Vitamin D deficiency is an overlooked and under-evaluated condition that is widespread in the athletic population, including the elite and professional basketball players. Although the symptomatology of this deficiency is well known in the general population (osteoporosis and increased risk of fracture), the symptom of muscle fatigue and athletic performance are not. There are many recent publications that spotlight the association between vitamin D deficiency, muscle fatigue, and athletic performance in professional athletes (Figure 1).
Figure 1: Vitamin D is an important supplement for basketball.
Vitamin D is a steroid hormone found in the diet and synthesized in the skin in the presence of ultraviolet radiation. Vitamin D has proposed roles in bone health, the inflammatory response, immunity, neuromuscular function, carcinoma risk, and overall mortality risk. The role of vitamin D in bone health has been studied extensively, and deficiency may predispose athletes to stress fractures. It also has a protective effect on the upper respiratory system. [1,2] New studies conclude that vitamin D can reduce the risk of developing COVID-19 as well as decrease the severity of the illness.
In addition to mediating bone health and inflammation, vitamin D may also influence neuromuscular function and athletic performance. Vitamin D is believed to directly affect skeletal muscle through a specific vitamin D receptor. Vitamin D has been shown to influence calcium storage in type II muscle fibers of which we use for sports performance. Sorensen and colleagues demonstrated an increased percentage and area of type II muscle fibers on muscle biopsies of patients receiving calcium and vitamin D supplementation.  Myon and colleagues assessed the influence of vitamin D supplementation on the athletic performance of elite ballet dancers.  In a randomized, double-blind trial, judo athletes supplemented with vitamin D3 demonstrated a 13% increase in muscle strength compared with a placebo group. The authors found an 18.7% increase in isometric quadriceps strength, a 7.1% increase in the vertical jump, and a decreased overall incidence of injury in the dancers receiving supplementation. 
Hypovitaminosis D (vitamin D deficiency) is very common among professional basketball players at the NBA Combine, affecting 73.5% of players. Dark skin pigmentation is a known risk factor for vitamin D insufficiency. The increased melanin found in the skin of darkly pigmented individuals may increase the amount of time to synthesize vitamin D up to 10-fold. [1,2] In the NFL study by Maroon and colleagues, black players had significantly lower vitamin D levels than white players.  In fact, all players in that study with deficient vitamin D levels were black players. Dark skin pigmentation would be considered a significant risk factor among professional basketball players, with up to 76.7% of players in the NBA.
The effect of vitamin D on athletic performance has been a subject of research since the 1930s. Vitamin D is believed to enhance neuromuscular function at the skeletal muscle level. [1,2] When we looked at the published NBA draft status as a surrogate measure of athletic performance for NBA Combine participants, a significant association was found between increasing levels of vitamin D and being drafted into the NBA. (Figure 2) NBA Combine participants drafted into the NBA would be expected to be superior in terms of their health and athleticism. 
Figure 2: Distribution of Vitamin D levels and drafted NBA players.
In summary, Vitamin D insufficiency was prevalent amongst male basketball athletes, with Vitamin D levels being lower in the autumn pre-season (October) than the summer off-season (July). Further, darker skin pigmentation significantly correlated with Vitamin D, indicating that individuals with darker skin tones may be at a greater risk of insufficiency/deficiency. Due to the importance of Vitamin D on health and sports performance, it is recommended that practitioners working with basketball athletes consider routine monitoring of Vitamin D status. Doing so may guide nutrition professionals in their recommendations for vitamin D supplementation, as standards in optimal intake are lacking. Additional supplementation may aid in improving athletic performance and overall health. Based on the current studies, it is suggested to have vitamin D blood levels over 30 ng/mL to improve health status and over 50 ng/mL for athletic performance. We have to keep in mind that Vitamin D blood levels over 100 ng/mL would be harmful to liver cells, which are the main storage for the vitamin. For that reason, it is recommended to keep Vitamin D blood levels between 50 ng/mL and 100 ng/mL.
Grieshober JA, Mehran N, Photopolous C, Fishman M, Lombardo SJ, Kharrazi FD. Vitamin D Insufficiency Among Professional Basketball Players: A Relationship to Fracture Risk and Athletic Performance. Orthop J Sports Med. 2018 May 21;6(5):2325967118774329.
Fishman MP, Lombardo SJ, Kharrazi FD. Vitamin D Deficiency Among Professional Basketball Players. Orthop J Sports Med. 2016 Jul 6;4(7):2325967116655742.
Sorensen OH, Lund B, Saltin B, et al. Myopathy in bone loss of aging: improvement by treatment with 1 alpha-hydroxycholecalciferol and calcium. Clin Sci (Lond). 1979;56(2):157-161.
Myon MA, Koutedakis Y, Wolman R, Nevill AM, Allen N. The influence of winter vitamin D supplementation on muscle function and injury occurrence in elite ballet dancers: a controlled study. J Sci Med Sport. 2014;17(1):8-12.
Maroon JC, Mathyssek CM, Bost JW, et al. Vitamin D profile in National Football League players. Am J Sports Med. 2015;43(5): 1241-1245.