Vitamin D is a fat-soluble vitamin that plays an important role in calcium homeostasis and bone metabolism. Vitamin D deficiency can lead to osteomalacia and rickets in children and osteomalacia in adults. The fortification of milk with vitamin D in the 1930s was effective in eradicating rickets in the world. However, subclinical vitamin D deficiency is still widely prevalent in both developed and developing countries with a worldwide prevalence of up to 1 billion.  This subclinical vitamin-D deficiency is associated with osteoporosis, increased risk of falls and fragility fractures. Many conflicting recent studies are now showing an association between vitamin D deficiency and cancer, cardiovascular disease, diabetes, autoimmune diseases, and depression.
Dermal synthesis and dietary intake (fatty fish livers, fortified food) are the major sources of ergocalciferol (D2) and cholecalciferol (D3), both of which are converted to 25-hydroxy-vitamin D2 (25-OH-D2) and 25-hydroxy-vitamin D3 (25-OH-D3) respectively in the liver by the enzyme hepatic enzyme 25–hydroxylase. Both 25-OH-D2 and 25-OH-D3 are then converted to the most active form of vitamin D (1,25 dihydroxyvitamin D) by the enzyme 1-alpha-hydroxylase in the kidneys. This active 1,25 dihydroxyvitamin D increases intestinal absorption of calcium and bone resorption and decreases renal excretion of calcium and phosphate. Vitamin D deficiency can result from several causes.
1. Decreased dietary intake and/or absorption.
Certain malabsorption syndromes such as celiac disease, short bowel syndrome, gastric bypass, inflammatory bowel disease, chronic pancreatic insufficiency, and cystic fibrosis may lead to vitamin D deficiency. Lower vitamin D intake orally is more prevalent in the elderly population. 
2. Decreased sun exposure.
About 50% to 90% of vitamin D is absorbed through the skin via sunlight while the rest comes from the diet. Twenty minutes of sunshine daily with over 40% of skin exposed is required to prevent vitamin D deficiency. Cutaneous synthesis of vitamin D declines with aging. Dark-skinned people have less cutaneous vitamin D synthesis. Decreased exposure to the sun as seen in individuals who are institutionalized, or have prolonged hospitalizations can also lead to vitamin D deficiency.  Effective sun exposure is decreased in individuals who use sunscreens consistently.
3. Decreased endogenous synthesis.
Individuals with chronic liver disease such as cirrhosis can have defective 25-hydroxylation leading to deficiency of active vitamin D. Defect in 1-alpha 25-hydroxylation can be seen in hyperparathyroidism, renal failure and 1-alpha hydroxylase deficiency.
4. Increased hepatic catabolism.
Medications such as phenobarbital, carbamazepine, dexamethasone, nifedipine, spironolactone, clotrimazole, and rifampin induce hepatic p450 enzymes which activate degradation of vitamin D.
5. End organ resistance.
End organ resistance to vitamin D can be seen in hereditary vitamin D resistant rickets.
Vitamin D deficiency is a global public health issue. About 1 billion people worldwide have vitamin D deficiency, while 50% of the population has vitamin D insufficiency. The prevalence of patients with vitamin D deficiency is highest in the elderly, obese patients, nursing home residents, and hospitalized patients. The prevalence of vitamin D deficiency was 35% higher in obese subjects irrespective of latitude and age. In the United States, about 50% to 60% of nursing home residents and hospitalized patients had vitamin D deficiency.  Vitamin D deficiency may be related to populations who have higher skin melanin content and who use extensive skin coverage, particularly in Middle Eastern countries. In the United States, 47% of African American infants and 56% of Caucasian infants have vitamin D deficiency, while over 90% of infants in Iran, Turkey, and India have vitamin D deficiency. In the adult population, 35% of adults in the United States are vitamin D deficient whereas over 80% of adults in Pakistan, India, and Bangladesh are Vitamin D deficient. In the United States, 61% of the elderly population is vitamin D deficient whereas 90% in Turkey, 96% in India, 72% in Pakistan, and 67% in Iran were vitamin D deficient.
Vitamin D plays a crucial role in calcium homeostasis and bone metabolism. With chronic and/or severe vitamin D deficiency, a decline in intestinal calcium and phosphorus absorption leads to hypocalcemia leading to secondary hyperparathyroidism. This secondary hyperparathyroidism then leads to phosphaturia and accelerated bone demineralization. This can further results in osteomalacia and osteoporosis in adults and osteomalacia and rickets in children.
