macronutrients **************** proteins

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)


Tissue protein

Amino acidpool


+

(N out)


(B)

(b) Negative N balancemetabolic stress



Tissue protein

Amino acidpool


+

(N out)


(C)

(c) Negative N balanceinadequate dietary protein


• 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.


Tissue protein

Amino acidpool


+

(N out)


(D)

(d) Negative N balancelack of an essential amino acid


• 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

macronutrients **************** proteins

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