Calories do measure food energy, but actually there’s no such “thing” as a Calorie.
If Calories were “things”, scientists could set them on a scale and weigh them; and dieters could just stop eating them -- with no further fuss. But since they aren't; they can’t. Calories are just a measure of the energy in various food substances.
Why should an ordinary dieter care in the slightest about this very abstract point?
Because in order to lose weight, you have to reduce the number of Calories in your diet. But the only way to reduce Calories is to reduce the amounts of those food substances you eat that have Calories in them.
Protein, carbohydrate, fat, and alcohol are the only important food substances that have Calories in them. (There are a few very unusual other substances that have Calories, but they are nearly nonexistent in most ordinary diets.) So these are the food substances you have to reduce.
But reducing the amounts of protein, carbohydrate, and fat has to be done in the right way, because otherwise your diet will become very, very, difficult – just like most diets actually are. (That’s also the reason most of those popular and simple-minded “high-this or low-that” diets stop working after a while.)
Most people know that dietary protein is necessary for tissue maintenance and repair. Protein can be burned for energy (and therefore it has Calories), but normally that isn’t its main purpose.
So eliminating protein entirely from your diet would eliminate Calories, but would also force your body into a very stressful “emergency” mode that would almost certainly include intense feelings of hunger, cravings, aches, pains, low energy levels, and other unpleasant symptoms just like the ones that actually destroy most diets. Reducing protein by too much would have nearly the same effect.
The same is true of carbohydrate. Carbohydrate from food is necessary for the brain, nervous system, and a few other tissues to function normally. A lack of enough dietary carbohydrate puts you into very unpleasant emergency states called “hypoglycemia” and “ketosis”. These states are not dangerous, but they are not normal and often include those intense feelings of hunger, cravings, aches, pains, low energy levels, or other unpleasant, diet-destroying symptoms.
Fat is no different. Chemically, there are many different kinds of fat. Most of these fats can't be used for anything but energy (Calories). They are the kinds of fat that can and should be eliminated from any weight-loss diet. But TWO of the fats are vital to your body for other things than just being burned for energy. These fats are ALA (alpha linolenic acid) and LA (linoleic acid). If you don’t get enough of these two, your body has to go into an emergency mode that also includes those intense feelings of hunger, cravings, aches, pains, low energy levels, or other unpleasant, diet-destroying symptoms.
The rest of this section explores this “dual-use” function of protein, carbohydrate, and fat, and discusses how dieters need to manage it in order to avoid those intense feelings of hunger, cravings, aches, pains, low energy levels, and other unpleasant symptoms that normally destroy diets.
(Note: Alcohol has no known physiological function other than as a source of energy (Calories). It can be safely eliminated from any diet.)
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