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How do we know if we are getting enough Vitamin D?

Fortunately, vitamin D is one of the few vitamins for which there is a reliable blood test. Ask your doctor to measure 25-hydroxy (D) levels. Unfortunately, in Ontario you now have to pay for the test, unless you have pre-existing osteopenia or osteoporosis, or a malabsorption syndrome like celiac disease. It is unwise to take more than 2,000 IU vitamin D without the advice of a health care practitioner. Whether you take vitamin D supplements or not, a blood test as part of your annual physical is a good idea.

The amount you need will vary depending on where you live, how physically active you are, your sunshine exposure and your food intake. In addition, it now appears as if there is a wide genetic variation in vitamin D needs from one individual to another. The current recommended daily allowance (RDA) for different age groups is shown in the table below. Data is taken form from Health Canada’s website. Search for Vitamin D and Calcium: Updated Dietary Reference Intakes.

* Note that Health Canada defines the UL as the highest average daily nutrient intake level likely to pose no risk of adverse health effects.

Dr Robert Heaney, a leading vitamin D advocate and one of the world’s top researchers in osteoporosis says that this bone-weakening disease cannot be prevented at currently recommended doses of vitamin D. Michael F. Holick, MD, Boston University School of Medicine, an internationally known vitamin D researcher suggests that, in the absence of sunlight exposure, a minimum supplemental dose of 1,000 IU per day is needed to maintain a healthy concentration in blood. And that will only be adequate for some people.

Recent research suggests that it is not how much you take, but how high your blood levels of 25-hydroxy D are. And the same amount of supplemental vitamin D will differ in its effect on blood levels from one individual to another. There is therefore no “one size fits all” approach one can take to vitamin D supplementation. What is important is that you take the right amount to give you good blood levels of 25-hydroxy vitamin D. Only serums levels of 25-hydroxy (D) above 250 nmol/L are considered toxic.

Taking vitamin D supplements

The human body is physiologically designed to rapidly manufacture vitamin D. In Toronto on a sunny summer day around noon, exposing the body in a swimsuit for as little as 15 minutes can produce 10,000 IU of vitamin D. Older or darker skin needs longer exposure, and black skinned people have been shown to be almost twice as likely to be deficient in vitamin D compared to light skinned individuals (1).

There are two forms of supplemental vitamin D – vitamin D3, which is identical to the natural vitamin D formed in the skin; and vitamin D2, a synthetic form. Vitamin D3 is the best form of vitamin D to take as it has been shown to be more potent than D2. Taking too much vitamin D can elevate levels of calcium in the blood, a rare but potentially serious condition that can lead to nausea and vomiting, or even death. Those with kidney disease or primary hyperparathyroidism should not take vitamin D without first consulting their doctor.

Interpreting vitamin D blood test results

Vitamin D experts believe that treatment should be sufficient to maintain blood levels similar to those found in humans living naturally in a sun-rich environment, that is, more than 100 nmol/L (or 40 ng/ml) all year around (2).

• Concentrations below 20 nmol/L indicate serious deficiency (3).  A consensus meeting at Harvard in 2005 concluded that the lower limit of normal (that is, the beginnings of functionality) should be set at 80 nmol/L.
• Concentrations greater than 50 nmol/L are needed to prevent secondary hyperparathyroidism (4). At this level leg, foot or hip pain is common, as is pronounced body sway.
• A minimum of 78 nmol/Lis required to prevent osteoporosis (5, 6, 7, 8).
• Recent studies suggest that levels above 130 nmol/L are needed to reduce the risk of several types of cancer, including breast, prostate, lung and colon by 50%
• However, much higher blood concentrations – 160 nmol/L or more – are commonly seen in those with adequate sun exposure, living in southerly parts of North America, and such levels may be required for optimal health.  Work with a knowledgeable physician or other health care practitioner if you want to optimize your blood levels.

IMPORTANT NOTE: Some American articles and scientific papers quote 25-hydroxy D levels in ng/ml rather than the more widely used international units(SI) of nmol/L. To convert ng/ml to nmol/L multiply by 2.5.

Aileen Burford-Mason, PhD, is a biochemist, immunologist and cell biologist and a widely recognized expert in the field of vitamins and their appropriate use in health maintenance, healthy aging and the prevention and treatment of disease. Respected for her balanced and scientifically-based approach, Aileen is known for her ability to take the latest findings in diet and supplement research and translate them into practical evidence-based guidelines for both lay audiences and health professionals. A go-to person for radio, TV and print journalists, Aileen is in regular demand for media commentary on the latest research and controversies in the field of dietary supplements. She is the author of Eat Well, Age Better, with Judy Stoffman (2012, Thomas Allen Publishers). Learn more about Aileen at aileenburfordmason.ca.

References

(1). Talward SA, et al “Dose response to vitamin D supplementation among postmenopausal African-American women” Am J Clin Nutr 2007; 86: 1657-1662
(2). Cannell JJ, et al. Diagnosis and treatment of vitamin D deficiency. Expert Opin Pharmacother. 2008 Jan;9(1):107-18
(3). McKenna MJ. Differences in vitamin D status between countries in young adults and the elderly. Am J Med 1992;93: 69-77.
(4). Malabanan A, Veronikis IE, Holick MF. Redefining vitamin D insufficiency. Lancet 1998;351: 805-6.
(5). Chapuy MC, Preziosi P, Maamer M, Arnaud S, Galan P, Hercberg S, et al. Prevalence of vitamin D insufficiency in an adult normal population. Osteoporos Int 1997;7: 439-43
(6). Chapuy MC, Arlot ME, Duboeuf F, Brun J, Crouzet B, Arnaud S, et al. Vitamin D3 and calcium to prevent hip fractures in elderly women. N Engl J Med 1992;327: 1637-42.
(7).Dawson-Hughes B, Harris SS, Krall EA, Dallal GE. Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J Med 1997;337: 670-6.
(8). Vieth R. Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. Am J Clin Nutr 1999;69: 842-56