Vitamin D may be absorbed from food by the intestines or may be produced by the skin when the skin is exposed to sunlight. In its active form, vitamin D acts as a hormone to regulate calcium absorption from the intestine and to regulate levels of calcium and phosphate in the bones.
Sunlight is important to skin production of vitamin D, and environmental conditions where sunlight exposure is limited may reduce this source of vitamin D. Lack of vitamin D production by the skin may occur with indoor confinement or working indoors during the daylight hours, or in climates with little exposure to sunlight.
Because vitamin D is a fat-soluble vitamin, conditions that reduce digestion or absorption of fats will decrease the ability of vitamin D to be absorbed from the intestines.
When the body is deficient in vitamin D, it is unable to properly regulate calcium and phosphate levels. If the blood levels of these minerals become too low, other body hormones may stimulate release of calcium and phosphate from the bones to the bloodstream to elevate the blood levels.
Rickets is a bone disease that affects children when these deficiencies occur. It causes progressive softening and weakening of the bones' structure. There is a loss of calcium and phosphate from the bones, which eventually causes destruction of the supportive matrix.
Rickets is fairly rare in the US. It is most likely to occur during periods of rapid growth, when the body demands high levels of calcium and phosphate. Rickets may be seen in young children 6 to 24 months old and is uncommon in newborns.
Nutritional causes of rickets occur because of a lack of vitamin D in the diet or in association with malabsorption disorders characterized by poor fat absorption.
A dietary lack of vitamin D may occasionally occur in people on a vegetarian diet who do not drink milk products or in people who are lactose intolerant (have trouble digesting milk products).
A dietary lack of calcium and phosphorous (rather than a lack of vitamin D) may also play a part in the nutritional causes of rickets. Rickets caused by a dietary lack of these minerals is rare in developed countries because calcium and phosphorous are present in milk and green vegetables.
Hereditary rickets is an inherited form of the disease caused when the kidneys are unable to retain phosphate. Rickets may also be caused by kidney disorders involving renal tubular acidosis.
Occasionally, it can also affect children who have disorders of the liver, do not adequately absorb fats and vitamin D, or cannot convert vitamin D to its active form.
Renal osteodystrophy occurs in people with chronic renal failure. The manifestation is virtually identical to that of rickets in children and that of osteomalacia or osteoporosis in adults.
