Vitamin D: The Sunshine Vitamin

In the early 20th century when the antibiotics had not yet arrived, exposure to sunshine was the only treatment known for TB. In fact, the patients would often be packed off to sunny locations for recuperation. The sunshine factor was also thought to account for the near absence of rickets in rural countryside and its preponderance in congested, industrial cities where people toiled in dingy workplaces. The current surge in interest in vitamin D is owing to the realization that the utility of this vitamin extends beyond building strong bones; it has now been proved to possess anticancer and immune-boosting actions.

Strictly speaking, vitamin D is not a vitamin at all, because with moderate exposure to ultraviolet radiation from the sun, we do not need to get it from food. Living in high latitudes, indoor living, darker skin pigmentation, and use of sunscreen can cause vitamin D deficiency. Breast-fed babies can be deficient in vitamin D as mother’s milk doesn’t contain adequate vitamin D, a condition made worse if the mother is already low on the vitamin.

Vitamin D is considered a prohormone, a precursor to a hormone. When ultraviolet rays from the sun strike the skin of our bodies, the skin cells in our body called keratinocytes trigger vitamin D synthesis. The endogenously made as well as vitamin D obtained from food are biologically inert (called vitamin D3). D3 is the major circulating form of Vitamin D and is what gets measured in the lab tests. D3 must be metabolized further in the liver and in the kidney to yield activated forms of D called calcidiol and calcitriol, respectively. It is this activated form that can form calcium-binding protein needed for the absorption of calcium from the intestines.

Vitamin D and Calcium Metabolism

Vitamin D facilitates the absorption of calcium and phosphate in the intestines. Calcium’s absorption from the gut is important to preserve homeostasis and for the maintenance of many physiological systems.

Vitamin D, calcium, and parathyroid hormone coordinate in a tightly regulated system. Parathyroid glands in the body regulate the calcium level in our bodies within a very narrow range (by withdrawing or depositing calcium on the bones) so that the nervous and muscular systems can function properly. If your body does not have sufficient calcium, it will start producing parathyroid hormone (PTH).  PTH causes your body to take calcium and phosphorous from your bones.  Insufficient vitamin D will weaken both your bones and teeth.