Vitamin C, or L-ascorbic acid, or ascorbate, is an essential nutrient for humans, a water-soluble vitamin. Humans, unlike most animals, are unable to synthesize vitamin C, so it is an essential dietary component.
Vitamin C is required for the biosynthesis of collagen (an essential component of connective tissue), L-carnitine, and certain neurotransmitters, it is also involved in protein metabolism.
Vitamin C is also an important physiological antioxidant and has been shown to regenerate other antioxidants within the body, including alpha-tocopherol (vitamin E). Vitamin C regenerates vitamin E by reducing vitamin E radicals formed when vitamin E scavenges the oxygen radicals.
Vitamin C plays an important role in immune function and improves the absorption of nonheme iron, the form of iron present in plant-based foods.
Approximately 70%–90% of vitamin C is absorbed at moderate intakes of 30–180 mg a day. At doses above 1 g a day, absorption falls to less than 50% and absorbed, unmetabolized ascorbic acid is excreted in the urine.
Insufficient vitamin C intake causes scurvy, which is characterized by fatigue or lassitude, connective tissue weakness, and capillary fragility.
Cells accumulate vitamin C. The total body content of vitamin C ranges from 300 mg (at near scurvy) to about 2 g.
High levels of vitamin C are maintained in cells and tissues, and are highest in leukocytes (white blood cells), eyes, adrenal glands, pituitary gland, and brain.
Relatively low levels of vitamin C are found in extracellular fluids, such as plasma, red blood cells, and saliva.
A sharp distinction between humans and “animals” is essential if we are to bend them to our will, make them work for us, wear them, eat them–without any disquieting tinges of guilt or regret. It is unseemly of us, who often behave so unfeeling toward other animals, to contend that only humans can suffer. The behavior of other animals renders such pretensions specious. They are just too much like us.
The protein digestibility-corrected amino acid score (PDCAAS) has been adopted by FAO/WHO as the preferred method for the measurement of the protein value in human nutrition.
PDCAAS = Amino Acid Score x Digestibility
The method is based on comparison of the concentration of the first limiting essential amino acid in the test protein with the concentration of that amino acid in a reference (scoring) pattern. This scoring pattern is derived from the essential amino acid requirements of the preschool-age child.
Although the principle of the PDCAAS method has been widely accepted, critical questions have been raised in the scientific community:
the validity of the preschool-age child amino acid requirement values (more than 4 times greater than the EAA requirement for an adult),
the validity of correction for fecal instead of ileal digestibility,
the truncation of PDCAAS values to 100%.
The reference scoring pattern was based on studies performed more than 25 years ago on a limited number of 2-year-old children recovering from malnutrition.
According to the current official recommendations, a 2-year old child needs ~ 3x higher essential-to-non-essential amino acid ratio, and needs essential amino acids in different proportions than adult. Methionine/cysteine is the limiting essential amino acids for adults, and for children it is lysine or tryptophan.
The use of fecal digestibility overestimates the nutritional value of a protein because amino acid nitrogen entering the colon is lost for protein synthesis in the body and is, at least in part, excreted in urine as ammonia.