How Much Vitamin D is Safe?
Vitamin D is a hot topic in health research. Most of the excitement revolves around its potential to prevent and treat chronic diseases. This reflection will consider the risks and benefits of vitamin D supplementation. On one hand, a lack of vitamin D can lead to a nutritional deficiency. On the other hand, too much vitamin D can have toxic effects. In the end, the question I will pose to the reader and answer for myself is: how much vitamin D supplementation should we take?
Vitamin D is a nutrient that regulates the absorption and metabolism of calcium and phosphate. It is the only vitamin that can be synthesized, but it is still considered a vitamin because the conversion of cholesterol to vitamin D can only take place when the skin is exposed to ultraviolet (UV) light. Deficiencies are common when people have little sun exposure and little access to vitamin D rich foods. Severe deficiencies manifest themselves as rickets in children and osteomalacia in adults. In both of these conditions, the bones become soft because they lack sufficient mineralized calcium and phosphate.
Vitamin D deficiency is a real concern at latitudes associated with long winters because the population is completely dependent on dietary sources of vitamin D for a large portion of the year. For example, people cannot synthesize vitamin D from November until February in most of Canada.1 In Saskatoon, Saskatchewan, sun exposure is likely inadequate from October until March. So, where do Canadians get their vitamin D in the winter? Milk products and alternatives are enriched with enough vitamin D so that the recommended daily intake (RDI) of vitamin D can be acquired if you eat and drink according to the recommendations in the Canadian Food Guide. However, there are two problems with this policy. The first (and most obvious) is that not everyone eats and drinks according to the Canadian Food Guide. Some ethnic groups that have immigrated to Canada do not drink milk or milk substitutes. The second problem is that the current RDI may not be sufficient for optimal health. While the current RDI will certainly prevent rickets and osteomalacia, new studies are indicating that higher levels of vitamin D may be required to prevent cancer and heart disease.2 Low vitamin D levels have also been linked with an increased risk of preeclampsia in pregnant women.3
So how can Canadians get more vitamin D? While there are some vitamin D rich foods such as liver and egg yolks, vitamin D supplements have become much more popular, partially because it is much easier to control the dose. Vitamin D supplements are now easily accessible at all pharmacies and many grocery stores. Vitamin D can be part of a multivitamin or by itself in chewable pills, liquid capsules, or liquid droppers. There is also a wide array of doses available, ranging from 400 – 50 000 International Units (IU) (10 – 1250μg).4
Naturally, when consumers start to read the labels showing different doses, they will be concerned with original question in this reflection. How much vitamin D is safe? The answer is not too little and not too much. Vitamin D has a typical U-shaped dose-response curve where adverse effects increase at both low and high doses. Hypercalcemia is the first known adverse effect seen with vitamin D toxicity, but the lowest observable adverse effect level (LOAEL) remains unknown.5 Hypercalcemia can lead to calcification of soft tissues and has been linked with cardiovascular disease.6 It is important to note that these toxic effects only occur with excessive dietary intake or direct injection, not sun exposure. This is because the sun’s UV radiation acts as a negative feedback system by breaking down excess vitamin D under the skin.5 Another common misconception is that vitamin D causes birth defects. Vitamin D is not a teratogen, but vitamin D induced hypercalcemia may cause birth defects involving vascular stenosis.7 Consumers need to be aware that is important avoid toxicity as well as deficiency.
Hathcock et al. published a risk assessment indicating that the no observable adverse effects level (NOAEL) for vitamin D is 10,000 IU (250 ug)/day.5 This dose is based on two randomized controlled trials that found no adverse effects in healthy men after 8 and 20 weeks. The authors recommend that an uncertainty factor should be applied to this NOAEL to account for variation in the population.
