From a New York Times online article:

The riders who had pedaled on an empty stomach, however, had incinerated about twice as much fat during each ride as the men who consumed the shake first. The riders all had burned about the same number of calories while pedaling, but more of those calories came from fat when the men did not eat first.

Those riders also showed greater improvements in insulin sensitivity at the end of the study and had developed higher levels of certain proteins in their muscles that influence how well muscle cells respond to insulin and use blood sugar.

Working out on an empty stomach could amplify the health benefits of the activity, according to a well-timed new study of the interplay of meal timing, metabolic health and moving. The study, which involved sedentary men and moderate cycling, suggests that whether and when we eat may affect how exercise affects us.

To read more: https://www.nytimes.com/2019/11/27/well/move/eating-food-exercise-fasting-insulin-weight-loss-fat.html