Vitamin D is a group of fat-soluble prohormones, the two major forms of which are vitamin D2 (or ergocalciferol) and vitamin D3 (or cholecalciferol). The term vitamin D also refers to metabolites and other analogues of these substances. Vitamin D3 is produced in skin exposed to sunlight, specifically ultraviolet B radiation.

Vitamin D plays an important role in the maintenance of organ systems.

Vitamin D deficiency can result from; inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders, or, rarely, by a number of hereditary disorders. Deficiency results in impaired bone mineralization, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis.

 

When kidneys begin to fail they can not produce enough active vitamin D3. This leads to decreased absorption of calcium from your diet and leads to less calcium available for healthy bone growth.  This also leads to activation of parathyroid hormone from the parathyroid glands in the neck. High levels of this hormone causes release of calcium from bones that in turn causes weak bones.

 Forms

Several forms of vitamin D have been described. The two major forms are vitamin D2 or ergocalciferol, and vitamin D3 or cholecalciferol.

Vitamin D2 is derived from fungal and plant sources, and is not produced by the human body. Vitamin D3 is derived from animal sources and is made in the skin when 7-dehydrocholesterol reacts with UVB ultraviolet light at wavelengths between 270–290 nm. These wavelengths are present in sunlight at sea level when the sun is more than 45° above the horizon, or when the UV index is greater than 3. Adequate amounts of vitamin D3 can be made in the skin only after ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen. With longer exposure to UVB rays, an equilibrium is achieved in the skin, and the vitamin simply degrades as fast as it is generated.

 Biochemistry

Vitamin D is a prohormone, that is, it has no hormone activity itself, but is converted to a molecule which does, through a tightly regulated synthesis mechanism.

 

 Production in the skin

The epidermal strata of the skin. Production is greatest in the stratum basale (colored red in the illustration) and stratum spinosum (colored orange).

The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer thinner epidermis. The epidermis consists of five strata; from outer to inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.

Vitamin D3 is produced photochemically in the skin from 7-dehydrocholesterol. The highest concentrations of 7-dehydrocholesterol are found in the epidermal layer of skin, specifically in the stratum basale and stratum spinosum. The production of pre-vitamin D3 is therefore greatest in these two layers, whereas production in the other layers is reduced.

Synthesis in the skin involves UVB radiation which effectively penetrates only the epidermal layers of skin. 7-Dehydrocholesterol absorbs UV light most effectively at wavelengths between 270–290 nm and thus the production of vitamin D3 will only occur at those wavelengths. The two most important factors that govern the generation of pre-vitamin D3 are the quantity (intensity) and quality (appropriate wavelength) of the UVB irradiation reaching the 7-dehydrocholesterol deep in the stratum basale and stratum spinosum.

A critical determinant of vitamin D3 production in the skin is the presence and concentration of melanin. Melanin functions as a light filter in the skin, and therefore the concentration of melanin in the skin is related to the ability of UVB light to penetrate the epidermal strata and reach the 7-dehydrocholesterol-containing stratum basale and stratum spinosum. Under normal circumstances, ample quantities of 7-dehydrocholesterol (about 25-50 mg/cm2 of skin) are available in the stratum spinosum and stratum basale of human skin to meet the body's vitamin D requirements, and melanin content does not alter the amount of vitamin D that can be produced. Thus, individuals with higher skin melanin content will simply require more time in sunlight to produce the same amount of vitamin D as individuals with lower melanin content.

Active vitamin D3 helps absorb calcium from your diet.

 

Nutrition

Very few foods are naturally rich in vitamin D, and most vitamin D intake is in the form of fortified products including milk, soy milk and cereal grains.

The U.S. Dietary Reference Intake for Adequate Intake (AI) of vitamin D for infants, children and men and women aged 19–50 is 5 micrograms/day (200 units/day). Adequate intake increases to 10 micrograms/day (400 units/day) for men and women aged 51–70 and to 15 micrograms/day (600 units/day) past the age of 70.

Milk and cereal grains are often fortified with vitamin D.

