When you take in more calories from food than you use up running your body systems (heart, lungs, brain, and so on) and doing a day's physical work, you end up storing the extra calories as body fat. In other words, you gain weight. The reverse is also true. When you spend more energy in a day than you take in as food, you pull the extra energy you need out of stored body fat, and you lose weight.
You don't have to be a mathematician to reduce this principle to two simple equations in which "E" stands for energy (in calories), and > stands for greater than, < stands for less than, and "W" stands for the change in weight:
If E in > E out: E total = +W
If E in < E out: E total = -W
Get the picture?
Our body requires a certain number of calories per day in order to maintain your current weight. This is known as your calorie maintenance level. It's the number of calories required by your body to do everything it needs to do (intense exercise, brushing your teeth, pumping blood, keeping organs functioning properly, etc.). Calories are what our bodies use for energy, so in order to do what needs to be done, a certain number of calories are needed.
As you already know, we supply our bodies with these calories through eating and drinking. If we end up consuming exactly the same number of calories that our bodies need each day, our weight would remain exactly the same. For example, if your calorie maintenance level was 2500 calories, and you consumed 2500 calories per day, your weight would not change. All of the calories you take in would end up getting used (or "burned"). This is how you maintain your weight, by giving your body only the calories that it needs. No more, no less.
However, if you do consume more calories than this maintenance level, your body will store the excess calories as fat. So, for example, if your maintenance level was 2500 calories, and you consumed 3000 calories per day, you would gain weight. You are giving your body more calories than it would end up burning. This is what causes weight gain.
On the other hand, if you do the opposite and give your body less calories than it needs, your body will convert your stored body fat into energy and use that instead. This is what causes weight loss. Sticking with the same example as before, if your daily maintenance level is 2500 calories, and you consume 2000 calories per day, you will lose weight.
Basically, consume the same number of calories that your body needs/burns each day and you maintain your weight. Consume more calories than your body needs/burns and you gain weight. And last but not least... consume less calories than your body needs/burns and you lose weight.
The One Fact: A caloric deficit is what causes weight loss.
Making FAT Loss Happen
Now that you understand the One Fact that causes weight loss, it's now time to learn the 3 ways to actually make the One Fact happen.
Consume Less Calories (Diet)
The first and most obvious way to make the One Fact happen is by consuming less total calories. If your body needs 2500 calories per day (just an example), and you give it 2000 calories per day, you are officially in a caloric deficit, meaning you are supplying your body with less calories than it needs to maintain your current weight. Doing this consistently will cause weight loss.
Burn More Calories (Exercise)
The second way to make the One Fact happen is by burning more calories. As I mentioned before, your daily calorie maintenance level is the number of calories that your body burns per day. So, for example, if your daily maintenance level is 2500 calories (again, just an example), and you consume 2500 calories per day, you will maintain your weight. Why? Because all 2500 calories consumed get burned by your body. There is no surplus and there is no deficit. It's even.
However, if you make your body burn additional calories each day through some form of exercise, there would then be a caloric deficit. A caloric deficit = weight loss.
Staying with the same example, if you consume 2500 calories per day, and you naturally burn 2500 calories per day, you maintain your current weight. If you then burn an additional 500 calories through exercise, a caloric deficit is created. Just like the deficit created if you consumed 500 less calories, the same deficit is created by burning 500 additional calories.
Either way, a caloric deficit then exists... which means the One Fact is happening... which means weight loss will happen.
Consume Less Calories AND Burn More Calories (Diet & Exercise)
In my personal opinion (and the opinion of practically every qualified expert), the best way of making the One Fact happen is a combination of both consuming less calories AND burning more calories. Studies have shown that people who utilize both diet and exercise were more often able to maintain their weight loss long term than those who did one or the other. Doing both is also more beneficial from a general health standpoint as well. If you're the type of person who just wants a clear-cut answer, here it comes... do both.
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