It was once commonly believed that all calories were essentially the same, and that controlling your weight was simply a matter of keeping the total number of calories in the right range. Many people still do believe this, even though it is clearly false.

The amount of calories in a serving of food tell you how much energy that serving can produce when you eat it. So at that level, it is true that all calories are the same. But that's a very simplistic view of the situation. Different kinds of food have wildly different effects on your body, even if you eat the same number of calories of each.

Protein takes a lot more energy to digest than simple carbohydrates do. So eating 100 calories worth of steak (protein) will have a different effect on your body than 100 calories worth of white bread (simple carbohydrate) will. And eating 100 calories of whole-grain bread (complex carbohydrate) will affect your body differently than the white bread too. 100 calories worth of salad dressing (fat) will have yet a different effect.

Clearly, when it comes to how they affect your body, not all calories are the same. Simple calorie counting accomplishes little if you don't control the types of calories you are consuming. Still don't believe me? Here's that experiment I was telling you about:

To prove to yourself that not all calories are the same when it comes to how they affect the human body, imagine this. Please don't actually try this experiment. You'll see why in a minute. Just think it through for now.

With the help of a scientific research team, we figure out the average number of calories you consume at each meal. Without changing anything else about your life, we replace your regular meals with a bowl of your favorite ice cream containing exactly the same number of calories. For the next week, you are going to eat exactly enough of your favorite ice cream to replace the calories of regular food you would normally consume.

What do you think this change would do to you? Aside from leaving you bored by eating the same thing all the time, this change would have ugly consequences for you. Your blood sugar levels would likely swing about wildly. If you were diabetic, this in itself could kill you (see why I said you shouldn't actually try this experiment?). Your moods would swing back and forth. Your digestive system would probably rebel. You would almost certainly put on weight. In short, you would be a mess. You know you would.

This thought experiment clearly shows that the source of the calories you consume matters a great deal. If you're overweight and trying to get back in shape, you need to find yourself a plan that recognizes that not all calories are the same. Fortunately, the creators of most modern fat loss plans do understand this distinction and give you dietary advice that recognizes this fundamental truth.