Assessment of serum vitamin Dlevels in apparently healthyindividuals.

Vitamin D is a fat soluble vitamin, steroidal in structure and hormonal in function.[1]Vitamin D was first discovered in early 1900s when children suffering from rickets got curedafter supplementation with fish cod liver oil.[2] It is the only vitamin that is produced in theskin with help of UV rays from sunlight.[3,4,5] The principle function of vitamin D is toregulate plasma calcium concentration by acting mainly on three target organs - on intestine(increases calcium absorption)[6], on the kidney (decreases urinary excretion of calcium)[7-9] and on the bone (increases bone resorption resulting in bone mineralization andremodeling).[10]

Besides increasing the calcium and phosphorus levels in the extracellular fluid to mineralizenew bone, vitamin D also has extra skeletal effects in the body. It modulates the transcriptionof cell cycle proteins that decreases cell proliferation and increases cell differentiation of anumber of specialized cells of the body (e.g. enterocytes, keratinocytes, etc.).[11-13]Vitamin D toxicity can also develop due to more than required supplementation which causesincreased calcium levels. Hypercalcaemia causes fatigue, loss of appetite, weight loss,excessive thirst, excessive urination, dehydration, constipation, irritability, nervousness,ringing in the ear (tinnitus, muscle weakness, nausea, vomiting, dizziness, confusion,disorientation, high blood pressure, heart arrhythmias, etc.The consequences of vitamin D deficiency are low calcium and phosphate, increasedparathormone(PTH) producing secondary hyperparathyroidism, a further reduction inphosphate levels and increased plasma levels of bone-derived alkaline phosphatase.Clinically, low vitamin D produces poor bone mineralization - rickets in children,osteomalacia in adults. Extra skeletal manifestations of vitamin D deficiency includedepression, increased risk of fracture, limb girdle muscle weakness, muscle twitching(fasciculation), pre-eclamsia,etc.[14]Despite ample sunshine in India, 70-100% Indians in different age groups are reported to bevitamin D insufficient or deficient.[15] But this deficiency is according to western referencescale. While osteomalacia and rickets have been studied extensively, there is littleinformation on vitamin D deficiency without obvious bone disease. As per western statistics,vitamin D deficiency is defined as a serum 25(OH)D level <50 nmol/L (<20 ng/mL), whereasa serum 25(OH)D level of >75 nmol/L (>30 ng/mL) is considered to be normal, and a level 2of 50-75 nmol/L (20-30 ng/mL) defines vitamin D insufficiency. According to this, majorityof Indian population are insufficient or deficient yet are asymptomatic.There was a retrospective analysis of the vitamin D levels measured at a major medical centerin Hyderabad, India. Vitamin D levels measured as a part of a routine health check werereviewed. A further analysis done on the basis of gender and geographic location of thepopulation in sample of 65 Indian females showed serum 25(OH)D level to be 12.12 ng/mL(range 3-47.5) while in 30 Indian males the mean level was 11.86 ng/mL.[16] In 2000, serum25(OH)D measured using sensitive and specific assay in apparently healthy subjects in Delhiand showed that significant hypovitaminosis D was present up to 90 per cent populationtested.[17] A study from eastern India in Kolkata, West Bengal conducted by Rudrajit Paul etal showed more than half of the study population had Vitamin D deficiency with 53%females and 33% males. In west India in study done in Maharashtra, 95% of total populationhad hypovitaminosis D.[18] The northern and southern states in India too had similar findingsof Vitamin D deficiency in majority of normal population.[19]Subclinical vitamin Ddeficiency has been reported to be highly prevalent in both urban and rural settings, andacross all socioeconomic and geographic strata.According to the western reference scale, 70-100 % Indian population who are found to bevitamin D deficient should have poor bone quality. We hypothesize that the incidence ofhypovitaminosis in Indian population is inflated because the western reference scale do notcorrespond to normal Indian values. Thus studies are required in India to estimate the serumvitamin D levels in normal and healthy population.