When making a list of the most effective and vital dietary compounds, it would be difficult to leave out vitamin D3. Because UV-B radiation is required for vitamin D production from pro-vitamin D3 (a precursor) in the skin, modern lifestyles are prohibitive to maintaining healthy levels without supplementation. In fact, vitamin D deficiency and insufficiency are conditions that are now collectively considered a world-wide epidemic with particularly detrimental consequences.
Deficient vitamin D levels have been shown to significantly impact health in both short and long-term studies. Long-term findings associated with vitamin D deficiencies have included cardiovascular disease, arthritis, diabetes, intestinal cancer, and osteoporosis. [3, 6] Short term studies have shown that deficient vitamin D levels can lead to “muscle weakness, bone pain, and fragility fractures” .
Vitamin D levels have long been associated with bone mineral density, calcium absorption, and other elements of bone health. Recent research has shown that vitamin D Receptors (VDRs) are present on more than just bone cells, but also throughout the immune system, digestive tract, muscle tissue, and many other tissues across the body. The presence of VDRs in these cell types has led to findings that vitamin D plays critical roles in regulating healthy tissues and optimizing their function.
A select number of these cell types are of crucial importance for athletes, not limited to their influence over bone mass density, immune function, and peak force generated by muscle contraction. 
One specific role of vitamin D includes the proliferation, hypertrophy, and regeneration of skeletal muscle through IGF-1, another protein influenced through the VDR cascade. Further evidence for vitamin D’s influence over muscle function is its role in calcium absorption and subsequent muscle performance. In studies examining calcium supplementation versus vitamin D plus calcium supplementation (like the combination found in our D1 Mass Stack), researchers found improvements in knee flexion, knee extension, and grip strength . In vivo studies have also shown that by depleting vitamin D binding of VDRs, muscle groups were 20% smaller than those found in unregulated and functional vitamin D binding sites. [1,2,3]
A crucial component of athletic mastery is time and quality of time spent on task. Ensuring your body’s readiness to perform at practice and in competition requires proper immune function. Deficient levels of vitamin D have been shown to increase susceptibility to infection and are associated with increased autoimmunity. Vitamin D has been used to treat infections throughout history (as seen with tuberculosis patients prescribed sunlight and cod liver oil), but recent studies have highlighted the significance of vitamin D and protection against infection. When using vitamin D in a double blind placebo controlled study to evaluate influenza susceptibility, researchers found vitamin D groups were 42% less likely to develop flu. As these and other studies have shown, sufficient levels of vitamin D support healthy immune responses, and should be a mainstay component of your supplementing routine. 
The optimal dose of vitamin D depends on several factors. Namely, you should be aware of how much time you spend outside, what time of day you go outside, how many high-vitamin D2/vitamin D3 foods you consume, your body fat percentage (vitamin D is fat-soluble and will decrease available levels), and how tan you are. A standard recommendation for ensuring you don’t become deficient in serum vitamin D levels is 15-25 minutes of direct midday sunlight across as much surface area as possible . Supplementation with vitamin D3 is regarded as a strong and safe way to ensure you are above insufficient levels and in the optimal window of 40-60ng/ml. Aiming for between 1,000 and 4,000 IUs of vitamin D3 will allow you to raise serum levels of vitamin D, though you should consult with your healthcare provider to see how your lifestyle habits shape your current levels; it’s as easy as a standard blood test. It is important to keep in mind the vitamin D toxicity levels shown in many studies, which is reached with doses higher than 10,000 IU. [1,2,4]
When you use D1 Lab’s stack, you’ll get 2,000 IU of third-party tested vitamin D3, along with calcium, key minerals, and 4 other clinically-tested doses of key supplements for peak performance.
- Ogan, Dana, Kelly Pritchett. Vitamin D and the Athlete: Risks, Recommendations, and Benefits. Nutrients. 2013.
- Holick, M F. Vitamin D and Bone Health. The Journal of Nutrition. 1996.
- Hamilton, Bruce. Vitamin D and Athletic Performance: The Potential Role of Muscle. Asian Journal of Sports Medicine. 2011.
- Patrick, Rhonda. The Nuances of Vitamin D--and How to Get Enough of It. Goop Wellness. 2020. https://goop.com/wellness/health/the-nuances-of-vitamin-d-and-how-to-get-enough-of-it/
- Aranow, Cynthia. Vitamin D and the Immune System. Journal of Investigative Medicine. 2012.
- Haimi, Motti; Richard Kremer. Vitamin D deficiency/insufficiency from childhood to adulthood: Insights from a sunny country. World Journal of Clinical Pediatrics. 2017.
- Pfotenhauer, Kim M.; Jay H. Shubrook. Vitamin D Deficiency, Its Role in Health and Disease, and Current Supplementation Recommendations. The Journal of the American Osteopathic Association. 2017.
- Sakamoto, Rosario Rose. Sunlight in Vitamin D Deficiency: Clinical Implications. The Journal for Nurse Practitioners. 2019.