|
| Vitamin D |
| It has been claimed that man's need for vitamin
D caused natural selection to favour loss of body hair an skin
pigment among Caucasian races as their ancestors migrated from
tropical to temperate climates. This is because vitamin D is
formed by the action of ultra violet rays of the sun upon the
skin. |
Vitamin D is now regarded as a vitamin which
works in a similar way to a hormone, being transported in the
blood and stimulating action in other parts of the body, in
the bones, intestines, kidney and possibly, in the muscles.
The active form of vitamin D appears to be synthesized in two
stages, in the liver and then in the kidney. This active form
stimulates the production of a special protein, which enables
calcium to be absorbed from food in the intestine. Parathyroid
hormone needs the presence of vitamin D to regulate the transfer
of calcium into and out of bone tissue and the blood. In essence,
vitamin D is concerned with the absorption, transport and metabolism
of calcium. |
|
| Dietary Sources |
| Vitamin D is found in a limited number
of animal foods and it is always associated with animal fats,
the richest being fish liver oil. Fat fish, dairy foods including
butter, milk, eggs and cheese. |
|
| Deficiency effects |
| A prolonged vitamin D deficiency during the growing
period of infancy, childhood and adolescence leads to the development
of rickets, a disease characterised by badly deformed legs and
square head in babies. Some Asian women, particularly those
who have borne several children suffer from osteomalacia. With
osteomalacia, the bones are more liable to fracture; there is
pain in the joints, particularly in the pelvis, lower back and
legs, and a general muscular weakness. |
|
| Excessive intakes |
| Excessive amounts of vitamin D in the body can
be poisonous. Calcium absorption from food is enhanced and the
blood calcium level rises to a dangerously high level. In these
circumstances, calcium may be deposited in soft body tissues,
including the kidneys and death may follow kidney failure. |
|
| Stability of Vitamin D |
| Vitamin D is a very stable vitamin. It is unaffected
by most domestic cooking and food processing temperatures. Being
soluble only in fat, it does not dissolve into water used for
washing or cooking food. |
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