> A question I have here is, why not try for low calories per litre instead of (or as well as) low calories per gram?
I think calories per gram is usually what people study due to some combination of: - this is the way somebody chose to measure “energy density” early on and it stuck for whatever reasons things stick - in cooking and/or conducting experiments, mass is pretty much always easier to measure than volume (even with liquids, in my opinion...) - we see this metric work pretty well—better than basically any other known single factor, is my impression—to predict satiety response, ad libitum caloric itake, diet adherence, and long-term weight changes in various experiments
I do know of a single-meal study that looked at how volumetric energy density (comparing potato chips vs. popcorn, which have similar energy per mass) predicted ad libitum caloric intake, and found that it does seem to independently matter. I don’t know of any other similar studies, though I won’t claim to be up to date on the literature.
>Plus at some point things leave the stomach and I don’t know what triggers that.
”gastric emptying” is the key term used in studies of this question (I haven’t really studied this myself)
>How does this whole thing work with fluids? Presumably they leave your stomach quite fast, so per-calorie they should contribute less to satiety than solids?
Right, that’s the usual finding. Drinking lots of water before or with meals does seem to promote satiety and lower ad libitum caloric intake, so water itself certainly counts for something, but liquids are generally nowhere near as filling per mass as solid foods, which agrees with the conventional wisdom around not drinking calories.
> A question I have here is, why not try for low calories per litre instead of (or as well as) low calories per gram?
I think calories per gram is usually what people study due to some combination of:
- this is the way somebody chose to measure “energy density” early on and it stuck for whatever reasons things stick
- in cooking and/or conducting experiments, mass is pretty much always easier to measure than volume (even with liquids, in my opinion...)
- we see this metric work pretty well—better than basically any other known single factor, is my impression—to predict satiety response, ad libitum caloric itake, diet adherence, and long-term weight changes in various experiments
I do know of a single-meal study that looked at how volumetric energy density (comparing potato chips vs. popcorn, which have similar energy per mass) predicted ad libitum caloric intake, and found that it does seem to independently matter. I don’t know of any other similar studies, though I won’t claim to be up to date on the literature.
>Plus at some point things leave the stomach and I don’t know what triggers that.
”gastric emptying” is the key term used in studies of this question (I haven’t really studied this myself)
>How does this whole thing work with fluids? Presumably they leave your stomach quite fast, so per-calorie they should contribute less to satiety than solids?
Right, that’s the usual finding. Drinking lots of water before or with meals does seem to promote satiety and lower ad libitum caloric intake, so water itself certainly counts for something, but liquids are generally nowhere near as filling per mass as solid foods, which agrees with the conventional wisdom around not drinking calories.