Vitamin D is a steroid hormone that exists
in two molecular forms: vitamin D-3 (cholecalciferol) found in animal skin, and
vitamin D-2 (ergocalciferol) found in yeast. These two forms are created by the
action of the sun's ultraviolet rays on the biological precursors
7-dehydrocholesterol and ergosterol. Vitamin D is essential for calcium and
phosphorus metabolism, and it is required for the normal development of bones
Vitamin D can be acquired either by
ingestion of preformed vitamin D or by the conversion of 7-dehydrocholesterol,
after exposure to ultraviolet light. Ingested vitamin D is absorbed with the
aid of bile. Vitamin D is then transported to the liver where it is converted
into 25-hydroxycholecalciferol. This compound is transformed in the kidney into
the physiologically active form 1,25-dihydroxycholecalciferol (1,25-DHCC).
1,25-DHCC is then transported to the intestinal mucosal cells, bone, and
skeletal muscle where it is stored, regulating calcium absorption and
mobilization. Vitamin D aids the absorption of calcium from the intestinal
tract by stimulating the synthesis of calcium-binding protein in the intestinal
mucous membrane. It also aids the resorption of phosphate in the renal tube.
Vitamin D mobilizes phosphate from the bone to maintain serum phosphate levels,
and stimulates the active phosphate transport.
Vitamin A, choline, vitamin C, unsaturated
fatty acids, and phosphorus assist absorption of vitamin D. Mineral oil or
insufficient sunlight can prevent vitamin D absorption.
Vitamin D's clinical application is in the
treatment of rickets and osteomalacia. Rickets can be prevented in newborns by
administering vitamin D in proper amounts early in, and throughout the growth
period. If rickets do occur, large doses of the vitamin are given. Osteomalacia
is prevented by adequate vitamin D, calcium, and phosphorus in the diet.
Vitamin D must come from food, adequate sunlight, or concentrated supplements.
The pain and weakness associated with vitamin D deficiency will usually
disappear after one to two months of treatment.
Vitamin D deficiency creates a deficient
deposition of hydroxyapatite in the bones. This is due to inadequate absorption
of calcium from the intestinal tract, and from the retention of phosphorus in
the kidney. This inadequate mineralization of the bones causes rickets in
infants and children, and osteomalacia in adults. Rickets can cause delayed
closure of the fontanelles, softening of the skull, soft fragile bones,
enlargement of the wrist, knee, and ankle joints, poorly developed muscles,
restlessness and nervous irritability. Delayed tooth development can be a sign
of rickets. Some children develop rickets with vitamin D supplementation. This
may be due to a genetic error in vitamin D metabolism, usually renal tubular
Insufficient sunlight can create vitamin D
deficiency by preventing the conversion of 7-dehydrocholesterol to
cholecalciferol. This type of deficiency is most common in countries with
limited sunlight, or where the population dresses in a manner that reduces the
Vitamin D taken in excess can cause
pathological changes in the body. Signs of vitamin D toxicity include excessive
calcification of bone, kidney stones, calcification of soft tissue, headaches,
weakness, nausea, vomiting, constipation, polyuria, and polydipsia.
- RDA for adult males: 200 IU
- RDA for adult females: 200 IU
- RDA for children 7 to 10 years: 200 IU
- RDA for infants: 200 IU
- RDA for pregnant and lactating women: 200 IU
Difficulties in establishing requirements
for vitamin D arise from the limited number of food sources available, lack of
knowledge of precise body needs, and degree of synthesis in the skin by
irradiation. The amount needed can vary between winter and summer in northern
climates. In addition, life-style determines the degree of exposure to sunlight
and would therefore influence individual need. This is especially true of the
elderly and invalids who do not go outside and therefore may need supplementary
vitamin D. Growth demands in childhood, during pregnancy, and during lactation
necessitate increased intake.
The daily recommendation for young adults
is 7.5 mcg and older adults 5.0 mcg. The RDA standard is 10 mcg, or 400
international units (IU), of cholecalciferol daily for children and for women
during pregnancy and lactation. One IU of vitamin D is equivalent to biologic
activity of 0.025 mcg of pure crystalline vitamin D-3 (cholecalciferol).
Adults over 22 years of age need only a
small amount of vitamin D. Under normal circumstances their need is met by the
vitamin D contained in an ordinary mixed diet and by exposure to sunlight.
Adults who work at night and those whose clothing or living customs shield them
from sunlight need somewhat more vitamin D in their diet.
No extra benefit is obtained from taking
more than 400 IU daily except for therapeutic reasons; then, dosages can range
from 1,500 to 2,800 IU daily.
|· Fish Liver Oils · Egg
Yolk · Herring · Kippers · Lard
||· Mackerel · Salmon
· Sardines · Shrimp · Tuna
|· Rickets In Children
· Soft Fragile Bones
· Enlarged Joints
· Deformation of Bones In:
· Tetanic Convulsions In Infants
· Osteomalacia In Adults
· Inability of the Body to Metabolize Calcium
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