Okay, let’s have a short rant about thermodynamics and fat loss.
If you want to lose fat, you must burn more kcal than you consume, and you must make sure that the kcal you burn come from fat. This is — or ought to be — utterly unsurprising.
Now, a lot of people will tell you that it takes a summed 3500 kcal energy deficit to lose a pound of fat — or that a summed 3500 kcal energy surplus will lead to a pound of fat gain. This is predicated on the idea that a pound of adipocytes contains roughly 3500 kcal worth of triglycerides, which is close enough to true that I’m not going to nitpick it to death… or at least, not today. You run into this claim in a surprisingly large number of places, but it’s bogus.
On a daily basis, your body tissue consumes as many kcal as it burns — at a metabolic level. This is one of those basic accounting-identity sorts of rules that makes people think metabolic things are simpler than they actually are. That energy has to come from somewhere. Much of it comes from diet — food and drink.
Let’s suppose you’re trying to lose fat and eating a kcal deficit. This is where much of the fiddly shit in various “diets” comes into play: Either by telling you to eat a kcal deficit, or by tricking you into eating a kcal deficit by playing satiety games. Today, you eat 2000 kcal but burn 2500. Where does the remaining 500 kcal come from?
Ideally, you’d like it to come from triglycerides released from your adipocytes. But there are other options: Depending on things like your cortisol and insulin levels, you might end up using glycogen stored in your skeletal muscle and (mainly) your liver, or breaking down protein by deamination. That protein probably comes from skeletal muscle, which isn’t helpful. So a lot of the fiddly diet shit not accounted for by the kcal deficit comes into play in building a metabolic environment that burns more endogenous fat than it does endogenous protein. Everything from ketogenic low-carb to carb cycling fits in here. HIIT and resistance training also helps by inhibiting deamination of muscle protein and pushing more dietary protein into muscular repair (and, if you’re doing it really right, even a bit of hypertrophy), which increases resting metabolism in addition to however many kcal you burned while you were actually lifting and/or sprinting.
But fat loss dieting isn’t over in a single day. Suppose your maintenance metabolism starts out at 2500 kcal/day and you eat 2000 kcal/day for months on end. You won’t be surprised to learn that your body doesn’t appreciate having to dip into its long-term energy reserves on a continual basis. If you do nothing about this, eventually you will find that your maintenance metabolic rate drops to roughly 2000 kcal/day. What a coincidence! Keeping your metabolism up falls largely to proper exercise, although there’s plenty of room for fiddly diet shit like carb refeeds here too.
So it’s technically true that, to lose a pound of fat, you have to eat a 3500 kcal deficit. But there’s a lot of metabolic shit involved on top of that simple equation.