The majority of patients with vitamin D deficiency are asymptomatic. However, even mild chronic vitamin D deficiency can lead to chronic hypocalcemia and hyperparathyroidism which can contribute risk of osteoporosis, falls and fractures especially in the elderly population. Patients with a prolonged and severe vitamin D deficiency can experience symptoms associated with secondary hyperparathyroidism including bone pain, arthralgias, myalgias, fatigue, muscle twitching (fasciculations), and weakness. Fragility fractures may result from chronic vitamin D deficiency leading to osteoporosis. In children, irritability, lethargy, developmental delay, bone changes, or fractures can be symptoms of vitamin D deficiency.
It is not recommended to screen asymptomatic individuals for vitamin-D deficiency. High-risk individuals shall be evaluated. Vitamin D sufficiency or deficiency is evaluated by the measurement of serum 25-hydroxyvitamin D. Optimal serum levels of 25-hydroxyvitamin D is still a matter of controversy. There are substantial differences in mineral metabolism amongst different races. African Americans, for example, have higher bone density and low fracture risk compared to other races. Further, the effects of calcium and vitamin-D supplementation in the non-Caucasian population have not yet been completely evaluated or reported. The International Society for Clinical Densitometry and International Osteoporosis Foundation recommend minimum serum levels of 25-hydroxyvitamin D of 30 ng/mL to minimize the risk of fall and fractures in older individuals.  There is insufficient data about the maximum safe up her level of serum 25-hydroxyvitamin D, however, at high levels such as above 100 ng/mL, there is a potential risk of toxicity due to the secondary hypercalcemia. In patients where vitamin-D deficiency has been diagnosed, it is important to evaluate for secondary hyperparathyroidism and levels of parathyroid hormone and serum calcium shall be checked.
Several preparations of vitamin D are available. Vitamin D3 (cholecalciferol), when compared with vitamin D2 (ergocalciferol), has been shown to be more efficacious in achieving optimal 25-hydroxyvitamin D levels, thus favoring vitamin D3 as a treatment of choice. 
Prevention of Vitamin D deficiency
Adults less than 65 years of age who do not have year-round effective sun exposure shall consume 600 to 800 international units of vitamin D3 daily to prevent deficiency. Older adults 65 years of age or more shall consume 800 to 1000 international units of vitamin D3 daily to prevent deficiency and to reduce the risk of fractures and falls.
Management of Vitamin D deficiency
The amount of vitamin D required to treat the deficiency depends largely on the degree of the deficiency and underlying risk factors.
A meta-analysis of 18 randomized controlled trials (RCT) including over 57,000 subjects found that intake of daily doses of vitamin D supplements decreased total mortality rates. In the Women’s Health Initiative, calcium and vitamin D supplementation decreased the risk of total cancer, breast cancer, and colorectal cancer while not changing total mortality. One RCT showed that calcium plus vitamin D substantially reduced all cancer risk in postmenopausal women. In a meta-analysis study from three randomized controlled trials, vitamin D supplementation was found to reduce the rate of COPD exacerbations in patients with vitamin D levels below 25 nmol/L. 
Vitamin D is a fat-soluble vitamin, hence, toxicity is possible, although rarely noted. Hypervitaminosis D results from excess oral intake and not due to excessive sunlight exposure. Toxicity has been reported at a serum 25-hydroxyvitamin D level of more than 88 ng/mL. Acute intoxication can lead to acute hypercalcemia that can cause confusion, anorexia, vomiting, polyuria, polydipsia, and muscle weakness. Chronic intoxication can lead to nephrocalcinosis and bone pain.
The severity of vitamin D deficiency is divided into mild, moderate, and severe. 
Mild deficiency: 25-hydroxyvitamin D less than 20 ng/mL
Moderate deficiency: 25-hydroxyvitamin D less than 10 ng/mL
Severe deficiency: 25-hydroxyvitamin D less than 5 ng/mL
Vitamin D deficiency is often overlooked in outpatient and inpatient settings. According to the U.S. Preventive Services Task Force (USPSTF), universal screening for vitamin D levels is not recommended; however, it is important to note that screening for vitamin D deficiency in asymptomatic high-risk individuals is paramount in preventing future complications. High-risk populations include nursing home residents, elderly patients, women with osteoporosis, African American/Hispanic individuals, hospitalized patients, patients with chronic kidney disease, chronic liver disease, and patients with malabsorption syndromes.
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