Agriculture Reference
In-Depth Information
Carrots, spinach, broccoli, peas, sweet potatoes, and squash are good sources of
provitamin A carotenoids. Ready-to-eat cereals may be fortified with vitamin A.
Carotenoids may be added to margarine and to diets for poultry or salmon—usually
to contribute desired color to edible products. Other foods have been fortified, and in
the Philippines, additions of vitamin A to coconut oil significantly improved human
retinol status. In addition, it has been shown that provitamin A concentrations in sev-
eral vegetables (e.g., carrots, cauliflower, yams, cassava, and rice) can be increased
by genetic selection or biotechnical amplification. Termed
biofortification,
, it may be
a useful public health measure for control of vitamin A deficiency in economically
poor cultures when applied to staple food crops. Pharmaceutical vitamin prepara-
tions commonly contain retinyl acetate or retinyl palmitate, and those consumed
orally may contain carotenoids as well.
v
i t a m i n
D
Ultraviolet (UV) irradiation of ergosterol in plants or yeast produces ergocalciferol
(vitamin D
2
), whereas UV irradiation of 7-dehydrocholesterol in skin produces pre-
vitamin D
3
, followed by thermal (body heat) conversion to cholecalciferol (vitamin
D
3
). Effective UV wavelengths are in the UVB range, 280 to 315 nm, although solar
wavelengths below 290 seldom reach the skin because they are usually screened out
by atmospheric ozone and molecular oxygen. Both vitamin D compounds are mod-
erately soluble in lipids and insoluble in water. They are best known for the ability of
their metabolically active forms to promote intestinal absorption of calcium and its
incorporation into bone, thus preventing rickets in growing young and osteomalacia
in adults. However, these active forms appear to have broader metabolic functions
and may inhibit the proliferation and growth of certain types of cancer, particularly
breast, colon, and prostate. In company with the hormones parathormone and cal-
citonin (as well as others), vitamin D maintains normal plasma Ca
2+
and phosphate
concentrations and thus has an impact on a variety of soft tissue events such as neu-
romuscular activity, reproduction, and immune function. Vitamin D
2
appears to be
only about one third as effective as vitamin D
3
in elevating human serum 25-hydroxy
vitamin D levels (used to assess vitamin D status).
Vitamin D
2
or D
3
from the diet is absorbed by dissolution in micelles within the
intestinal lumen, passive diffusion into enterocytes, and incorporation into chylomi-
crons, which enter the general circulation via the lymph. Chylomicrons may deliver
vitamin D to the liver and extrahepatic tissues, or some may be transferred to and
transported by D-binding protein (DBP). Vitamin D
3
, formed following irradiation
of the skin, slowly diffuses into the blood and is bound to DBP for transport. To
become metabolically active, ergocalciferol and cholecalciferol undergo conver-
sion in the liver to 25-hydroxy ergocaliferol and 25-hydroxy cholecalciferol, respec-
tively, followed by conversion in the kidneys to 1,25-dihydroxy ergocalciferol and
1,25-dihydroxy cholecalciferol (calcitriol). This last renal conversion also results in
a 24,25-dihydroxy cholecalciferol, which appears to have metabolic functions alone
or in combination with calcitriol.
Vitamin D activity is expressed in IU, with 1 IU equivalent to 0.025 µg cholecal-
ciferol. Dietary vitamin D comes primarily from animal products, such as liver, fish,
