The optimal serum level of vitamin 25(OH) D has not been established, however vitamin D deficiency is typically defined as < 50 nmol/L ( < 20 ng/mL), insufficiency defined as 50-80 nmol/L (20-32 ng/mL), and optimal levels 100 nmol/L (> 40 ng/mL). It has been noted that at levels < 40 ng/mL (100 nmol/L), the body relies on daily replenishment of vitamin 25(OH)D to meet its requirements and it's difficult to obtain this amount in the average diet.
The research available to support vitamin D's ability to increase performance is very limited, showing possible benefit in muscular strength, sprinting capacity, and VO2 max. Increased levels of inflammation from intense training (aerobic) have also been associated with low vitamin D levels. Vitamin D plays a key role in active muscle, as well as preventing stress fractures, supporting the notion that correcting vitamin D insufficiency may improve future performance. The direct cause of low or insufficient vitamin status in athletes training a high intensity is not clear and is most likely multi-factorial, due to inflammatory processes, muscular damage, increased protein synthesis requirements, increased immune activity, race, genetics or other unknown causes. Athletes competing in indoor sports may be at higher risk. (Note - player 'G' whose levels were 144 nmol/L was supplementing with vitamin D at the time of the assessment. He was the only player supplementing at the time of assessment).