"If you want to lose weight, eat less." That's what popular wisdom has to say about fat reduction, but the truth is a lot more complicated than that. While it's true that restricting your calories usually produces some short-term loss, it can lead to long-term problems. That's because your body is designed to hold onto fat in case of a famine. When you restrict your food intake too much, you can actually end up gaining weight in the long run as your system tries to "fix" what it sees as a problem.

Calorie Restriction Theory

The basis of calorie restriction is pretty simple. People who advocate this kind of dieting imagine your body as a big rubber bag; food goes in and gets used for energy, but if you're not using enough energy, the food gets turned into fat and the bag expands. To cut down on fat, just decrease what goes in. It takes about 3,500 excess calories to make a pound of fat, so by reducing your intake by 500 a day, you should lose a pound a week.

Starvation Mode

Unfortunately, things are a little more complicated than this theory suggests. While reducing the amount you eat does cause weight loss, it can backfire. This is especially common if you decide to cut out a large number of calories. It may seem like reducing your intake even more will cause you lose more weight and to lose it faster, but you could actually find yourself gaining.

This is due to the fact that your body enters a phase called starvation mode when your food intake drops below a certain level. This level varies for everyone, but is usually between 900 and 1,200 calories per day. If you're a very active person or have a naturally high metabolism, however, it could be much higher. When you use a diet that restricts you below this threshold, your body believes that it's going through a famine and that resources are scarce.

Instead of getting rid of that pesky fat, it starts holding onto it even more tightly. Instead of attacking fat for energy, it begins to use up lean muscle tissue, especially if you're not doing any resistance training to build up that tissue. You lose weight, but you probably won't lose much fat.

You'll also probably notice that your energy levels start to drop. You may feel tired and listless throughout the day. This is due to your body cutting down on the amount of energy it needs in an attempt to get through the famine. As your metabolism slows down in response to perceived starvation, your weight loss will probably slow down and start to plateau. In the end, severe calorie restriction and fad dieting leaves you tired, flabby and with a reduced metabolic response.

Long-term Effects of Dieting

Problems with calorie restriction don't stop there, of course. In the long term, repeatedly using highly-restrictive diets to take off those last few pounds can actually lead to more weight gain. Recent studies in the weight loss field have shown that even people who successfully lose large amounts of weight tend to gain it back. That's because the calorie restriction causes their bodies to believe that they need to return to their "normal" size as soon as food is more readily available.

If you've been using a strict diet, after you lose the desired amount of weight, you'll probably find yourself slipping back into your old eating habits. You may even find that you crave unhealthy foods like refined sugar and fried, salty snacks. People who've lost weight through restrictive diets are often actually hungrier than people who are naturally thin, since the previously-fat people's bodies are still trying to put the weight back on.

Over time, your weight may even go up. Long-term weight loss studies have shown that repeated yo-yo dieting encourages people to increase their bodies' internal upper weight threshold, potentially getting fatter than they were before they started dieting. This tendency is highest in people who lost large amounts of weight very quickly through calorie restriction or other faddish methods.

What's the Solution?

All this information can be disheartening. After all, most people want to take off fat and stay healthy and fit, not slow down their metabolisms and potentially gain more. The good news is that long term, sustainable weight loss isn't impossible. It's just a little more complicated than "eat less, lose more." Studies have shown that minor calorie restriction, especially if it's done by reducing your intake of unhealthy foods, can be quite effective, but that activity may be the real key to reducing weight.

While exercise alone won't burn enough calories to whittle your waistline quickly, it will help you keep your energy level up and maintain muscle mass. Resistance training activities are especially important for this, so it's vital to make sure you're taking in enough energy to stay active. Over time, you can replace fat with lean muscle, which actually burns more calories and can increase your metabolism, instead of starving your body and slowing down your metabolic rate. The more energetic and active you are, the better your results are likely to be.

Reasonable Expectations

It's also important to maintain reasonable expectations when you go to lose weight. Doctors at the U.S. National Institutes of Health say that most successful dieters take off about 8 to 15 percent of their highest body weight. That means that a 200 pound person can expect to effectively lose 16 to 30 pounds. This isn't an upper limit, of course, but it's what most people lose without the risk of gaining it back.

If you want to lose more, those figures can be disheartening, but there is hope. Slower weight loss backed by sound nutrition and an activity level powered with plenty of energy increase the chances of major loss. This plan might not get you into a bikini by the end of the summer, but it's the best-documented way to take that weight off and keep it off.

If you're hoping to take off some fat, don't be tempted by plans that tell you to eat nothing but grapefruit and coffee or subsist on spicy lemonade. Instead, cut those calories only a little, ramp up your activity level, and make sure that you've got the energy to keep moving. It might take a little longer, but you'll do a lot better at losing fat, permanently.