Macronutrients Macronutrients **************** **************** Proteins Proteins
Dec 31, 2015
MacronutrientsMacronutrients
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ProteinsProteins
Protein Nutrition and Protein Nutrition and MetabolismMetabolism• 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
• Protein malnutrition, also called Kwashiorkor, is a common problem in less developed countries where meat, fish and other good sources of protein are scarce
• 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
Indispensible Amino Acids Indispensible Amino Acids
also calledalso called
Essential Amino AcidsEssential Amino Acids
Branch chain AAs Aromatic AAs
Val
His
Lys
Ile
Leu
Met
Thr
Trp
Phe
Basic AAsOther AAs
Indispensible Amino AcidsIndispensible Amino Acids
• 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
• infancy and childhood
• pregnancy
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.
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, See “Protein DRI 2002” table on p. 3,
Macronutrient-III handoutMacronutrient-III handout
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 cf., Table 4.2, Macronutrient-III handout, p. 5handout, p. 5
• 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
• 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
• 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
Table 4.2. Amino Acid as Percent Protein in FoodsProtein Sulfur
food Lys AAs Thr Trp Leu
Ideal 5.5 3.5 4.0 1.0 7.0
Egg 6.4 5.5 5.0 1.6 8.8
(12.8% protein)
Milk (cow) 7.8 3.3 4.6 1.4 9.8
3.5% protein
Beef (hamburger) 8.7 3.8 4.4 1.2 8.2
Chicken 8.8 4.0 4.3 1.2 7.2
20.6% protein
Soybeans 6.9 3.4 4.3 1.5 8.4
34.9% protein
Black beans 6.4 2.6 3.4 1.0 8.7
23.6% protein
Lentils 6.1 1.5 3.6 0.9 7.0
25.0% protein
Cornmeal 2.9 3.2 4.0 0.6 3.0
9.2% protein
Oatmeal 3.7 3.6 3.3 1.3 7.5
14.2% protein
Collagen 3.4 0.9 1.8 0.0 3.0
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
When assessing protein content in the diet,another factor which has to be taken intoaccount is protein digestability. In general,animal protein is more digestible than proteinsof plant origin.
Table 4.4. Digestibility of Food Proteins Food Digestibility of Protein (%) Eggs 97Meats, poultry, fish 85 - 100Milk 81Wheat 91 - 95Corn 90Soybeans 90Other legumes 73 - 85
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
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
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
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 (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
(a) Positive N balancegrowth, lactation,recovery from metabolic stress
DietaryProtein(N in)
Tissue protein
Amino acidpool
Excretion asurea + NH4
+
(N out)
Purines, heme, etc.energy
(A)
Adapted from Devlin, 5/e (2002) fig. 26.1
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
• 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)
DietaryProtein(N in)
Tissue protein
Amino acidpool
Excretion asurea + NH4
+
(N out)
Purines, heme, etc.energy
(B)
(b) Negative N balancemetabolic stress
Adapted from Devlin, 5/e (2002) fig. 26.1
DietaryProtein(N in)
Tissue protein
Amino acidpool
Excretion asurea + NH4
+
(N out)
Purines, heme, etc.energy
(C)
(c) Negative N balanceinadequate dietary protein
Adapted from Devlin, 5/e (2002) fig. 26.1
• 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.
DietaryProtein(N in)
Tissue protein
Amino acidpool
Excretion asurea + NH4
+
(N out)
Purines, heme, etc.energy
(D)
(d) Negative N balancelack of an essential amino acid
Adapted from Devlin, 5/e (2002) fig. 26.1
• 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).
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 MealShort Term Effects of High Protein Meal
• During absorptive phase following eatinga 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
• 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
Fig. 26.8 Fig. 26.12