What is vitamin D?

Vitamin D is a fat-soluble vitamin that is naturally found in a few foods and also produced by the body when ultraviolet (UV) rays from sunlight start the formation of vitamin D in the skin.¹ In foods and dietary supplements, vitamin D has two main forms, D2 (ergocalciferol) and D3 (cholecalciferol), that differ only in their chemical structures. However, the majority of evidence indicates that vitamin D3 raises blood levels to a greater extent and maintains these higher levels longer than vitamin D2, even though both forms are well absorbed in the gut.² Most people are also able to get some of their vitamin D needs through exposure to sunlight. Type B UV (UVB) radiation in sunlight penetrates uncovered skin and converts 7-dehydrocholesterol in the skin to previtamin D3, which then becomes vitamin D3. UVB radiation does not penetrate glass, however, so exposure to sunshine indoors through a window does not produce vitamin D. Older people and people with dark skin are less able to produce vitamin D from sunlight.³

What are the functions of vitamin D?

Vitamin D supports calcium absorption in the gut and maintains adequate blood levels of calcium and phosphate to allow the normal bone development process of deposition of minerals on the bone matrix. It is also needed for bone growth and bone remodeling, following injury. In addition, Vitamin D is involved in the reduction of inflammation as well as regulation of neuromuscular and immune function, and glucose metabolism.³ Finally, Vitamin D is needed for the normal development and growth of muscle fibers and low vitamin D levels can negatively affect muscle strength and lead to muscle weakness and pain.³

Does vitamin D supplementation alter outcomes?
Although there is considerable debate over the definitions, Vitamin D deficiency is defined by the Endocrine Society as a level < 20 ng/ml while Vitamin D insufficiency is defined as a level of 21–29 ng/ml.⁴ Applying these definitions, the occurrence of vitamin D insufficiency in the general population is as high as 50 to 80%.⁵ A systematic review of studies examining Vitamin D levels and outcomes after surgery reported that the majority (but not all) the studies showed significantly worse outcome in patients with low vitamin D status.⁶ In a small study, preoperative supplementation of Vitamin D in spine surgery patients decreased inflammation and the intensity of pain after surgery.⁷ Vitamin D supplementation might also play an important role in prevention of skeletal muscle injuries following exercise.⁸

What is the suggested dose of Vitamin D?

The Food and Nutrition Board at the National Academies of Sciences, Engineering, and Medicine has established that the maximum daily intake unlikely to cause adverse health effects (i.e. upper limit) for adults older than 18 years is 100 micrograms or 4000 International Units.¹

1. NIH. Vitamin D Fact Sheet for Health Professionals. Accessed March 14, 2021. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
2. Logan VF, Gray AR, Peddie MC, Harper MJ, Houghton LA. Long-term vitamin D3 supplementation is more effective than vitamin D2 in maintaining serum 25-hydroxyvitamin D status over the winter months. Br J Nutr. Mar 28 2013;109(6):1082-8. doi:10.1017/S0007114512002851
3. Medicine Io. Dietary Reference Intakes for Calcium and Vitamin D. The National Academies Press; 2011:1132.
4. Holick MF, Binkley NC, Bischoff-Ferrari HA, et al. Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. J Clin Endocrinol Metab. Jul 2011;96(7):1911-30. doi:10.1210/jc.2011-0385
5. Rosen CJ. Clinical practice. Vitamin D insufficiency. N Engl J Med. Jan 20 2011;364(3):248-54. doi:10.1056/NEJMcp1009570
6. Iglar PJ, Hogan KJ. Vitamin D status and surgical outcomes: a systematic review. Patient Saf Surg. 2015;9:14. doi:10.1186/s13037-015-0060-y
7. Krasowska K, Skrobot W, Liedtke E, et al. The Preoperative Supplementation With Vitamin D Attenuated Pain Intensity and Reduced the Level of Pro-inflammatory Markers in Patients After Posterior Lumbar Interbody Fusion. Front Pharmacol. 2019;10:527. doi:10.3389/fphar.2019.00527
8. Zebrowska A, Sadowska-Krepa E, Stanula A, et al. The effect of vitamin D supplementation on serum total 25(OH) levels and biochemical markers of skeletal muscles in runners. J Int Soc Sports Nutr. Apr 9 2020;17(1):18. doi:10.1186/s12970-020-00347-8