The purpose of this study is to test whether a ketone ester drink will improve exercise in people with heart failure (HF) compared to a placebo. In HF, patients are limited in their ability to do all the things they want to do, and exercise as much as they would like, due to becoming tired and short of breath early. There may be several reasons why these symptoms occur. There is some evidence that in addition to problems with the heart, patients with HF also have problems with their arteries and muscles that affect their ability to exercise. Ketones have been shown to improve exercise capacity in healthy volunteers, which may be related to effects on the heart function or on muscles. An infusion of ketones through an intravenous (IV) line has also been shown to significantly improve heart function, but whether a drink can produce similar improvements in HF patients is not known. This drink has been given status by Food and Drug Administration as "generally regarded as safe". The use of DeltaG in this study is experimental. DeltaG has not been approved by the Food and Drug Administration (FDA) for the use being evaluated in this study.

This is a randomized, crossover trial of ketone ester versus placebo in 20 patients with heart failure with preserved ejection fraction. Participants come for a baseline assessment. If they meet eligibility criteria, they are randomized to receive ketone ester first or placebo first. After approximately a week washout period, participants return for the last visit to receive the product they did not receive at the previous visit.