I think you're right about the concept of sharing energy with gut microbes. I calculate that a 160 pound man would lose about 250 calories to gut microbes each

I think you're right about the concept of sharing energy with gut microbes. I calculate that a 160 pound man would lose about 250 calories to gut microbes each day if he were consuming sufficient protein, fat, and minerals to sustain efficient gut microbe multiplication activity. A diet poor in supportive nutrition would curtail gut microbe activity resulting in more efficient energy absorption into the bloodstream. That is, a higher proportion of the energy consumed would be available for metabolic needs and fat storage. In my own experience, if I consume extra calories, I just have more bowel movements and don't gain any weight unless the extra calories are carbohydrate.