Vitamin D also known as calciferol is one of the fat-soluble vitamins that naturally occurs in certain foods, added to some foods, and is also available as a supplement for diet. Vitamin D synthesis also triggers endogenously when the skin absorbs ultraviolet (UV) rays from the sunlight. Vitamin D that we obtain from food, supplements, and sun exposure is naturally inert and needs to undergo two hyrooxylation steps for activation in the body. The first hydroxylation that occurs in the liver, transforms vitamin D to 25-hydroxyvitamin D, also known as calcodiol. The second hydroxylation process occurs mainly in the kidney and escalates the physiologically active 1-25-dihydroxyvitamin D, also known as calcitriol.
Vitamin D stimulates absorption of calcium in the gut and regulates adequate serum phosphate and calcium concentrations to ensure normal bone mineralization. Moreover, vitamin D prevents the hypocalcemic tetany, which leads to cramps, spasms, and involuntary contraction of muscles. Normal levels of vitamin D is important for bone remodeling by osteoclasts and osteoblasts as well as for contributing to bone growth. Bones might start to get brittle, thins or misshapen due to insufficient amounts of vitamin D in the body. Adequate amounts of vitamin D along with calcium prevents osteomalacia in adults and rickets in children.
Vitamin D has other major roles in the body as well that include the reduction of inflammation and modulation of process like immune function, neuromuscular function, glucose metabolism, and cell growth. A variety of genes encoding protein which help in the regulation of cell proliferation, apoptosis, and differentiation must need vitamin D. several tissues in the body have vitamin D receptors and convert D2 to D3. The two main types of vitamin D i.e., D2 and D3 differ only in their chemical structures in side-chain. Small intestine absorbs both the forms of vitamin D very well. Absorption happens by passive diffusion and a mechanism that includes intestinal membrane carrier proteins.
The simultaneous presence of fat in small intestine stimulates the absorption of vitamin D but some amount of vitamin D does not absorb even in the presence of dietary fat. Vitamin D absorption in the gut does not suffer any alterations neither due to obesity nor aging. In chronic cases, vitamin D toxicity results in renal failure and calcification of soft tissues, cardiac arrhythmia, and can also be fatal. The toxicity of vitamin D results due to the consumption of dietary supplements that contain exceeding amounts of vitamin D due to manufacturing errors.
The inappropriate manufacturing practices might lead to the excessive amounts of vitamin D and sometimes physicians also incorrectly prescribe these supplements. Serum vitamin D concentrations of vitamin D2 is the significant indicator of vitamin D status at present. The serum levels of vitamin D2 reflects the amount of vitamin D that our body produces endogenously and that we obtain from dietary supplements and foods we eat. Vitamin D2 has fairly long circulation time in the blood so it is an appropriate measure of vitamin D status. As vitamin D deficiency is not a part of routine bloodwork, but it has immense importance because it can lead to bone fractures and long-lasting body aches.