Thousands of articles are written every year on how to lose weight, and each of them suggests a different way of going about it. The truth, however, is that any excess weight you may have is related to the number of calories you take in and the number you use up each day. It’s as simple as that, and no one can prove otherwise. It’s like the gas you put in your car: you put in so much and you travel so far. In the case of your body, you eat a certain amount of food, and it sustains you for a certain length of time. If you eat more than you use up, the excess calories are stored as fat.

Let’s consider the calories you take in and use up each day in detail. As it turns out, there are three main types of calorie burns. The first is referred to as the Basal Metabolic Rate, or BMR. It is the biggest of the three, and is your resting metabolism; in other words, it is the number of calories you burn when you’re just sitting around. It is used for fueling your cells, keeping your heart beating, your lungs functioning, your brain operating, and so on. About 60 to 80 percent of the overall calories that you burn each day are BMR calories.

The second largest calorie burn comes from the thermionic effect of eating. It might seem a little strange, but it takes calories to burn calories — in other words, to digest your food. For protein it takes about 25 calories for every 100 calories consumed, and for carbohydrates and fat it takes about 10 to 15 calories for every 100 calories consumed. And that can add up to a substantial number of calories. It accounts for between 10 and 30 percent of the total calories you burn each day.

Finally we come to the one that might surprise you, namely exercise and general body movement. Most people think that exercise makes a large contribution, but it doesn’t. This doesn’t mean that it isn’t important — it is. You can easily burn off 500 calories in an intensive workout, but in general, for most people, exercise and body movement accounts for only about 10 to 15 percent of the calories they burn off in a day.

Now let’s apply the above to a typical woman and man. First of all we need BMR formulas, and they are as follows:

Adult male: 66+ (6.3 times your body weight in pounds) + (12.9 times your height in inches) – (6.8 times your age in years).

Adult female: 655 + (4.3 times your weight in pounds) + (4.7 times your height in inches) – (4.7 times your age in years).

As an example, we’ll assume you are a 40-year old woman who weighs 140 pounds and is five feet-two. Substituting in the appropriate numbers we find your BMR is 1360. For your thermionic calories we multiply .15 times the total number of calories that you took in during the day. Assume it was 1900; your thermionic calories are therefore 285. The contribution from exercise and body movement is a little harder to determine exactly, as it depends on how hard and how long you exercised during the day. We’ll assume that you didn’t work out; in this case your contribution is probably about 200 calories. And this gives you a grand total of 1845 calories.

If you consumed 1900 calories during the day you took in an excess of 55 calories. This doesn’t seem like much, but in two months (if you do this each day) you will gain a pound, and over a year you’ll gain 6 pounds. (I’m using the fact that there are 3500 calories in a pound here.)

Let’s do the same thing for a 40-year old six-foot man who weighs 170 pounds. Substituting into our BMR formula we get 1793. In this case we will assume a calorie intake of 2600; multiplying it by .15 gives 390 for his thermionic calories. For exercise and body movement let’s assume he uses up 300 calories (this means he did not work out during the day). His grand total is therefore 2483, and if he took in 2600 calories during the day, he would have an excess of 117 calories. This will add up to a pound of weight every 30 days, and about 12 pounds in a year.

It’s easy to see from this that weight gain can slip up on you if you’re not careful. Even a few excess calories each day can add up to a considerable weight gain over the year. The best way to keep tab on how well you are balancing your input and output calories is, of course, to weigh yourself every so often, and if you see that you are gradually gaining weight you should take steps to change things as soon as possible. You will have to either cut down slightly on your intake of calories or exercise more. And I’d like to emphasize that, even though exercise doesn’t appear to make a large contribution to the overall calories we use up in a day, it can be very important. As I mentioned earlier, a good workout can use up 500 calories, and this could easily offset any excess you might have.