Vitamin D is the sunshine vitamin produced on this earth for more than 500 million years. This is produced in humans by sunlight. Sunlight is the main source of vitamin D, and adequate sun exposure alone is sufficient to produce enough vitamin D to maintain the physiological demand without producing toxicity. According to Holick's rule, exposure to sunlight on the face and both arms for 25 min, 3 times a week, should maintain adequate vitamin D status. In Indonesia, it was found that after exposure to sunlight at this specific time and duration for 6 weeks, the mean 25(OH)D levels of participants increased from 59 nmol/L at the baseline to 84 nmol/L.In India, exposure to sunlight for 30 minutes, between 11 am and 2 pm, three times a week is sufficient to maintain adequate serum vitamin D concentration. Lack of adequate sun exposure as a consequence of urbanization prone human races to a threat of hypovitaminosis D. Vitamin D (25-hydroxyvitamin D) insufficiency and its impending consequence on health is now the subject of important concern and controversy. The optimal value of vitamin D substantially differs among populations depending upon environmental, cultural and racial factors. However, the definition of vitamin D insufficiency is still not out of controversy, mostly cutoffs of \<50 nmol/L of serum 25-hydroxyvitamin D (25(OH)D) level is considered as "vitamin D deficiency" state, 50 to 74.9 nmol/L denotes "insufficiency" and ≥75 nmol/L considered as "sufficiency".

With the falling of serum25 (OH)D beyond a certain point, absorption of calcium from the gut decreases, which leads to a surge of parathyroid hormone (PTH) secretion, which in turn stimulates the conversion of 25(OH)D to 1,25(OH)2D to keep calcium at a normal limit. Thus, a higher value of PTH could be considered as a surrogate marker of vitamin D insufficiency. Serum 25(OH) D below the level of 75-100 nmol/L has an inverse correlation with PTH concentration; beyond this point, serum intact PTH concentration remains in a plateau state.

Based on these definitions, the whole world faces a pandemic of vitamin D deficiency/insufficiency. According to recent studies, the prevalence of vitamin D deficiency has been found between 30% and 100% in different communities. This picture is similar in the USA, Europe, South East Asia and the Middle East. In India, the prevalence of Vitamin D deficiency is around 70%-100% in the general population. Baidya et al. (2012) studied 40 healthy Indian physicians in Calcutta. Around 97.5% of subjects had low vitamin D. Mean age and meant 25(OH)D levels of that cohort were 52.22 ± 10 years and 13.02 ± 4.77 ng/ml (32.50±11.91 nmol/L). Vitamin D deficiency was found in 92.5% of participants. Though available data in Bangladesh are confined to female subjects only, the scenario is not different. More than ninety-nine percent of Bangladesh female garment workers were vitamin D insufficient in a study.

This picture is alarming because, in addition to its deleterious effect on the musculoskeletal system, vitamin D deficiency is also linked with many other medical issues like cardiovascular disease, malignancy, neuromodulation, diabetes, and metabolic syndrome several autoimmune diseases (such as multiple sclerosis, Crohn's disease and ulcerative colitis) and certain infectious diseases, such as tuberculosis. Vitamin D supplementation is also a key component in the hierarchy of management of Chronic Kidney Disease-Mineral and Bone Disorder.

However, the caveat of all of those studies done in Bangladesh and neighbouring countries like India and Pakistan is that the cutoff value used to determine vitamin D deficiency or insufficiency has not yet been validated for the corresponding population. Before reaching a firm conclusion about epidemics of vitamin D deficiency and planning for supplementation programs, the cutoff value of vitamin D deficiency/insufficiency of the Bangladeshi population must be validated. The optimal level of vitamin D for the Bangladeshi population should also find out.

Rationale: Maintenance of adequate vitamin D levels undoubtedly is an important human health issue. It helps prevent many chronic diseases, which in fact are in addition to its principal role in maintaining appropriate blood calcium levels. To get the optimum beneficial effects of vitamin D, including prevention of many potential adverse health effects caused by deficient vitamin D, circulating 25(OH)D levels should be within the normal range. Vitamin D varies depending on various factors like exposure to the sun, age, race, body mass index, lack of exercise etc. The cutoff value for deficiency and insufficiency also varies between studies. Several studies using prescribed cutoff values of vitamin D deficiency reveal an epidemic of vitamin D deficiency in our country, despite abounded sunlight due to the proximity of the equator. However, no study was done previously in Bangladesh to determine the Optimal value of vitamin D representative of this population. The deficiency and insufficiency levels are ignorant and when to intervene is also not known. That is why it is important to conduct this exploratory study. It will help to assume the optimal reference range of serum vitamin D for the Bangladeshi population. That will give a clearer picture of the actual burden of vitamin D deficiency in Bangladesh and guide them to treat them and take necessary preventive measures at the national level.