Want to train harder? Check your vitamin D levels. A recent study published in The European Journal of Preventive Cardiology has found higher levels of vitamin D are associated with better exercise capacity. It’s well established that vitamin D is important for healthy bones, but there is increasing evidence that it plays a role in heart and muscle function, too. In particular, cardiorespiratory fitness – the ability of the heart and lungs to supply oxygen to the muscles during exercise – is improved with higher levels of vitamin D.