Essential source for building and maintaining body proteins.
Requires essential amino acids and a N2 source (non-essential amino acids)
There is a wide range between the minimum amount of protein required and the toxic (maximum) amount
The RDA is 50% higher than what is normally required to allow for differences in protein quality and digestibility
Protein Intake = Catabolism + skin/hair Loss + Growth
occurs all the time, for 70 kg man ~ 23 grams/day
Catabolism increased with burns, trauma, severe infection, and starvation. Clinically catabolism can be diminished by providing a source of energy (glucose).
(also lactation and blood loss too) ~ 5 grams/day
, there is typically no growth in a fully grown 70 kg man.
In babies as much as 70% of protein intake is going towards growth. (total protein requirements are greater) Human infant protein requirements are reflected in the content of human breast milk, which is well suited for newborns in both total protein content and quality (quality = content/mix of essential amino acids) However premature infants need more protein than is available in human breast milk but less than is in cow milk (the large amount of protein in cow’s milk is too much for their underdeveloped kidneys and liver) so preemies get formula.
Therefore the minimum protein intake for adult is 25-30 grams
US diet men average 118 (43-235) women average 64 (19-130)
Do not see protein deficiency in US. When seen it is usually linked to special diets (e.g. IV glucose only)
Eating excess protein will not increase protein synthesis
Sources of Protein in the Diet
Relatively few natural foods lack protein (fruit is the exception)
Refined foods (fats, oils, sugars, and starch) contain no protein
Staple foods (corn, wheat, potatoes, rice) provide excellent sources of protein
Combining foods (protein complementation) with different qualities of proteins increases the total protein quality of the meal. (beans and rice)
General Scheme of Nitrogen Metabolism
Dietary protein is digested and absorbed (what is not digested and absorbed is lost in the stool)
Essential amino acids and non-essential amino acids are used to create and replace body proteins
The left over amino acids are deaminated and their carbon skeletons used for energy
The amino groups are transformed into Urea which goes out in the urine.
Protein and Disease
in adults the tolerance of excess protein is high and does not become a problem until >40% of energy comes from protein. Newborns, especially preemies, the range between minimum and toxic protein intake levels is much smaller. Excess protein may cause liver injury, and the large renal solute load of Urea N2, Na+, and K+ may exceed renal capacity.
lack of protein results in growth failure, deterioration of intestinal mucosa, anemia, impaired wound healing, thin and easily removed hair, hypoproteinemia, and fatty liver. Low serum albumin would be due to increased protein losses rather than decreased synthesis therefore not specific to protein deficiency but to other causes.
There is a large reserve of potentially consumable muscle protein in the body relative to normal requirements. In a typical adult about ˝ of total protein (6kg) can be utilized before muscle weakness becomes incompatible with life.
During starvation, muscle protein is broken down to provide glucose for the body. Even if one has zero protein intake catabolism is taking place.
"Kwashiorkor" – is the multiple nutrient deficiency syndrome; not due to specific protein deficiency
Effects of Disease on Protein Requirements and Tolerance
obligatory loss due to: surgery, burns, trauma, nephrotic syndrome
treatment: give extra protein to keep up with losses
tolerance in general due to liver or renal disease
treatment: low protein diet providing only essential amino acids
tolerance to one specific amino acid
treatment: a diet that lacks that amino acid but has all the others.