We can easily accept that 'calories in/calories out' is valid while still holding that changing your hormonal balance is more important.
It's (tautologically) true that if I consume more calories than I expend (expend meaning 'use on anything other than making fat') then I will gain fat. If I consume fewer calories than I expend then (tautologically) I need to burn some calories from body stores.
The crucial points are:
- 1) My body can change the amount of
- 2) Hormonal balance
can influence the amount of energy
If I normally expend 2500 calories per day, that doesn't mean that if I consistently consume only 2000 calories per day, my body won't ensure that I henceforth only consume 2000 calories per day.
Similarly, hormonal balance can alter the amount of energy available and thus the amount of energy expended. If I have low insulin I may have endless supplies of fat energy available to burn, if I have high insulin I may have no fat energy to burn, I may indeed have a caloric deficit that can only be met exogenously (from consuming more food) or from reducing the amount of calories I expend.
This is shown in a great study cited in Good Calories, Bad Calories (regrettably I don't have my copy on me), where a group of women are shown to be able to lose fat easily on a hypocaloric fat and protein diet, whereas they maintain or even continue to gain fat while eating the same or fewer calories on a carbohydrate based (and thus insulinemic) diet.
So the final conclusion: yes calories in/calories out is the ultimate factor as to whether fat is lost or gained, but hormonal balance can itself determine whether more calories go in or out. If you are burning more fat then more calories are going out, by necessity, it's an analytically true (but unrevealing) statement.