BMI is bullshit, you will never catch me defending BMI or its proponents, but calories, and by extension, calorimeters are actually a pretty effective tool for understanding how much energy a human can derive from a piece of food, I know the dismissive tagline is "You'll notice your body is different from a tub of water placed above a burn chamber" but also, in many significant ways it is identical to a burn chamber and a tub of water, at least when it comes to specific processes. Like, I think the way we think about calories isn't great, there is an absolute over-focus on simply counting at the input level, but overcorrecting into 'the Calorie as a unit is entirely meaningless' isn't the way forward either, IMO. It seems like giving up on the idea that we can use things to learn about other things, when, in fact, comparisons to similar but easier to understand systems are how we've gone about understanding basically everything we currently understand about the universe, which is quite a lot.