Milk and cereal grains are often fortified with vitamin D.

 

In food

Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis in the skin, and it is important for individuals with limited sun exposure to include good sources of vitamin D in their diet.

In some countries, foods such as milk, yogurt, margarine, oil spreads, breakfast cereal, pastries, and bread are fortified with vitamin D2 and/or vitamin D3, to minimize the risk of vitamin D deficiency. In the United States and Canada, for example, fortified milk typically provides 100 IU per glass, or one quarter of the estimated adequate intake for adults over the age of 50.

Fatty fish, such as salmon, are natural sources of vitamin D.

 

Fatty fish, such as salmon, are natural sources of vitamin D. Fortified foods represent the major dietary sources of vitamin D, as very few foods naturally contain significant amounts of vitamin D. Natural sources of vitamin D include:[1]

 

Fish liver oils, such as cod liver oil, 1 Tbs. (15 mL) provides 1,360 IU.

 

Deficiency

Vitamin D deficiency can result from: inadequate intake coupled with inadequate sunlight exposure, disorders that limit its absorption, conditions that impair conversion of vitamin D into active metabolites, such as liver or kidney disorders, or, rarely, by a number of hereditary disorders. Deficiency results in impaired bone mineralization, and leads to bone softening diseases, rickets in children and osteomalacia in adults, and possibly contributes to osteoporosis.

 

Diseases caused by deficiency

The role of diet in the development of rickets was determined by Edward Mellanby between 1918–1920. In 1921 Elmer McCollum identified an anti-rachitic substance found in certain fats could prevent rickets. Because the newly discovered substance was the fourth vitamin identified, it was called vitamin D. The 1928 Nobel Prize in Chemistry was awarded to Adolf Windaus, who discovered the steroid, 7-dehydrocholesterol, the precursor of vitamin D.

Vitamin D deficiency is known to cause several bone diseases including:

Prior to the fortification of milk products with vitamin D, rickets was a major public health problem. In the United States, milk has been fortified with 10 micrograms (400 IU) of vitamin D per quart since the 1930s, leading to a dramatic decline in the number of rickets cases.

Vitamin D malnutrition may also be linked to an increased susceptibility to several chronic diseases such as high blood pressure, tuberculosis, cancer, periodontal disease, multiple sclerosis, chronic pain, depression, schizophrenia, seasonal affective disorder and several autoimmune diseases (see role in immunomodulation).

 

Groups at greater risk of deficiency

Vitamin D requirements increase with age, while the ability of skin to convert 7-dehydrocholesterol to pre-vitamin D3 decreases. In addition the ability of the kidneys to convert calcidiol to its active form also decreases with age, prompting the need for increased vitamin D supplementation in elderly individuals.

The American Pediatric Associations advises vitamin D supplementation of 200 IU/day (5μg/d) from birth onwards. Health Canada recommends 400IU/day (10μg/d). While infant formula is generally fortified with vitamin D, breast milk does not contain significant levels of vitamin D, and parents are usually advised to avoid exposing babies to prolonged exposure to sunlight. Therefore, infants who are exclusively breastfed are likely to require vitamin D supplementation beyond early infancy, especially at northern latitudes. Liquid "drops" of vitamin D, as a single nutrient or combined with other vitamins, are available in water based or oil-based preparations ("Baby Drops" in North America, or "Vigantol oil" in Europe). However, babies may be safely exposed to sunlight for short periods, as little as 10 minutes a day without a hat can suffice, depending on location and season. The vitamin D found in supplements and infant formula is less easily absorbed than that produced by the body naturally and carries a risk of overdose that is not present with natural exposure to sunlight.

Obese individuals may have lower levels of the circulating form of vitamin D, probably because of reduced bioavailability, and are at higher risk of deficiency. To maintain blood levels of calcium, therapeutic vitamin D doses are sometimes administered (up to 100,000 IU or 2.5 mg daily) to patients who have had their parathyroid glands removed (most commonly renal dialysis patients who have had tertiary hyperparathyroidism, but also to patients with primary hyperparathyroidism) or with hypoparathyroidism.[17] Patients with chronic liver disease or intestinal malabsorption disorders may also require larger doses of vitamin D (up to 40,000 IU or 1 mg (1000 micrograms) daily).

