[w-j-w]

That's a bold claim. Calories as a concept can be derived from the principals of Thermodynamics, to which the human body adheres. The conception of calories does however ignore the body's natural response to calorie restriction, and is therefore incomplete.

[8note]

Calories can't explain why I can't live off of drinking gasoline, despite the bomb calorimeter showing it to be highly caloric

Calories are an simplified approximation, where your body handles things in the same way as a fire does.

Calories are bunk in that there's a random function between the thermodynamics your body adheres to, and the thermodynamics the food adheres to. One with properties like the total calories your body has access to must be less than the total in, but for a specific food, or a specific meal, the total calories could be larger than the food in the meal, if it makes other calories easier to process, or even a negative, if it makes you puke.

[kelseyfrog]

I know this is a bit hyperbolic to prove a point but it brings to the surface a point which so far goes unaddressed: the modulation and variation of metabolic efficiency.

Between the consumption of glucose and gasoline, there exists a spectrum of efficiency in the conversion of thermodynamic calories to useful bodily energy.

Take glucose for example. It's thermodynamic and metabolic calorie values are nearly equal (15.5MJ/kg or 3.7kcal/g). On the opposite end is gasoline, thermodynamically it's 46 MJ/kg but it's 0kcal/g if one were to ingest it. There simply isn't a metabolic pathway to extract energy from gasoline in human physiology.

I have no reason to doubt that the ratio of thermodynamic to metabolic efficiency varies amongst individuals, over time, and in the presence or absence of other compounds. When 9-14kcal/day supports the maintenance of 1kg of body mass in equilibrium, it's clear that, ceteris paribus[1], minute changes in metabolic efficiency can explain rather large differences in human mass.

1. Truly. Even/especially in the absence of differences in consumption.

[errantmind]

Food calories / caloric consumption, as an idea, are a gross, erroneous oversimplification of bodily processes to the point of being bunk science.

[TheSpiceIsLife]

This is correct.

Any weight loss / weight gain regime that doesn't consider, at the very least, thyroid hormones is bollox.

Steroidal sex hormones can play a role, particularly female sex hormones.

Allergies too.