If you have ever been on a low-carb diet, you probably noticed two things:

Low-carb diets help you take off 3 to 8 pounds (or 2 or 3 kilos) very quickly, and

Going off your low-carb diet for even one meal will put that lost weight right back on.

Whether you are elated by losing a significant amount of weight quickly or depressed about putting it all back on, what you need to know about weight loss on low-carb diets is that the low-carb component of the diet, over the long run, just helps you with appetite control. Whether you are on a low-carb diet or not, you have to cut 3,500 calories out of your diet to lose a pound of fat (that is, you have to defer eating 7,700 kcal to lose a kilo of fat), and you have to eat at least 3,500 calories to get that pound of fat back. So why do you gain back all the weight you lost in the first week of a low-carb diet just by eating a doughnut?

The answer lies in the body’s production of glycogen. The liver stores an emergency carbohydrate fuel for intense physical activity. Glycogen also stores the glucose needed for the brain. In a typical adult man, the store of glycogen in the liver and the muscles weighs about 3/4 of a pound (320 grams or so). Each molecule of glycogen, however, is bound to four molecules of water.

If you are on a low-carb diet, and your liver is depleting its glycogen, it’s getting rid of water weight, too. You may feel lighter and your waist may be slimmer. But if you eat any carbohydrate at all, your liver and muscles are going to replenish their stores.

Half a muffin contains enough carbohydrate for your liver to recharge its 3/4 of a pound of glycogen. Eat a burger and fries, and you’ve recharged the glycogen in your muscles, too. But the glycogen in your muscles and liver does not just store the carbohydrates you eat and don’t burn off. It stores water with them, too.

So why go low-carb if you want to lose weight? If you can stay on a low-carb diet for two weeks, you’ll lose many of the cravings you have for high-calorie foods. It’s calorie reduction, not carb reduction, that makes it possible for you to lose weight over the long term.

What happens when you go off your diet? You will have an almost immediate weight gain of 3 to 8 pounds (a little under 2 to a little over 3 kilos). You still have to eat a lot of additional calories, however, to restore lost fat. If you get over the initial disappointment of a relatively small weight gain, you can get back on your low-carb diet and keep the majority of your lost weight off.

Water weight returns easily. Fat is hard to get back.

So, if you go off your low-carb diet plan for a meal, or a day, or even a week, don’t panic. You can get back to where you were. Just remember that the first pounds you lose and the first pounds you gain back are water, not fat.