Blood calcium levels must be maintained within a narrow range for normal nervous system functioning and maintenance of bone density. Blood calcium levels are especially vital in childhood during bone growth. Vitamin D as calcitriol is an essential part of the regulation of blood calcium and phosphorus levels, as seen in Figure 4-2. Bone growth and regulation is also assisted by vitamin A, vitamin C, vitamin K, the hormone calcitonin, parathyroid hormone, and magnesium.
Calcium levels are sensed by the parathyroid glands. If blood levels of calcium fall too low, the parathyroid glands secrete parathyroid hormone. The parathyroid hormone stimulates production of an enzyme in the kidneys. This enzyme increases the transformation of calcidiol to calcitriol. Calcitriol is a potent hormone that increases blood calcium levels. Increased blood levels of calcitriol cause increased absorption of calcium from food in the intestines. The kidneys also reduce losses of calcium in the urine in
Figure 4-2 Vitamin D as calcitriol and blood calcium.
response to increased levels of calcitriol. In addition, calcium can be mobilized from bones if dietary levels of calcium are insufficient. Increased release of calcium from the bones requires parathyroid hormone in addition to calcitriol.
Vitamin D and Immunity
Calcitriol has a powerful ability to aid the functioning of the immune system. Immune system cells such as T cells and macrophage cells need calcitriol. Macrophages have the ability to make calcitriol from calcidiol. In autoimmune diseases an excess of macrophages may produce an excess amount of calcitriol.
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