ATP (Adenosine triophosphate) is the currency for energy required for cellular activities. Digestion needs to take place for the energy in food to be converted into ATP for synthesis in the body whether for building up or growth. Conversion depends on the kind of food taken in. Carbohydrates are converted faster than proteins and fats because they are sugars already.
It takes about six to eight hours more or less for digestion to take place from the mouth through the oesophagus up until the intestines. But you need to take note the digestion of food for energy starts to happen immediately It gets into the mouth. There's saliva and digestive enzymes in the mouth That starts to collect nutrients. At every stage It passes through, nutrients are being collected by the body until it gets to the rectum where it is popped out.