Agriculture Reference
In-Depth Information
yogurt contain somewhat less, mostly as methyl- and hydroxocobalamin. Vitamin
supplements usually contain cyanocobalamin or hydroxocobalamin (dark red in pure
form, pink when diluted). Recommended dietary allowances range from 0.9 µg per
day for 1- to 3-year-old children to 2.8 µg per day for lactating women.
v
i t a m i n
C
Vitamin C (ascorbic acid or ascorbate) is not manufactured by tissues of inverte-
brate and fish species that have been studied. It is likewise a dietary essential for
humans, guinea pigs, fruit bats, some birds, and most nonhuman primates because of
a missing tissue enzyme, l-gulanolactone oxidase, required for its synthesis. It can be
synthesized in the liver or kidneys of other terrestrial animal species that have been
investigated, including some amphibians, reptiles, and other mammals. Ascorbic acid
functions as an antioxidant and as a cofactor for hydroxylating enzymes that partici-
pate in synthesis of collagen, carnitine, and neurotransmitters (e.g., norepinephrine
and serotonin) and in synthesis and catabolism of tyrosine. Its role in these reactions
is to maintain iron and copper atoms in their reduced state within metalloenzymes.
Absorption of lower dietary concentrations in the small intestine involves a
sodium-dependent active transport system in the brush border of the enterocyte;
higher intakes may be absorbed by simple diffusion. Prior to absorption, ascorbate
may be oxidized to dehydroascorbate, followed by reduction (requiring glutathione)
within the enterocyte back to ascorbate. Transport across the basolateral membrane
into the portal blood is carrier mediated. Free ascorbic acid is the predominant form
in the plasma.
Good food sources include citrus fruits and juices, asparagus, broccoli, kale,
papaya, and strawberries. Vitamin supplements may contain ascorbic acid, cal-
cium ascorbate, sodium ascorbate, ascorbyl palmitate, or ascorbyl polyphosphate.
Recommended dietary allowances range from 15 mg per day for 1- to 3-year-old
children to 120 mg per day for lactating women.
mIneRAls ARe not Just Rocks
Recognition that certain mineral elements are required nutrients was associated with
discovery of their regular presence in body tissues. In 1874, J. Forster concluded
that elements consistently found by analysis in animal tissues must be essential for
life and thus should be present in the diet. However, connections between some ele-
ments and human or animal health were apparent even earlier. The English physician
Thomas Sydenham noted in 1664 that oral iron salts restored normal skin color in
anemic humans, and V. Menghini reported in 1747 that blood contains iron. J. G.
Gahn, of Sweden, discovered in 1748 that bones contained phosphorus as phosphate
of lime. The “calcareous” nature of bone had been noted for some time, but previ-
ously bone was considered some peculiar kind of earth. Calcium, the element, was
not discovered until 1808 (simultaneously) by H. Davy and J. Berzelius. An asso-
ciation of calcium with the health of bone was made in 1842 by Charles Chossat, a
Swiss physiologist and physician, who observed that the addition of calcium carbon-
ate to a diet for pigeons prevented bone fragility. The German chemist, Justus von
