Vitamin D

Facts about Vitamin D

Vitamin D functions as a fat soluble Vitamin and hormone, promoting calcium absorption and keeping our calcium and phosphorus levels in check. We need it not only for bone and teeth formation but also for our joints, muscles, nerves and cardiovascular system. Vitamin D is also said to contribute to healthy moods and might protect against age-related diseases.

Sunlight activates a form of cholesterol that can be converted into Vitamin D. However, this process is affected by age, skin colour, season of the year, time of day, clouds, pollution and the use of sun-block. We won't get Vitamin D indoors through a window either. A deficiency is therefore quite common. Despite the positive effects of sunlight, we need to be aware of the risk of skin ageing and cancers. Exposure to UV rays without sun-block should therefore be limited to 30 minutes between 10am and 3pm.

Vitamin D helps to maintain sufficient insulin levels in our blood. Similar to Vitamin A, it attaches to a "protein-taxi" for transport and is mainly stored in the liver with smaller amounts in body tissues. The most important compounds of about 10 in this vitamin group are ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). If taken in large amounts, Vitamin A can interfere with Vitamin D activity.

Benefits of Vitamin D

Strong bones and teeth are known benefits. But what's more, due to its part in cell growth and development, Vitamin D plays a role in looking after our immune system, brain and nervous system, our joints and muscles, and might even help to slow down the ageing process. Vitamin D is also said to contribute to healthy moods and might help in prevention and treatment of:

  • atherosclerosis
  • burns
  • cancer
  • celiac diseases
  • Crohn's disease
  • depression
  • diabetes
  • epilepsy
  • glucose intolerance
  • hearing loss
  • heart disease
  • hypertension
  • inflammation
  • migraine
  • multiple sclerosis
  • obesity
  • osteoarthritis
  • osteoporosis
  • rheumatoid arthritis
  • rickets
  • skin disorders
  • tuberculosis

Athletes appear to perform better with high levels of Vitamin D. However, it can interact with medicines, for example with steroids, which reduce calcium absorption and in turn affect Vitamin D metabolism.

Vitamin D Deficiency

Although our daily requirements can easily be met if our body is exposed to sunlight, a deficiency is far more common than we think. Age, skin colour, season of the year, time of day, clouds, pollution and the use of sun-block reduce our body's ability to produce this vitamin. Furthermore, the ability of our liver and kidneys to convert Vitamin D to its active form and how well our body can absorb it from our digestive tract influence our Vitamin D level.

A study in 1998 by Thomas et al found that 37% of the total group surveyed were deficient in Vitamin D, even though their reported diets should have provided the currently recommended levels. Vegetarians and people with milk allergy or lactose intolerance are also linked to insufficient dietary intake.

Human milk does not provide all of our early requirements. A deficiency can cause rickets in children, which is a disease causing defective bone growth, skeletal deformation, as well as weak muscles. In adults, this deficiency is called osteomalacia, causing soft bones and weak muscles; its symptoms can easily go unnoticed in the beginning. Osteoporosis is a long-term effect of insufficient Vitamin D and calcium.

A deficiency might cause:

  • cancer
  • depression
  • diabetes
  • heart disease
  • hypertension
  • multiple sclerosis
  • osteodystrophy
  • osteomalacia
  • osteoporosis
  • rickets
  • weak immune system
  • weight gain

Vitamin D Toxicity

Too much sunlight cannot cause Vitamin D toxicity and dietary intake is very unlikely to cause toxicity but is possible if very high doses are consumed over long periods of time. However, toxicity is rather caused by excessive supplement intake. Based on new data, researchers and some clinicians agree that Vitamin D supplements are not the most likely supplements to cause toxicity as originally believed. New guidelines and recommendations adjust slowly.

Symptoms of toxicity might be:

  • anorexia
  • blindness
  • confusion
  • deafness
  • dehydration
  • depression
  • diarrhoea
  • drowsiness
  • fatigue
  • headache
  • heart rhythm problems
  • itching
  • kidney stones
  • lassitude
  • muscle weakness
  • nausea
  • poor appetite
  • thirst
  • urination (increased)
  • vomiting
  • weight loss

Foods high in Vitamin D

Only very few products contain Vitamin D. Although plants are able to produce some Vitamin D when they are exposed to UV-light, most of our intake comes from fortified products like milk and cereals. However, grains may interfere with Vitamin D absorption and milk used to make cheese or yoghurt is usually not fortified. Currently in Australia fortification is mandated for edible oil spreads (margarines) but only voluntary for milk, yoghurts, and cheese.

The most important compounds of about 10 in this vitamin group are ergocalciferol (vitamin D2 that can only be found in products of animal origin) and cholecalciferol (vitamin D3 that can be found in yeast and fungi). Vitamin D is unstable to light and air. Good sources include:

  • butter
  • cereals (fortified)
  • egg yolk
  • fish liver oil (cod)
  • high-fat fish (halibut, herring, mackerel, salmon, sardines, shark, tuna)
  • liver
  • milk (fortified)
  • mushrooms

It is still believed that our requirement is usually met by fortified foods and exposure to sunlight. However, the Dietary Reference Intakes (DRIs) for Vitamin D, developed by the Food and Nutrition Board (FNB) at the Institute of Medicine of The National Academies, have been revised based on more information and higher-quality studies: they now state 15mcg for adult men and women up to the age of 70 years, after which the amount increases to 20mcg for adult men and women. The Tolerable Upper Intake Level for Vitamin D is 100mcg. Australia and New Zealand state as their Nutrient Reference Values (NRVs) 5mcg for adult men and women up to the age of 50 years, after which the amount increases to 10mcg up to the age of 70 years with a further increase to 15mcg thereafter. The tolerable Upper Intake Level for Australia and New Zealand state 80mcg.

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