Vitamin D or cholecalciferol is a fat-soluble vitamin. Vitamin D is unique because it is synthesized by the body when sunlight falls on the skin (as Vitamin D3), giving it the name “sunshine” vitamin. It also occurs naturally in some foods.

Vitamin D deficiency is considered a global health issue affecting nearly one billion persons across the world

VITAMIN D SYNTHESIS AND ABSORPTION
When a person is exposed to sunlight, vitamin D3 is formed from the precursor 7-dehydrocholesterol (7-DHC) in the skin. Vitamin D3 enters the blood and is transported to the liver, where it is converted to 25-hydroxyvitamin D3 (calcidiol), the major circulating form of vitamin D). In the next step, in the kidneys, the 25-hydroxyvitamin D3 is converted to 1, 25-dihydroxyvitamin D3 (calcitriol), the most potent form of vitamin D). Most of the effects of vitamin D in the body are due to the activity of 1, 25-dihydroxyvitamin D3

When taken as food or as a nutritional supplement, vitamin D is absorbed into blood from the small intestine (duodenum). It is then transported by its binding protein to the liver to be stored or metabolized

DIETARY SOURCES OF VITAMIN D
Most foods do not contain sufficient amounts of vitamin D. Foods that are naturally rich in vitamin D include
• Oily fish (such as tuna, sardines, trout, pilchards, herring, salmon and mackerel).
• Egg yolk, red meat and liver
• Only vegan sources of vitamin D are UV irradiated yeast and mushrooms

Some foods are fortified with vitamin D (i.e. they have vitamin D added to them). Examples include baby milk formula, margarines and certain cereals. In some countries all milk is fortified with vitamin D

FUNCTIONS OF VITAMIN D
• Regulation of concentration of calcium and phosphate in the bloodstream,
• Bone health
• Adequate amounts of vitamin D are essential to prevent rickets in children and osteomalacia in adults. Along with calcium, it helps to protect the elderly from osteoporosis
• Nerve and muscle function
• Possible role in reducing inflammation
• Influences genes that regulate the growth of cells

RISK FACTORS OF VITAMIN D DEFICIENCY
• Pregnant and breastfeeding women (due to increased demand)
• Inadequate exposure to sunlight – colder countries, sick and elderly, sunscreen use, covering the body completely when outside
• Darker skinned persons (synthesize lesser amounts)
• Liver or kidney disease
• Crohn’s disease
• Coeliac disease
• Medications – anti-HIV, anti-epilepsy medications
• Vegan or vegetarian diet

CLINICAL FEATURES OF VITAMIN D DEFICIENCY
In children, vitamin D deficiency causes rickets, where the bones become soft, seen especially among African American infants and children. In adults, vitamin D deficiency leads to osteomalacia, mainly characterized by bone pain and muscle weakness.

RICKETS IN INFANTS AND CHILDREN
• Soft skull and leg bones
• Outwardly curved or bow legs
• Muscle weakness and pain
• Delayed walking
• Stunted growth
• Delayed teething
• Irritability
• Recurrent respiratory infections
• Muscle cramps and fits (severe deficiency)
• Heart muscle weakness (cardiomyopathy) in severe deficiency

OSTEOMALACIA IN ADULTS
• Generalized weakness and tiredness
• Vague aches and pains
• Difficulty getting up from a chair or climbing stairs
• Waddling type of gait
• Bone pain on light pressure, particularly in lower back, hips, pelvis, thighs and feet
• Occurrence of hairline fractures

In addition to bone and muscle health, vitamin D deficiency is associated with several conditions such as diabetes, coronary heart disease, bowel and breast cancer and Alzheimer’s disease. The exact significance of these associations remains unclear

DIAGNOSIS OF VITAMIN D DEFICIENCY
• Vitamin D deficiency may be suspected from a thorough history and examination.
• The diagnosis may be confirmed by a simple blood test. In general, vitamin D levels less than 12 ng/mL are considered low for bone or overall health, and levels above 50 ng/ mL are too high. Levels of 50 nmol/L or above (20 ng/mL or above) are adequate for most people
• Low levels of serum calcium and phosphate
• Abnormal liver function or renal function tests
• Wrist x-ray in children to assess bone growth and development

TREATMENT OF VITAMIN D DEFICIENCY
Vitamin D deficiency is treated with supplements which may be in the form of vitamin D2 or vitamin D3 depending on patient’s age and severity of condition. The supplements may be in tablet form, liquid form or injections. Both vitamin D2 and vitamin D3 were until recently considered to be equally effective, but recent studies have demonstrated that vitamin D3 is twice as effective in increasing vitamin D levels compared to vitamin D2. Therefore, these findings could lead to altered guidelines as to how foods should be fortified and choice of supplements doctors may prescribe to take to treat vitamin D deficiency

Following treatment of vitamin D deficiency and reaching sufficient body stores, maintenance treatment may be necessary long-term, to prevent further deficiency in the future.

VITAMIN D REQUIREMENTS
Daily vitamin D requirements depend on age. Average daily requirement advised by the Food and Nutrition Board (a national group of experts) for different ages are as below

PREVENTION OF VITAMIN D DEFICIENCY
• Get some sun – 2-3 exposures of sunlight on bare skin every week from April to September will provide sufficient amounts of vitamin D for the entire year. Each exposure should be 20-30 minutes to bare arms and face without causing redness or sunburn
• Include foods rich in vitamin D in the diet
• Consider taking vitamin D supplements if you are at risk of vitamin D deficiency

PROGNOSIS
The outlook following treatment is usually very good. However, it may take months for bones to recover and for pain to improve. If untreated or treatment is delayed, the condition can affect the strength and appearance of bones, resulting in permanent bone deformities