[foobarian]

Yes the first one. The energy contained in ingested food is far and away the dominant metabolic input.

There is no need to get into the weeds of various details of how that breaks down; the main problem is that calories are way too cheap in the modern day and age, and people's habits and instincts developed in far different eras lead to easily ingesting more than is expended. Yes the OP is not wrong, but it's not a very useful argument unless we're going to roll back the modern civilization.