Vitamin D

Synonym: calciferol

Characteristics: It is a fat-soluble vitamin that occurs naturally in several foods, is added to others, and is available as a dietary supplement. It is also produced endogenously when ultraviolet rays from sunlight hit the skin and trigger the synthesis of vitamin D. Vitamin D obtained from sunlight, food and supplements is biologically inert and must undergo two hydroxylations in the body to calcidiol and calcitriol to be activated. Vitamin D promotes the absorption of calcium in the intestine and maintains adequate concentrations of calcium and phosphorus in the serum. It is also needed for bone growth and bone remodeling. Without enough vitamin D, bones can become brittle or deformed. Sufficient vitamin D prevents rickets in children and osteomalacia in adults. Along with calcium, vitamin D helps protect older adults from osteoporosis. Vitamin D has other roles in the body, such as reducing inflammation and modulating such processes as cell growth, neuromuscular and immune function, and glucose metabolism.

Absorption: In foods and dietary supplements, vitamin D occurs in two main forms, D2 (ergocalciferol) and D3 (cholecalciferol). Cholecalciferol (D3) is the form found in animal products and fish oil, while ergocalciferol (D2) is the typical synthetic form of provitamin D found in dietary supplements and fortified foods. Both forms are well absorbed in the small intestine. The simultaneous presence of fat in the intestine increases the absorption of vitamin D, although some vitamin D is absorbed even without dietary fat. Neither aging nor obesity affect the absorption of vitamin D from the intestine.

Dietary supplements: Dietary supplements may contain vitamins D2 or D3. Vitamin D2 is produced by UV irradiation of ergosterol in yeast, and vitamin D3 is typically produced by irradiation of 7-dehydrocholesterol from lanolin obtained from sheep's wool. A lichen-derived version of vitamin D3 is also available. Both vitamins D2 and D3 increase serum calcidiol levels, and most steps in the metabolism and action of vitamins D2 and D3 are identical. However, most evidence shows that vitamin D3 increases serum calcidiol levels to a greater extent and maintains these higher levels longer than vitamin D2.

Natural sources: Few foods contain vitamin D naturally. The best sources include fatty fish (such as salmon, tuna and mackerel) and fish liver. Beef liver, egg yolks and cheeses contain small amounts of vitamin D3. Mushrooms provide variable amounts of vitamin D2, even being exposed to UV light to increase its content. Older people and people with dark skin produce less vitamin D from sunlight. UVB rays do not penetrate glass, so exposure to sunlight indoors through a window does not produce vitamin D. About 5-30 minutes of sun exposure, especially between 10:00 AM and 4:00 PM, either daily or at least twice a week on the face, arms, hands, and legs without sunscreen usually results in sufficient vitamin D synthesis.

Effect: Vitamin D has a critical role in bone growth and development, but it also affects other processes throughout the body, including inhibiting the growth of abnormal cells. Other uses of vitamin D include supplementation during pregnancy, treatment of vitamin D deficiency caused by certain medications or diseases such as celiac disease, Crohn's disease, and cystic fibrosis, prevention of breast, prostate, and colorectal cancer, and various metabolic bone disorders. Evidence suggests that higher levels of circulating vitamin D may be associated with a better prognosis in patients with breast, colon and rectal cancer, reducing mortality in the elderly.
Deficiency: Vitamin D deficiency can develop in humans when the usual intake is below the recommended allowance, there is limited exposure to sunlight, the kidneys are unable to convert calcidiol to its active form, or the absorption of vitamin D from the digestive tract is insufficient. People who have trouble absorbing dietary fat, are obese, or have undergone gastric bypass surgery may require increased vitamin D supplementation. Diets low in vitamin D are more common in people who have a milk allergy or lactose intolerance, and those who consume an ovo-vegetarian or vegan diet. Older people are at increased risk of developing vitamin D deficiency because the skin's ability to synthesize vitamin D decreases with age. In addition, older people spend more time indoors than outdoors and may have insufficient vitamin intake in their diet. More melanin pigment in the epidermal layer of the skin results in darker skin and reduces the skin's ability to produce vitamin D from sunlight. Long-term exclusive breastfeeding without vitamin D supplementation can cause rickets in infants. In children, vitamin D deficiency is manifested by poor mineralization of bone tissue. Severe rickets can cause failure to thrive, delayed development, tetanic convulsions, heart involvement, and dental abnormalities. In adults and adolescents, the deficiency manifests itself in weak bones.

Recommended daily dose: adult: 15 µg (600 IU) for men, 15 µg for women (15 µg during pregnancy, 15 µg during breastfeeding). For >70 years 20 µg (800 IU).

Adverse Effects: Because vitamin D increases calcium absorption in the gastrointestinal tract, vitamin D toxicity leads to marked hypercalcemia. In turn, hypercalcemia can lead to nausea, vomiting, muscle weakness, neuropsychiatric disorders, pain, loss of appetite, dehydration, polyuria (increased urination), excessive thirst, and kidney stones.

Interactions: The weight loss drug orlistat, along with a low-fat diet, may reduce the absorption of vitamin D from food and supplements. Statins reduce cholesterol synthesis. Because endogenous vitamin D comes from cholesterol, statins can reduce vitamin D synthesis. Additionally, high intakes of vitamin D, especially from dietary supplements, can reduce the effectiveness of atorvastatin, lovastatin, and simvastatin. Corticosteroids are often prescribed to reduce inflammation, which can reduce calcium absorption and impair vitamin D metabolism. Thiazide diuretics reduce urinary calcium excretion. Combining these diuretics with vitamin D supplements (which increases intestinal calcium absorption) can lead to hypercalcemia, especially in older adults and individuals with reduced renal function or hyperparathyroidism. Vitamin D supplementation may reduce the effectiveness of calcium channel blockers. Vitamin D together with estrogens prevents osteoporosis. Valproate increases the metabolism of vitamin D and therefore its long-term use can lead to osteomalacia (loss of bone mass).

Pregnancy: safe in usual doses.

Breastfeeding: safe in usual doses.

Toxicity: Excessive amounts of vitamin D are toxic. In extreme cases, vitamin D toxicity causes kidney failure, calcification of soft tissues throughout the body (including coronary vessels and heart valves), cardiac arrhythmias, and even death. Excessive sun exposure does not cause an overdose of vitamin D.