Two hormones are responsible for much of our energy metabolism, insulin, and glucagon.
Insulin is released in the fed state when it senses elevated blood glucose levels. Insulin moves glucose into the cell to be used for energy. Cells can only use so much glucose before the body has to find other storage for it. Namely in fat cells and in organs. Both are detrimental to the body and healthy longevity. As you can see this would occur in the fed state. Insulin is a storage hormone. It elevates at an imbalanced level and performs these actions when we eat too much, too long, snack between meals, consume simple sugar and refined carbohydrates. This imbalance can lead to type 2 diabetes, obesity, elevated triglycerides, inflammation, fatty liver, and other serious health conditions.
Glucagon is a catabolic hormone. When blood glucose levels are low, glucagon is released to mobilize the stored energy. First looking for glycogen then looking for fat. If insulin levels are elevated, glucagon has no reason to do its job. It never mobilized fat from storage to use as fuel.
The body has to be in a fasted state for this hormone to work efficiently. It can take 4-6 hours after a meal for blood glucose levels to drop and another 4 hours or so to burn through the stored glycogen in the liver and muscles. So in essence it can take 10-12 hours of no food for glucagon to come out and do its job.
Bring these hormones into balance by not snacking between meals, reducing sugar and refined carbohydrate intake, fast overnight for 12 hours. As these hormones become more sensitive and under medical supervision, you can begin to extend the fasting time to 14-24 hours.
https://pubmed.ncbi.nlm.nih.gov/32673591/