Vitamin D is produced in the skin from exposure to ultraviolet B (UVB) radiation and is thus commonly referred to as the sunshine vitamin. Vitamin D is classified as a fat-soluble vitamin even though technically speaking it is a prohormone (any substance that can be converted to a hormone) with its hormonally active form, calcitriol, playing a key role in calcium metabolism and bone health.

The active form of vitamin D, derived from sunlight and dietary sources, works in conjunction with the parathyroid hormone and the hormone calcitonin to regulate serum calcium and phosphorus concentrations, generally by enhancing intestinal absorption of these minerals, facilitating normal bone mineralization, and protecting against debilitating osteoporotic and stress-related fractures. Vitamin D, which is obtained from sun exposure, food, and supplements, must undergo two chemical reactions within the body before it’s available for use. Initially, vitamin D is converted to 25-hydroxyvitamin D [25(OH)D], also known as calcidiol, within the liver and then forms physiologically active 1,25-dihydroxyvitamin D [1,25(OH)2D], also known as calcitriol. As a steroid hormone, calcitriol also regulates more than 50 genes in tissues throughout the body, including muscle and nerve tissue, a mechanism of action thought to have a positive impact on athletic performance especially as it relates to the neuromuscular system.

Performance benefit: Vitamin D–deficient athletes may benefit from stronger bones and increases in muscle strength and speed by increasing vitamin D stores through whole food intake, supplementation, and sun exposure. Vitamin D is found naturally in such food sources as cod liver oil (1 tbsp. =1,360 IU), salmon (3.5 oz [99 g] = 360 IU), and egg yolk (1 = 20 IU); fortified in several products such as milk (1 c = 98 IU); and as a supplement. The recommended daily amount of vitamin D, ranges from 600 IU for adults under age 70 to 800 IU for age 71+.

Athletes with serum 25(OH)D levels below 30 ng/dL, as revealed by a simple blood test, are encouraged to discuss ways to increase vitamin D via whole food intake, sun exposure, or supplementation with a doctor or a registered dietitian. A serum 25(OH)D concentration of 40 ng/mL or greater has been shown to be achievable with a daily supplementation protocol that incorporates 4,000 IU of vitamin D3.

Health concerns: Chronically low serum 25(OH)D concentrations (<10 ng/dL) can lead to rickets in children and osteomalacia in adults, both characterized by soft bones and increased susceptibility to fracture. At the other end of the spectrum, toxicity symptoms, indicated at serum 25(OH)D levels ranging from 200-240 ng/mL or supplementation doses of 10,000-40,000 IU/day, include weakness, muscle pain, bone pain, loss of appetite, nausea, intestinal cramps, headache, metallic taste, and in most severe cases renal impairment. Athletic performance has shown to decline with a supplementation protocol of 5,000 IU or more.

Additional performance benefits: By reducing the production of proinflammatory cytokines and increasing the production of anti-inflammatory cytokines, vitamin D may help speed the recovery process between hard workouts.