What about Stomach Efficiency?

If let's say i take in 100 calories, will my body use those 100 calories??or does it go through something and you're left with 80? I'm not so sure about this. I would like to know since i can be assured that the amount i'm eating doesn't go to waste, trying to gain weight here.

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A life in Christ is a life of restfulness. There may be no ecstasy of
feeling, but there should be an abiding, peaceful trust.
Steps to Christ, p. 70.

Well, some of the calories are used to digest the food and other involuntary bodily functions - heart beating, lung pumping, etc.
But that all falls under your Basal Metabolic Rate (BMR). And should already be factored in to the number of calories you need for whatever weight you are aiming for.

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