At a glance
Why get tested?
To check for vitamin D deficiency or to investigate a problem related to bone metabolism, mineral levels in the blood or parathyroid function.
When to get tested?
Your doctor may request a vitamin D 25 OH measurement as part of a general check-up because vitamin D deficiency appears to be very common in Australia. Vitamin D 25 OH measurement may also be requested if you have an abnormal calcium, phosphate, and/or parathyroid hormone level, as part of the investigation of some forms of bone disease or if you have kidney disease or a disease of the gastrointestinal tract that may result in malabsorption.
A blood sample drawn from a vein in your arm.
What is being tested?
Vitamin D is vital for the growth and health of bone; without it, bones will be soft, malformed, and unable to repair themselves normally, resulting in the disease called rickets in children and osteomalacia in children and adults. Vitamin D also helps to control the absorption of the minerals calcium, phosphate and (to a lesser extent) magnesium from food as it passes through the intestine. Vitamin D may also be important in preventing many other diseases including disorders of the , some forms of cancer and heart and blood vessel disease and it has been shown that the Vitamin D receptor (VDR) can affect a wide range of other gene expression, and may be involved in many cellular functions.
Vitamin D is a fat soluble vitamin found naturally in only a few foods, liver and fatty fish such as salmon, and so the major source of the vitamin is from sunlight skin exposure. A recommended time of sun exposure is difficult to say as it depends on a number of factors including skin type, latitude, time of year and time of day.
Vitamin D is now also contained in fortified foods; it is added to milk, cereals and a variety of other foods, to ensure adequate intake by the general population.
The body is able to form vitamin D by exposure to sunlight. This is why vitamin D is sometimes described as the sunshine vitamin - it is formed from 7-dehydrocholesterol in the skin when the skin is exposed to light. Vitamin D can also be ingested - either in a few foods or in vitamin supplements. Vitamin D produced in the bodies of humans and other animals is slightly different to that produced in plants – the animal form is known as vitamin D3 or cholecalciferol and the plant form as vitamin D2 or ergocalciferol. All supplements sold in Australia are now vitamin D3, however both forms vitamin D2 and D3 are active in the human body.
Both vitamin D2 and vitamin D3 are converted in the liver into 25-hydroxy-vitamin D. This is the main active form of vitamin D stored in the body. When a doctor asks for a vitamin D 25 OH level to be tested on a person’s blood, this is the form the laboratory will measure. The test for 25-hydroxy-vitamin D is used to check that the body has an adequate supply of vitamin D.
Occasionally, the doctor will ask the laboratory to measure another active form of vitamin D known as 1,25 dihydroxy-vitamin D. 1,25 dihydroxy-vitamin D is a form of vitamin D produced in a number of tissues, mainly the kidney, but also skin, colon, pancreas, adrenal and brain, from 25-hydroxy-vitamin D, via an enzyme 1 alpha-hydroxylase also known as (CYP27B1). It is tested in special circumstances such as in people with kidney disease who may not be able to make enough 1,25-dihydroxy-vitamin D. The doctor may also ask the laboratory to measure 1,25-dihydroxy-vitamin D if he or she suspects you have a condition in which the body produces too much of this form of the vitamin such as in sarcoidosis or some lymphomas.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
Are there other uses for vitamin D?
Yes, there is a topical form of vitamin D cream that is used to treat psoriasis.
How much vitamin D should I be getting each day?
This depends on your age and sex. The Australian and New Zealand Bone and Mineral Society (ANZBMS) and Osteoporosis Australia (OA) guidelines (2012) recommend that when there is minimal sun exposure vitamin D intake from dietary and supplementation sources for adults should be at least 600 IU (15ug) per day for people aged equal to or less 70 years and 800 IU (20ug) per day for those aged above 70 years.
The amound of daily sun exposure required to aid in maintaining adequate vitamin D levels depends on a number of factors including skin type, latitude, time of year and time of day. The following table gives recommendations of daily sun exposure to around 15 per cent of the body surface (e.g. hands, arms and face) for moderately fair-skinned people. People with dark skin are likely to need 3-6 times more sun exposure.
||Time of year and day
||december-january, 10am or 2pm
||july-august, 12 pm
Source: Nowson et al, Australian guideline MJA 2012; 196: 1-7
If I am vitamin D deficient, do I need to take a supplement containing calcium as well as vitamin D?
Many supplements contain both vitamin D and calcium, because both are needed for bone health. However, for most people with vitamin D deficiency, it is not necessary to take a supplement that also contains calcium. It is possible that such combined supplements are bad for the heart and blood vessels and it is probably better to ensure your diet contains enough calcium. However, there are some people for whom combined calcium and vitamin D supplements may be of overall benefit, such as frail elderly people in nursing homes in whom it is very important to prevent bone fractures.
Is fortifying foods with vitamin D a good practice?
Yes. The amount of vitamin D produced by the body may be insufficient, especially when there is limited exposure to sunlight and routine use of sunscreens. Since dietary vitamin D is found naturally only in a few foods dietary intake would not be sufficient for most people. In Australia it is mandatory that edible oil spreads (e.g. margarine) are fortified with vitamin D. Voluntary fortification of vitamin D is allowed for some products such as skim milks, powdered milk, yoghurts, dairy desserts, butter, breads, cereals and cheese.
Is vitamin D a necessary component of calcium supplements?
Since absorption of calcium is dependent on vitamin D, many manufacturers of calcium supplements add vitamin D to assure optimal calcium uptake. If you have adequate amounts of vitamin D from other sources, the additional vitamin D is not necessary. The amount of vitamin D in these tablets is not likely to lead to excess vitamin D or be harmful either.
Who are the adult groups at increased risk of vitamin deficiency?
- People who have low or no exposure to the sun, for example those who:
- are housebound or who are confined indoors for long periods (e.g. the elderly, hospitalised patients)
- wear concealed clothing (e.g. for cultural reasons)
- work in enclosed environments (e.g. factory and office workers) or night-shift workers
- People with naturally dark skin; the melanin in dark skin doesn't absorb as much UV radiation and their bodies are not able to make as much vitamin D
- People with fair skin and those who are at risk of skin cancer who avoid sun exposure
- People with a disability or chronic disease that affects vitamin D metabolism
- People who are obese