A calorie is a calorie.
Not necessarily. It's just as much, if not more, about hormones too.
Insulin hormone:
If type 1 diabetics don't inject their insulin, they could eat 10,000 calories of whatever they want, even refined sugar, in a day and not gain one ounce of body fat and actually burn fat instead. Not that this is healthy or recommended, but it's what happens.
Why? Because the Insulin hormone isn't there to tell the body how to deal with all that blood glucose, how to burn it, how to store it, etc. So the body is deprived of energy and switches to making and burning ketones by breaking down fat stores instead. This of course can lead to diabetic ketoacidosis which life-threatening. But you get the point.
Cortisol hormone:
You can eat zero carbs and still have very high blood glucose levels if your cortisol levels are high. Cortisol promotes gluconeogenesis. This can lead to insulin resistance and excess body fat storage, even on a zero carb diet. Your cortisol levels can be naturally higher in the morning. Skipping breakfast and drinking coffee on an empty stomach will send cortisol levels even higher. Eating only one large meal a day in the late afternoon or evening can raise your cortisol levels too, but not so much if eating your one meal in the morning.
FGF21 Hormone
This hormone promotes body fat burning and it's primarily released by low-protein feeding. Levels of this hormone seem to go up the most after low-protein meals and not so much during fasting. It doesn't matter if you eat only carbs or only fat, although it seems to go up more if you eat only sweet, refined carbs, probably as a way to signal that you're eating too many of these carbs.
Some in the medical/scientific community are saying that this is how/why The Sugar Diet fad works in helping some healthy, relatively lean males get leaner. It doesn't seem to work that well for females or obese individuals.
FGF21 doesn't go up as much while water fasting even though you're eating zero protein. It's the low protein feedings that elevate levels of this hormone the most.
High circulating levels of this hormone makes you crave protein while reducing sweets and alcohol cravings. The increase in body fat burning is probably to spare muscle mass and other tissues when eating only low-protein meals.
In this study, participants switching from their normal diet to a low-protein diet saw their FGF21 hormone levels increase and had to consume 800 more calories to avoid losing body weight, from fat. After going back to their normal protein intake, they had to eat 800 less calories to avoid gaining weight, from fat.
They didn't lose lean body mass while eating low protein for 5 weeks in this study, although going longer with this low protein intake might have eventually caught up to them.
Dietary protein restriction elevates FGF21 levels and energy requirements to maintain body weight in lean menPublished: 06 March 20250.83 g protein (9% protein) per kg body weight per day for 5 weeks
https://www.nature.com/articles/s42255-025-01236-7
