[_heimdall]

If a calorie is a calorie, would you agree that I would be just as well of eating 1800 calories of fat rather than a balanced diet? Could I stick to 1800 calories of gasoline?

We can't simply burn food and deem that an accurate analog to how the body processes and utilizes different food. Ask anyone that live(d) primarily on rabbit meat.

And to be clear, the 80:20 rule is am extremely broad and inaccurate rule of thumb that isn't useful when applied to something specific. You can't claim that calories in equals 80% of calories out because Paredo.