Diagnostic Atlas of Variation - education toolkit
Vitamin D is synthesized from cholesterol by the action of sunlight on the skin. It is also found in oily fish, in some plants and may be added to some foodstuffs e.g. breakfast cereals. In the northern hemisphere low sunlight levels, especially in winter, make it difficult to synthesize sufficient vitamin D, and deficiency is common. Those whose exposure to sunlight is limited and those with darker skin are at increased risk of deficiency. Recommended use of sunscreen is a further barrier to vitamin D synthesis. Current advice is for adults and children over 1 year to consider taking 10microg (400 IU) of vitamin D daily in order to protect their musculoskeletal health.
Vitamin D deficiency causes osteomalacia in adults. Lesser deficiency may be symptomless, but diverse health problems ranging from asthma to heart disease, from autoimmune diseases to cancers have been associated with low levels of vitamin D though causal links are yet to be established.
Vitamin D toxicity is serious but rare. Hypercalcaemia, demineralisation of bone, soft tissue calcification and renal damage are unlikely to be caused by over the counter supplements taken as advised.
Measuring vitamin D is often unnecessary dependent upon patient symptoms and other abnormal biochemistry. Deficient patients may have low serum calcium though secondary hyperparathyroidism may sustain calcium within the normal range. Serum phosphate tends to decrease and alkaline phosphatase to increase.