The use of sunscreen with a sun protection factor (SPF) of 8 inhibits more than 95% of vitamin D production in the skin. Recent studies showed that, following the successful "Slip-Slop-Slap" health campaign encouraging Australians to cover up when exposed to sunlight to prevent skin cancer, an increased number of Australians and New Zealanders became vitamin D deficient. Ironically, there are indications that vitamin D deficiency may lead to skin cancer. To avoid vitamin D deficiency dermatologists recommend supplementation along with sunscreen use.

The reduced pigmentation of light-skinned individuals tends to allow more sunlight to be absorbed even at higher latitudes, thereby reducing the risk of vitamin D deficiency. However, at higher latitudes (above 30°) during the winter months, the decreased angle of the sun's rays, reduced daylight hours, protective clothing during cold weather, and fewer hours of outside activity, diminish absorption of sunlight and the production of vitamin D. Because melanin acts like a sun-block, prolonging the time required to generate vitamin D, dark-skinned individuals, in particular, may require extra vitamin D to avoid deficiency at higher latitudes. At latitudes below 30° where sunlight and day-length are more consistent, vitamin D supplementation may not be required.

Overdose

Vitamin D stored in the human body as calcidiol (25-hydroxy-vitamin D) has a large volume of distribution and a long half-life (about 20 to 29 days). However, the synthesis of bioactive vitamin D hormone is tightly regulated and vitamin D toxicity usually occurs only if excessive doses (prescription or megavitamin) are taken. Although normal food and pill vitamin D concentration levels are too low to be toxic in adults, because of the high vitamin A content in codliver oil it is possible to reach poisonous levels of vitamin A, if taken in multiples of the normal dose in an attempt to increase the intake of vitamin D. Most cases of vitamin D overdose have occurred due to manufacturing and industrial accidents.

Exposure to sunlight for extended periods of time does not cause Vitamin D toxicity. This is because within about 20 minutes of ultraviolet exposure in light skinned individuals (3–6 times longer for pigmented skin) the concentration of vitamin D precursors produced in the skin reach an equilibrium, and any further vitamin D that is produced is degraded.

The symptoms of vitamin D toxicity or (hypervitaminosis D) are a result of hypercalcemia (an elevated level of calcium in the blood) caused by increased intestinal calcium absorption. Gastrointestinal symptoms of vitamin D toxicity can develop including anorexia, nausea, and vomiting. These symptoms are often followed by polyuria (excessive production of urine), polydipsia (increased thirst), weakness, nervousness, pruritus (itch), and eventually renal failure. Other signals of kidney disease including elevated protein levels in the urine, urinary casts, and a build up of wastes in the blood stream can also develop.

 

Role in cancer prevention and recovery

The vitamin D hormone, calcitriol, has been found to induce death of cancer cells in vitro and in vivo. Although the anti-cancer activity of vitamin D is not fully understood, it is thought that these effects are mediated through vitamin D receptors expressed in cancer cells, and may be related it its immunomodulatory abilities. The anti-cancer activity of vitamin D observed in the laboratory has prompted some to propose that vitamin D supplementation might be beneficial in the treatment or prevention of some types of cancer.

In 2005, scientists released a study which demonstrated a beneficial correlation between vitamin D intake and prevention of cancer. Drawing from a meta-analysis of 63 published reports, the authors showed that intake of an additional 1,000 international units (IU) (or 25 micrograms) of vitamin D daily reduced an individual's colon cancer risk by 50%, and breast and ovarian cancer risks by 30%. Research has also shown a beneficial effect of high levels of calcitriol on patients with advanced prostate cancer.

Research has suggested that cancer patients who have surgery or treatment in the summer and therefore get more vitamin D have a better chance of surviving their cancer than those who undergo treatment in the winter when they are exposed to less sunlight.