The various functions of Vitamin D

I] Vitamin D and bone health

The most important and primary function of Vitamin D is the regulation of serum Calcium levels in the body and is considered to be the essential factor for the proper absorption of Calcium from the intestine. Vitamin D and Parathyroid Hormone are the two most important regulators of Serum Calcium. Vitamin D also plays an important role in regulating adequate Phosphorous levels in the body. Vitamin D mediates the active transport of calcium ions across the intestinal mucosa. Calcium is absorbed from the intestine by both passive as well as active transport. The active transport of calcium is carried out by Calbindin, a protein which attaches to the calcium ions and transports the same across the villi of the intestine. Vitamin D is the essential factor for the production of this Calbindin and hence is the most important factor for calcium absorption. It is hence labelled as the “enabling factor” for transport of calcium across the intestine. The quantitative relationship between serum Vitamin D levels and calcium ion absorption has been studied in details but still need to be clarified further. It is a known fact that the serum Vitamin D concentration and calcium ion absorption are directly proportional only up to a specific concentration of serum vitamin D and thereafter the calcium absorption remains stable in spite of an increase in the vitamin D levels. Whenever either dietary calcium is low or serum vitamin D levels are lower than normal, the absorption of calcium ions is hampered and hence the serum calcium levels reduce. To counteract this hypocalcaemia, the parathyroid hormone is released from the parathyroid gland. This parathyroid hormone functions by causing bone resorption by activating the osteoclasts which shift the calcium from bones to the circulation. This bone resorption leads to an increase in the serum calcium levels and thus normalising the effect of the low dietary calcium and vitamin D levels. 

Hypovitaminosis D and Bone Health

The very mechanism mentioned above is the cause of all the skeletal disorders resulting from low Vitamin D levels. The bone resorption occurring as a result of the release of Parathyroid Hormone leads to a reduction in the bone strength and poorly mineralised bones. This leads to Rickets in small children and Osteomalacia and Osteoporosis in adults. As a result of the osteomalacia and osteoporosis caused in the people having low vitamin D levels, the incidence of fractures is known to increase significantly. The clinical manifestation of Hypovitaminosis D is characterised by myalgia (muscle pains), poor muscle strength and bone pains. Low bone mineral density is frequently found in all patients having low Vitamin D levels. In children, Vitamin D deficiency causes Rickets, a disease that leads to soft bones with very poor bone mineralisation. In recent years, especially in the western countries, milk has been fortified with Vitamin D as a move to prevent Rickets in children. It has been documented that infants exclusively breastfed for prolonged periods of time by mothers having vitamin D deficiency are at a higher risk of developing Rickets. Hence we have seen Paediatricians regularly prescribe Calcium- Vitamin D supplements to newborn infants.  

II] Role of Vitamin D in Cardiovascular Disease

A definite association between Hypovitaminosis D and cardiovascular disease has been established. A high percentage of persons deficient in vitamin D also had cardiovascular diseases. Low Vitamin D levels have been associated with left ventricular hypertrophy and abnormalities of renin–angiotensin system activation . It is now proved that patients with vitamin D plasma levels below the recommended 30 ng/ml have higher systolic and diastolic blood pressure level. The Health Professional Follow-Up Study at Harvard has conclusively stated that “men who were deficient in vitamin D were twice as likely to have a heart attack as men who had adequate levels of vitamin D”. There have been many studies with the aim of establishing whether Hypovitaminosis D can also be considered to be a cause of cardiovascular diseases but this fact has not been yet established. Hence a causal relationship between low Vitamin D and cardiac diseases needs further investigation.

III] Role of Vitamin D in obesity

Vitamin D and obesity has been studied in quite some detail. Leptin is a hormone closely related to obesity. It has been labelled as the “obesity hormone” by some and as the “starvation hormone” by others. It is the hormone which indicates satiety to the body and signals it to stop eating once the adequate amount of food has been ingested. Studies on the association between Vitamin D levels and Leptin have revealed that more amount of Leptin is released in the body of individuals having normal Vitamin D levels than those who are deficient. This means that Vitamin D deficient individuals are at greater risk of obesity than those having adequate Vitamin D levels. Secondly, reduced Vitamin D levels also leads to insulin resistance leading to overeating and obesity. Increased Parathyroid hormone levels are known sequelae of reduced Vitamin D levels. Parathyroid hormone is known to reduce the rate of fat metabolism, thereby leading to obesity. Increased Parathyroid levels also cause increased inflow of calcium into adipocytes and hence reduce the speed of lipolysis leading to obesity.