Vitamin D Blood Test
There is growing support for having an annual vitamin D blood test. Canadian labs now use 75-200 nmol/L (30-80 ng/ml) as the acceptable range
The Vitamin D Council recommends that levels of 125 nmol/L (50 ng/ml) be maintained year-round, in both children and adults.
To view comments about vitamin D dosage and supplements visit our vitamin D deficiency symptoms page.
Osteoporosis treatment guidelines emphasize the importance of Vitamin D for building strong bones and also for maintaining a strong immune system. The symptoms of vitamin D deficiency are so subtle that we may not notice that our intake is inadequate until our bones are brittle and health problems appear. The best way to determine if your vitamin D levels are sufficient is to take the vitamin D blood test 25(OH)D to measure 25-hydroxyvitamin D.
There are four things that should be noted about this blood test:
- The 25(OH)D test should not be confused with a test for 1,25-dihydroxyvitamin D. (Levels of 1,25(OH)2D do not typically decrease until vitamin D deficiency is severe.)
- Serum concentrations of 25(OH)D are reported in both nanograms per milliliter (ng/mL) and nanomoles per liter (nmol/L). (1 ng/mL = 2.5 nmol/L)
- The test measures the vitamin D circulating in the blood and not that which is stored in other parts of the body.
- In the northern hemisphere, blood levels will peak around September and be the lowest around March. An acceptable summer score is not indicative of year round health.
In 2007, a group of Vitamin D and nutrition researchers published an editorial arguing that we should strive for vitamin D blood test results of 75 nmol/L (30 ng/ml) or more... and that 400 IU/day of vitamin D did little to help us reach this level. They also stated that approximately 1,700 IU of vitamin D are needed daily to raise blood levels from 20 to 32 ng/mL. (Vieth R, Bischoff-Ferrari H, Boucher BJ, Dawson-Hughes B, Garland CF, Heaney RP, et al. The urgent need to recommend an intake of vitamin D that is effective. Am J Clin Nutr 2007;85:649-50.)
The debate over the optimal blood levels is as vigorous as the debate over the vitamin D dosage required from supplements. As this debate is likely to continue for some time, it is useful to get an overview of all the recommendations... so that we can choose a target for our vitamin D blood test and a vitamin D strategy that will optimize our well-being.
Experts Recommending Greater Levels
In 1997, the US Institute of Medicine- Food and Nutrition Board identified > 37.5 nmol/L (>15 ng/ml) as adequate for bone and overall health in healthy individuals. But Health Canada has identified 75-200 nmol/L (30-80 nb/ml) as the acceptable range and the Vitamin D Council recommends that we maintain a 125 nmol/L (50 ng/ml) vitamin D blood level to optimize our immune system.
This, of course, has implications for how much vitamin D we need to get through supplements... especially for people living in northern regions. Here are some of their arguments and recommendations regarding vitamin D blood tests.
Dr. Michael Holick (>75 nmol/L or 30 ng/ml)
Dr. Michael Holick, author of "The UV Advantage" recommends that we maintain bloods levels above 75 nmol/L (30 ng/ml). He found that north of the 42 latitude (north of Boston) that 1,000 IU of vitamin D a day was unable to raise blood levels above 30 ng/ml.
He recommended that people take 2,000 IU of vitamin D in the winter when the sun does not provide enough ultraviolet radiation to support Vitamin D production through the skin. This recommendation is now supported by Osteoporosis Canada for women over fifty.
Dr. Holick notes that 100% of the vitamin D produced from sunlight is bound to the vitamin D binding protein while only 60% of the vitamin D that is ingested from food or supplements is bound to the vitamin D binding protein. Thus, the vitamin D that is made in the skin has a longer half-life in the circulation than vitamin D that is ingested.
Many people concerned about their immune system (and especially those with a gastrointestinal disease) choose to use a sunlamp rather than rely on supplements.
Vitamin D Council (125-200 nmol/L or 50-80 ng/ml)
The Vitamin D Council recommends that vitamin D levels of 50-80 ng/ml be maintained year-round, in both children and adults. The Council bases its recommendation on research conducted by Bruce Hollis which found that the body does not reliably begin storing vitamin D3 in fat and muscle tissue until 25(OH)D levels get above 125 nmol/L (50 ng/ml).
The average person starts to store cholecalciferol at 100 nmol/L but at 125 nmol/L virtually everyone begins to store it for future use. Hollis argues that at levels below 125 nmol/L (50 ng/ml) the body uses up vitamin D as fast as we can make or ingest it.
The Council suggests that 2,000-5,000 IU of vitamin D are needed in the Fall and winter with the higher level needed in the northern regions where the "vitamin D winter" is the longest. It also points out the lack of evidence for any toxicity or vitamin D side effects below 10,000 IU a day.
Laboratory Assays Are Now Standardized
Until recently, there has been considerable variability in the assays used by laboratories. This means that compared to the actual concentration of 25(OH)D in a sample of blood serum, a falsely low or falsely high value could be obtained.
A standard reference material for 25(OH)D became available in July 2009 and will now permit standardization of values across laboratories.
The U.S. National Health and Nutrition Examination Survey (NHANES III) found that when researchers gave a vitamin D blood test to over 13,000 people in the United States, that 73% had blood levels lower than 75 nmol/L (30 ng/ml). The Vitamin D Society in Canada reports that 97 percent of Canadians may be vitamin D deficient at some point in the year.
Many health experts believe that administering an annual vitamin D blood test will soon become a regular medical practice. But there is no need to wait for these tests to become a common practice.
Patients can work proactively with their health care providers to ensure that a 25(OH)D test is not only included in their check-ups... but that they have a vitamin D strategy to maintain the optimal blood level of this essential vitamin.
A vitamin D blood test in the middle of winter will tell you if your bloods levels are adequate to support calcium absorption. Six months of 5,000 IU vitamin D will cost approximately $10.