After a jointly funded investigation in 2010, the Canadian and American governments applied an uncertainty factor of 10 to this NOAEL for infants (0-6 months) to establish a tolerable upper intake level (UL) of 1000 IU/d. The UL increases with age: 1500 IU/d for infants (7-12 months), 2000 IU/d for children (1-3 years), 3000 IU/d for children (4-8 years), and 4000 IU/d for children (9+ years) and all adults.8,9 The ULs for vitamin D in Europe are based on the same studies and are very similar: 1000 IU/d for infants (0-12 months), 2000 IU/d for children (1-10 years), and 4000 IU/d for children (10+years) and adults.10
In the United Stated and Canada, the RDIs are 400 IU for infants (0-12 months), 600 IU for children and adults (1-70 years), and 800 IU for seniors older than 70.8,9 The Canadian government advises that dietary intake is adequate for most Canadian but seniors over the age of 50 should take daily vitamin D supplements with 400 IU. These recommendations are comparatively higher than the guidelines set by the World Health Organization (200 IU for adults and 600 IU for seniors) and most European countries (200-400 IU for adults and seniors).11 These differences are due to differences in perceived health benefits and basic assumptions about sun exposure and dietary habits.
The question remains, are these government recommendations adequate for optimal health? Hathcock et al. report that despite many hours of sun exposure, vitamin D levels in outdoor workers at the end of the summer are equivalent to a daily intake of 2800-4000 IU.5 This supports that a UL of 4000 IU is reasonable. However, considering that humans have worked outdoors for the vast majority of their existence, it seems reasonable to me that our biological systems are best adapted to sunshine abundant conditions and a higher RDI may be warranted. While there is no decisive evidence that daily doses higher than the RDIs are beneficial, there is strong evidence that daily doses below 4000 IU pose no harm. The most reasonable safe solution that I have formulated for myself is to spend time outdoors in the summer and to take a daily vitamin supplement of 1000-2000 IU in the winter (October-March).
In summary, this reflection has looked at the risks and benefits of vitamin D supplementation. I believe that dietary sources of vitamin D and sunlight exposure are adequate for me in the summer, but taking vitamin D supplements of 1000-2000 IU/d in the winter is warranted. What do you think is an appropriate amount of supplementation for optimum health?
- Sharma, S., Barr, A. B., Macdonald, H. M., Sheehy, T., Novotny, R., & Corriveau, A. (2011). Vitamin D deficiency and disease risk among aboriginal Arctic populations. Nutr Rev, 69(8), 468-478. doi: 10.1111/j.1753-4887.2011.00406.x
- Holick, M. F. (2007). Vitamin D deficiency. N Engl J Med, 357(3), 266-281. doi: 10.1056/NEJMra070553
- Marya, R. K., Rathee, S., Manrow M. (1987). Effect of calcium and vitamin D supplementation on toxaemia of pregnancy. Gynecol Obstet Invest, 24:38–42. doi: 10.1159/000298772
- Haines, S. T., & Park, S. K. (2012). Vitamin D supplementation: what’s known, what to do, and what’s needed. Pharmacotherapy, 32(4), 354-382. doi: 10.1002/phar.1037
- Hathcock, J. N., Shao, A., Vieth, R., & Heaney, R. (2007). Risk assessment for vitamin D. American Journal of Clinical Nutrition, 85(1), 6-18. hyperlink
- Brandenburg, V. M., Vervloet, M. G., & Marx, N. (2012). The role of vitamin D in cardiovascular disease: From present evidence to future perspectives. Atherosclerosis, 225(2), 253-263. doi: 10.1016/j.atherosclerosis.2012.08.005
- Kaushal, M. and Magon N. (2013). Vitamin D in pregnancy: A metabolic outlook. Indian J Endocrinol Metab, 17(1): 76–82. doi: 10.4103/2230-8210.107862
- The United State Office of Dietary Suppliments. http://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
- Health Canada. http://www.hc-sc.gc.ca/fn-an/nutrition/vitamin/vita-d-eng.php
- EFSA Panel on Dietetic Products, Nutrition and Allergies (2012). Scientific Opinion on the Tolerable Upper Intake Level of vitamin D. EFSA Journal, 10(7):2813 doi: 10.2903/j.efsa.2012.2813
- Doets, E. L., et al. (2008) Current micronutrient recommendations in Europe: towards understanding their differences and similarities. European Journal of Nutrition. 47(1):17-40. doi: 10.1007/s00394-008-1003-5