Calciferol - Vitamin D

Vitamin D consists of two forms of calciferol, cholecalciferol which is vitamin D3 (animal origin) and ergocalciferol which is vitamin D2 (plant origin). Vitamin D is a fat-soluble vitamin which acts like a hormone, regulating the formation of bone and the absorption of calcium and phosphorus from the intestine. It helps to control the movement of calcium between bone and blood, and vice versa.

Vitamin D absorption from food takes place in the upper part of the small intestine with the aid of bile salts. It is stored in the liver where it is metabolised into calcidiol. Further metabolic reactions take place in the kidneys where it is converted to calcitriol and then absorbed into the blood where it is stored in fat and muscle. Calcitriol is a hormone that helps regulate the body's calcium needs. Vitamin D is stored efficiently in body tissues and remains available for an extended period.

UV rays from the sun trigger vitamin D synthesis in skin from 7-dehydrocholesterol. Ten to fifteen minutes of sun exposure at least two times per week to the face, arms, hands, or back without sunscreen is usually sufficient to provide adequate vitamin D. It is very important for individuals with limited sun exposure to include good sources of vitamin D in their diet. As people age, skin cannot synthesize vitamin D as efficiently and the kidney is less able to convert vitamin D to its active hormone form. 30% to 40% of older adults with hip fractures have been shown to be deficient in vitamin D, so older adults may benefit from supplemental vitamin D.

FOOD SOURCES

Fish liver oils, such as cod liver oil, oily fish such as salmon, mackerel, sardines, tuna, egg, shiitake mushrooms, butter, meat, milk.

SUPPLEMENTATION

Fish oil capsules and synthetic cholecalciferol. 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 D3 is about three times more potent biologically than vitamin D2, and remains in circulation for much longer. For this reason supplements that specify a high level of animal-derived cholecalciferol (D3) should always be chosen in preference to those containing cheap but relatively ineffective plant-derived ergocalciferol (D2).

SIGNS OF DEFICIENCY

Without vitamin D, bones can become thin, brittle, or misshapen. Vitamin D sufficiency prevents rickets in children and osteomalacia in adults, two forms of skeletal diseases that weaken bones.

A recent study has linked vitamin D deficiency with an increased risk for cancer and autoimmune diseases, such as rheumatoid arthritis MS, and lupus.

Deficiency is seen more often in northern countries, or where tradition dictates that the body is well covered by clothes, such as in parts of the Islamic world. Epidemiologic studies suggest that a higher dietary intake of calcium and vitamin D, and/or sunlight-induced vitamin D synthesis, correlates with lower incidence of cancer. You can ask your doctor for a simple blood test called the '25-hydroxy vitamin D test' if you think you may be deficient.

SIGNS OF TOXICITY

Chronic excess of vitamin D can cause high levels of calcium and phosphorus in the blood and excessive excretion of calcium in the urine leading to calcification of soft tissues (calcinosis), blood vessels and kidney tubules (hypercalcemia). Other signs are: anorexia, arrhythmia, diarrhoea, hypertension, itching skin, kidney damage, mental retardation, nausea, physical retardation, premature arteriosclerosis, seizures, sore eyes, unusual thirst, weight loss.

MINIMUM DAILY REQUIREMENT

Estimates of daily adult requirements range from 20mcg to 50mcg, much of which is supplied by skin exposure to ultraviolet light in many populations. Dosages up to 60 mcg (2,400 IU) /day in healthy adults are believed to be safe.