^That's a reductio ad absurdum.
No someone will not gain fat on a 500 calorie diet.
But there's a very high chance that someone will gain more fat/lose less fat on a 2000 calorie dextrose diet than on a 2500 meat/fat/vegetables diet.
Look at it this way: total calories in = normal body function + tissue regeneration + muscle mass + fat mass
If someone is below maintenance calories, at least one of these parameters needs to go down.
If someone is above maintenance calories, at least one needs to go up.
Which one goes up and which one goes down largely depends on how hormonally healthy you are.
Unhealthy = preference for fat gain/retention above all others
No someone will not gain fat on a 500 calorie diet.
But there's a very high chance that someone will gain more fat/lose less fat on a 2000 calorie dextrose diet than on a 2500 meat/fat/vegetables diet.
Look at it this way: total calories in = normal body function + tissue regeneration + muscle mass + fat mass
If someone is below maintenance calories, at least one of these parameters needs to go down.
If someone is above maintenance calories, at least one needs to go up.
Which one goes up and which one goes down largely depends on how hormonally healthy you are.
Unhealthy = preference for fat gain/retention above all others