Calories are pretty much the standard metric and it makes sense since they're the energy unit. You burn the food and measure the heat it generates, boom: calories. Protein burns and makes heat, fat does, sugars/carbs do. It makes sense to measure calories because they're measurable across components using the same methods. Mass doesn't seem relevant at all and skews, like you mentioned, based on caloric (there they are again) density, water content, cell structure, etc. Someone could eat a truly massive amount of leafy greens and get less sustenance (Calories+nutrients) than a small steak or modest bowl of porridge would give.
Discussion
Calories are standard only because they are easy to measure.
Doesn't take into account the metabolic costs of digestion or the myriad antinutrients plants contain. Or their interactions.
Weight gain in rats as a fraction of mass of food consumed for a particular diet is a much better metric.
(Yes, rats nutrition is quite relevant to human nutrition. After the primates and the tree shrews, Rodentia are our closest living relatives).
I'll agree with that
Yeah, I am aware of why we use calories, and I understand it's importance, but I think the use case in this above example is kinda redundant, that's really what I was getting at.
However, I still don't think it's a great metric when thinking about nutrition, too reductive, it also doesn't take into account the variety of other vitamins, minerals and chemicals consumed from certain food groups which the body might benefit or draw harm from. Nutrition is kinda hard to draw scientific concensus on, I would go so far as saying it is something of a "sudo science", (or at least it has been in the past, things seem to be improving) especially as it seems that there are certain food groups which benefit certain portions of humanity better than others. 🤷🏻♂️
"Don't consistently eat more calories than you burn in a day" is a big part of the story, but it's not the whole story.