Before we start, let’s just define a few terms:
A Calorie is the amount of energy needed to heat 1 gram of water by 1 degree Celsius. That’s all it is. It’s not a tangible, physical thing: it’s a unit of measure. (In the context of food, we’re technically talking about Kilocalories, but who’s counting?...)
Basal Metabolic Rate (or BMR) is the amount of energy our bodies burn while at rest (sometimes referred to as Resting Metabolic Rate, or RMR), which is to say the amount of energy (re: calories) they burn to do everything other than move (including heartbeat, cellular growth, breathing, cellular repair, etc.).
Metabolic Flexibility is the body’s ability to switch back and forth between burning glucose (sugar) and burning fat—depending on availability and need—to supply its energy needs. For a wonderful and in-depth discussion on Metabolic Flexibility, check out J. Stanton’s post at gnolls.org.
Here is a list of the cells in our bodies that require an uninterrupted source of energy to function properly: all of them.
Cutting calories for fat loss is based on the assumption that, should we eat fewer calories than we expend, our bodies will make up the difference from our fat stores.
But will it?
The true answer is: maybe.
If you do not have good Metabolic Flexibility, then your body’s response to running out of glucose to burn will not be to run to your body fat stores, but rather to make you hungry (specifically for glucose). If you go too long without eating again, your body will start finding its own supply of energy by breaking down your muscle tissue and converting it to glucose. And if you somehow manage to fight through all this and continue eating a calorie-restricted diet, your body will start saying “Ok, looks like we don’t have enough food to eat: let’s keep storing fat for the day that ALL the food is gone. And you guys (points to all the cells whose function doesn’t keep you alive right now): sorry, but you’ll just have to stop doing as much” (which, if you're keeping score at home, means we need to burn fewer calories to stay alive).
Let’s break this down with some fake numbers. Say you require 1,800 calories per day to function based on your activity level and BMR. Calorie math tells you to eat 1,200 calories per day to lose weight. You may lose some weight (though, as discussed above and here, it will not necessarily be the weight you want to lose…), but it will not last because, eventually, your body will start down-regulating the processes that aren’t needed for your immediate survival (don’t forget – every cell in your body needs an uninterrupted supply of fuel). Now your metabolism has slowed, so your daily caloric need is less, so the difference in how many calories you need each day (we’ll call it 1,200 now) and the number of calories that you eat is equal rather than the triumphant 600 you originally fought so hard for.
And be aware: with this method, even if you lose some weight at first, you have not gotten ANY healthier. In fact, you’ve very likely become decidedly less healthy because your body has been down-regulating all the processes that keep you healthy in the long run (for example autophagy, which is basically when specialized cells go through and clean out non-functioning and/or damaged cells).
You will also surely gain the weight back because these types of diets are not sustainable in the long run. To add to this, you will start your weight gain from a less-healthy place than you started before, so your chances of gaining extra weight back is increased, and your chances of losing that weight will be decreased.
Everyone knows the term “yo-yo dieting,” but I find that a little inaccurate since yo-yos go up and down between the same two spots. This process, with which so many of us are familiar, is more like a bouncy ball going up the stairs: always fluctuating in height, but continually trending upward.
Yes, a caloric deficit is technically necessary to lose weight. But that does not mean you can achieve a caloric deficit simply by trying to eat less or exercise more. Your body is incredibly smart and adaptive; if you try to throw a caloric deficit at it, it will simply lower the number of calories it needs (re: slow your metabolism) and increase your "Emergency - find food now!" signals.
The key, as we've said, is to give your body the signal that it does not need to hold onto all that excess fat.