Humans don't digest anything (except complex carbohydrates, via saliva in the mouth). Gut bacteria digest food. What is available for the host human to absorb after the bacteria are done changes significantly -- not by some little correction factor, by up to an order of magnitude -- depending on a number of factors such as food particle size, prevalence of cell walls and connective tissue, the exact ratio and distribution of gut bacteria species, and so forth, for a given "energy content" of food. (A human will typically absorb as much chemical energy from a 4-oz. medium-well hamburger patty as from a 16-oz. rare steak, and an much from a 2-oz. piece of cake as from a 6-oz hunk of black bread.)
What the human body then does with that chemical energy depends on a number of genetic, environmental, and experiential factors. Having lost a significant amount of weight lowers energy demand, permanently, by up to 30%. Food availability to the mother during gestation affects the metabolic efficiency of the offspring. Hormones and hormone analogs in _microgram_ quantities effect the efficiency and completeness of energy absorption by the gut and whether abdominal fat stores the glucose. (Subcutaneous fat responds to glucose levels, not hormone levels.) Oddly, there is a strong correlation between maternal soy consumption during pregnancy and non-obese offspring: but then soy is an estrogen mimic. Most plastics also shed endocrine mimics.
The "fuel" model of food is overly simplistic. The conflation of extreme overweight and obesity is overly simplistic (yes, obese people can diet and exercise to normal weight -- 5% of the time; the other 95%, other mechanisms keep the fat from turning into energy). The worldwide obesity crisis cannot be solved by diet, exercise, and willpower, because it is not caused by overeating, lack of exercise, and self-indulgence. _Overweight_ can be so addressed; obesity cannot.