Patients with heart failure with preserved ejection fraction (HFpEF) find their ability to walk and carry out the American Journal of Medicine suggests lack of vitamin D may be partly responsible. Vitamin D is thought to play a role in cardiovascular and muscle function, but its relationship with exercise capacity has been unknown. Researchers measured vitamin D levels in 112 HFpEF patients and 37 matched controls, all of whom took a six-minute walk test and had their peak oxygen capacity measured. Vitamin D levels were found to be significantly lower in HFpEF patients than in controls. More than 90 percent of HFpEF patients had insufficient vitamin D levels or were frankly deficient. These low levels were associated with more severe exercise intolerance. This study laid the groundwork for a randomized, controlled clinical trial to evaluate whether vitamin D supplementation could be effective in improving exercise tolerance.