girls with adequate vitamin D had significant higher bone mass and muscle strength. Questions have been raised about teenage swimmer Ye Shiwen's incredible performances at London 2012, but such research has yet to prove that high levels of vitamin D could be a contributing factor to her success. We know low levels may impact muscle strength and bone turnover, but if an athlete has adequate levels there is no evidence of an enhanced benefit to performance, so natural talent and athletic ability is more likely, Stevenson told CNN. Meanwhile, studies have shown that young athletes in northern European countries such as Britain, Finland and Germany often suffer from vitamin D deficiency. Proper levels can reduce the risk of stress fractures, inflammatory injuries and upper respiratory tract infections. You can get vitamin D in food sources -LRB- such as eggs, dairy and fish -RRB- but it's not as effective as sunshine production, Stevenson said. Those at risk are athletes training at 35 degrees or above latitude -- northern Europe, north China, North America -- as UVB rays are inadequate during the winter months from November to April. When we measured our elite tennis players we only found one athlete who was marginal with vitamin D, as he had been injured and was rehabbing indoors. There needs to be more research evidence looking at vitamin D and injury prevention -- most of