Vitamin D

Vitamin D is a water-soluble vitamin whose natural form is known as cholecalciferol. Synthetic versions are in the form of ergocalciferol which is metabolised in the same way as the natural substance. Vitamin D is broken down in the body to form a hormone called calcitriol.

Vitamin D can be synthesized in the body by the action of sunlight on a cholesterol-like molecule (7-dehydrocholesterol) in the skin. Blood levels of vitamin D are therefore naturally higher in the summer and lower in winter.

Sometimes the amount of vitamin D is expressed in International Units (IU) rather than micrograms. 1mcg vitamin D = 40IU.

Why you need it

Vitamin D is essential for the absorption of dietary calcium and phosphate in the small intestine and for their deposition in bone. Under the regulation of parathyroid hormone, vitamin D also mobilises calcium and phosphorus from bone during remodeling. It is therefore vital for the development and maintenance of healthy bones and teeth. If vitamin D is lacking during childhood, deformed bones (rickets) result. In adults, vitamin D deficiency leads to weakened, softened bones - a condition called osteomalacia.

Vitamin D also has an important action on skin health and may be effective in treating some forms of psoriasis.

Research

Lack of vitamin D can cause secondary hyperparathyroidism (high levels of parathyroid hormone) that in turn promotes osteoporosis. Four out of five people with hip fracture have evidence of vitamin D deficiency. Adding calcium supplements to the diet of elderly people reduces their risk of a vertebral fracture by 20 per cent, while giving them both calcium and vitamin D supplements reduces their risk of hip fracture by up to 40 per cent.

Bone thinning (osteoporosis) in menopausal women is partly regulated by the amount of vitamin D they obtain from their diet or from supplements. In a study of postmenopausal women with low vitamin D levels, increasing intakes to 500IU daily (= 12.5mcg) for one year significantly reduced late wintertime bone loss and increase bone density in the spine. Osteoporosis in older males can also be reduced and the risk of fractures decreased by taking 500mg calcium plus 700IU (17.5mcg) vitamin D daily.

Vitamin D has been found to reduce growth and division of prostate cancer cells in the laboratory. Researchers feel vitamin D may be a useful treatment for prostate cancer in the future and that its actions need further investigation.

Synthetic forms of vitamin D (e.g. calcipotriol, tacalcitol) are effective in treating psoriasis when applied directly to the skin. It is thought that natural vitamin D is beneficial in psoriasis, too.

Excess intake of fluoride causes mottling of teeth known as fluorosis. This was thought to be irreversible, but a study in which children with fluorosis were given vitamin C (500mg), calcium (250mg) and vitamin D (800IU = 20mcg) daily showed that marked reversal of dental mottling.

How much you need

The EC RDA for vitamin D is 5mcg (200IU). People over the age of 50 usually need at least double this amount (10mcg = 400IU) as blood levels fall with increasing age.

People living in high altitudes, in Northern latitudes, and those who cover up their skin in sunlight, or who stay indoors all day may not be exposed to enough sunlight to meet their vitamin D needs. They must then rely on getting vitamin D from their diet.

Symptoms that may be due to lack of vitamin D include

  • constipation
  • muscle weakness
  • lowered immunity with increased susceptibility to infections
  • poor growth
  • bone pain
  • bone deformities (in rickets)
  • deafness (in osteomalacia)

Foods containing vitamin D include

  • oily fish - sardine, herring, mackerel, salmon, tuna
  • fish liver oils
  • fortified margarine
  • liver
  • eggs
  • fortified milk
  • butter

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