Vitamin D – should you take it?

Vitamin D – should you take it? 
By Lindsay Kobayashi
Posted: April 24, 2014

Egg yolks contain a small amount of vitamin D
iStockphoto.com

My hunch is that it depends. Vitamin D is a nutrient that helps our bodies regulate the metabolism of calcium and phosphate (1). Most vitamin D comes from sunlight, while it is also found in certain foods including fatty fish, mushrooms, egg yolks, vitamin-D fortified foods. For example, milk in many countries is always fortified with vitamin D, and some brands of breakfast cereals and orange juice are fortified as well (2).  Vitamin D can also be obtained through taking vitamin D supplements found at your local grocery or health food store. The classic health consequences of inadequate vitamin D are rickets in children, and low bone mineral density and osteoporosis in older adults (3). Low vitamin D has also been associated with increased risk for many other health conditions including breast, prostate, and colorectal cancer, multiple sclerosis, and cardiovascular disease (4-6). However, the quality of scientific evidence for these relationships varies because it is actually quite challenging methodologically to study the cause-effect relationship of vitamin D on health.

Because definitive high-quality evidence is lacking, the actual beneficial effect of vitamin D on health has been heavily debated in recent years. Like many other dietary or lifestyle factors that have been linked to health outcomes with scientific uncertainty (examples: coffee, alcohol, vitamin C, herbal supplements), the available information about whether to take vitamin D supplements can be very confusing. Here is where we stand right now:

In 2011, the American Institute of Medicine released an expert report on the dietary reference intakes for vitamin D (3). They stated that, for people aged 1 to 70 years old including pregnant and lactating women, the recommended dietary allowance (RDA) is 600 IU per day of vitamin D. For adults aged over 70 years the RDA is 800 IU per day. Intake should not exceed 4000 IU per day for people aged 9 years and over. The full RDA guidelines can be found here. Interestingly, their expert panel concluded that current scientific evidence is insufficient to conclude that vitamin D plays a causal role in non-bone-related health conditions (3). Now, this statement may or may not mean that vitamin D has no effect on health aside from bone conditions, simply that our current knowledge is insufficient.

Supplements can be a good source of vitamin D

iStockphoto.com

Fast forward to today, and it doesn’t seem like our evidence base has evolved much. An ‘umbrella’ review of evidence on the link between blood plasma concentrations of vitamin D and 137 unique health outcomes was published in the British Medical Journal earlier this month (7). The review was the largest synthesis of knowledge to date, and the authors unfortunately had to conclude that:

“Despite a few hundred systematic reviews and meta-analyses, highly convincing evidence of a clear role of vitamin D does not exist for any outcome, but associations with a selection of outcomes are probable”

The authors concluded that vitamin D supplementation is probably linked to decreased dental caries (cavities) in children, reduced parathyroid hormone concentrations in patients with chronic kidney disease requiring dialysis, and to an increase in maternal vitamin D concentrations at term, and an increase in birth weight (7). These are very specific conditions that apply only to children, pregnant mothers, and chronic kidney disease patients. The authors also concluded that the evidence is ‘suggestive’ for a correlation between higher blood vitamin D concentrations and a lower risk of several conditions including colorectal cancer, non-vertebral fractures, cardiovascular diseases, depression, high body mass index, and type 2 diabetes (7). However, a major point to note is that these are correlations, which means that although vitamin D has been associated with these health conditions, it may not cause them. Because of the limitations of current research, including the difficulty in measuring the actual vitamin D intake of people, and how much of this actually gets absorbed and has a biological effect, the timing between vitamin D intake and disease onset, and determining the actual dose of vitamin D that may protect against disease, we don’t have definitive answers right now.

So, what should we do about our own health? It is clearly too soon to make any strong recommendations about population-level vitamin D supplementation. Following the current RDA for vitamin D is good, and achieving this level for yourself may include supplementation if you don’t eat many foods containing vitamin D. Always talk to your family physician if you have any concerns about your own health or vitamin D intake. And finally, as always, keep yourself informed with high quality information to make decisions for your own health.

References
1)      National Health Service. Vitamins and minerals – vitamin D. http://www.nhs.uk/Conditions/vitamins-minerals/Pages/Vitamin-D.aspx (accessed 21 April 2014).
2)      National Institutes of Health. Vitamin D: Fact sheet for consumers. http://ods.od.nih.gov/factsheets/VitaminD-QuickFacts/#h3 (accessed 21 April 2014).
3)      Committee to Review Dietary References Intakes for Vitamin D and Calcium, Institute of Medicine: Dietary Reference Intakes for Calcium and Vitamin D. Edited by Ross AC, Taylor CL, Yaktine AL, Del Valle HB. Washington, DC: The National Academies Press; 2011.
4)      Holick MF. Vitamin D deficiency. N Engl J Med 2007;357:266-81.
5)      Munger KL, Zhang SM, O’Reilly E, Hernán MA, Olek MJ, Willett WC, et al. Vitamin D intake and incidence of multiple sclerosis. Neurology 2004;62(1):60-5.
6)      Wang TJ, Pencina MJ, Booth SL, Jacques PF, Ingelsson E, Lanier K, et al. Vitamin D deficiency and risk of cardiovascular disease. Circulation 2008;117:503-11.
7)      Theodoratou E, Tzoulaki I, Zgaga L, Ioannidis JPA. Vitamin D and multiple health outcomes: umbrella review of systematic reviews and meta-analyses of observational studies and randomised trials. BMJ2014;348:g2035

Source PLOS Blogs