absorption, breakdown, and excretion of protein metabolites

short term effects of high protein meal
Short Term Effects of High Protein Meal
  • During absorptive phase following eating a high protein, low carb meal get:
  • increase in glucagon secretion (due to low blood glucose)
  • also have increase in insulin secretion, but much lower than found following typical carbohydrate-containing meal
slide35
Increased insulin is sufficient to promote protein synthesis, but not high enough to prevent gluconeogenesis
  • Overall outcome is that amino acids can be used for protein synthesis and gluconeogenesis, oxidized for energy, or possibly stored as glycogen and fat
slide37

Fig. 26.8

Fig. 26.12