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
•
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
also called
Essential Amino Acids
Indispensible Amino Acids
Branch chain AAs
Aromatic AAs
Val
Phe
Leu
Trp
Ile
Other AAs
Basic AAs
Lys
His
Thr
Met
• 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,
Macronutrient-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 handout, 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 Foods
Protein
food
Ideal
Egg
(12.8% protein)
Milk (cow)
3.5% protein
Beef (hamburger)
Chicken
20.6% protein
Soybeans
34.9% protein
Black beans
23.6% protein
Lentils
25.0% protein
Cornmeal
9.2% protein
Oatmeal
14.2% protein
Collagen
Sulfur
AAs
3.5
5.5
Thr
4.0
5.0
Trp
1.0
1.6
Leu
7.0
8.8
7.8
3.3
4.6
1.4
9.8
8.7
8.8
3.8
4.0
4.4
4.3
1.2
1.2
8.2
7.2
6.9
3.4
4.3
1.5
8.4
6.4
2.6
3.4
1.0
8.7
6.1
1.5
3.6
0.9
7.0
2.9
3.2
4.0
0.6
3.0
3.7
3.6
3.3
1.3
7.5
3.4
0.9
1.8
0.0
3.0
Lys
5.5
6.4
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 into
account is protein
digestability. In general,
animal protein is more digestible than proteins
of plant origin.
Table 4.4. Digestibility of Food Proteins
Food
D igestibility of Protein (%)
Eggs
Meats, poultry, fish
Milk
Wheat
Corn
Soybeans
Other legumes
97
85 - 100
81
91 - 95
90
90
73 - 85
Nitrogen Balance
Some important relationships to remember:
•
Protein = 16% N
Therefore:
0.16 x g protein = g N
6.25 x g N = g protein
or
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 =
intake/6.25) – [(1.25 x urinary urea N) + 4]
(grams)
(Protein
(grams)
NOTE:
•
1.25 corrects for the fact that not all urinary N is
of urea
in form
4 grams added are estimate of N loss by
urinary routes
non-
•
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 balance
growth, lactation,
recovery from metabolic stress
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)
(b) Negative N balance
metabolic stress
Adapted from Devlin, 5/e (2002) fig. 26.1
(c) Negative N balance
inadequate 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.
(d) Negative N balance
lack 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 Meal
During absorptive phase following eating a high
protein, low carb meal get:
•

increase in glucagon secretion (due to
blood glucose)
low
also have increase in insulin secretion,
much lower than found following
typical
carbohydrate-containing meal
but

•
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