Vitamin D is a hormone widely recognized to be essential to many body systems and physiological processes. It may also be one of the most common nutrient deficiencies globally, with insufficient or deficient levels found in nearly 40% of Canadians.3-5  Vitamin D is critical for healthy immune function and may reduce the risk and/or severity of infections, especially those of the respiratory tract, and particularly in individuals prone to infection.6-8  Many recent studies have linked lower levels of this vitamin to an increased risk of cardiovascular disease, including hypertension, peripheral vascular disease, and coronary artery disease.9-12

Vitamin D is needed to help absorb dietary calcium, and it is also important for bone health and muscle strength. Vitamin D has been associated with overall physical fitness and performance, with lower levels linked to frailty, especially in older adults.13-17  Supplementation at clinically relevant doses has been associated with a well-established reduction in osteoporotic fracture.18,19  Given that essentially every tissue in the body has a receptor for vitamin D, it is hard to overestimate its importance.20  Many practitioners recommend supplementation to achieve blood levels of 50–80 nmol/L for 25-OH-vitamin D. This level often requires doses of 1000–2000 IU per day, and more for those with common genetic variants affecting vitamin D metabolism.21-23