Everybody's who's tried to lose more than a small amount of weight, over 10%, runs into a similar frustration. They begin a diet, start exercising, and experience some immediate and encouraging results.

After a week or so, the scale says you've lost a few pounds. Your clothes fit better. Great, keep on going.

However, after a few months -- assuming you've kept up your self-control and not given in to any cravings -- you're still watching what and how much you eat and still exercising.

But your scale is broken. It's stuck on the same number it was last week, and the week before.

You've hit the dreaded plateau.

At this point, many people quit in frustration. All of a sudden they're giving up their favorite foods and sweating a lot - for nothing.

Or so it seems.

Our bodies have a weight-related "set point." We can quickly lose and gain about 10% of our current weights, but after that the body starts resisting change.

So the first 10% of body weight goes easily -- if you call giving up the foods you love and putting in lots of gym time "easy."

But your body wants to hang on to the remaining 90%.

So then dieters quit. They go back to eating fattening foods and keeping the couch warm instead of running outside. So they regain the weight they lost, depressed at their "failure."

Eventually, driven again by a desire to feel healthier and look better, they try again, and again hit that plateau and quit. That's the reason for the yo-yo dieting syndrome.

Interestingly, this applies just as much to people trying to gain weight. They can put on some pounds, but then hit a plateau no matter how many protein shakes they drink or weights they lift. If they quit, they lose it fast.

The set point makes sense, however. A 10% weight loss, especially one caused by a large drop is caloric intake, is a signal to the body that you are experienced a shortage of food. Why else would you stop eating as much as you could?

Throughout our million years of walking upright, we human beings have never experienced a prolonged surplus of food. Our bodies evolved to conserve fat to keep us alive through those long winters when both animal prey and edible vegetation were scarce.

The exact physical mechanisms that maintain our set points are complex, involving our thyroid glands, insulin, glucagon, our hypothalamus, our autonomic nervous systems, our vagus nerves, stomachs and probably other parts of physiologist that we don't yet understand.

The good news is that with time our set points change. For example, adults tend to gain one pound per year from age 20 to age 50.

The key phrase here is "with time." If you want to lose weight, a short term crash diet is only good for 10% of your current weight.

If you want to lose more, it's going to take time, usually around six months.

So people on diet and exercise programs to lose weight must understand that their progress will not be one steep curve. They will hit plateaus that can last up to six months. They must continue eating a healthy diet and continue to exercise moderately.

Eventually, your body will realize that even though it's lighter than it was, you're still eating an adequate amount of food and you haven't starved to death, so the new weight is acceptable.

It becomes your new set point, and if you continue your program you'll again see the pounds drop off.

Yes, after losing another 10%, you'll plateau again.

If you're extremely overweight, however, you just have to remember that you didn't put on all those pounds in one month. And they're not going away in one month.

You have to think long term. You must realize that you should eat a healthy, non-fattening diet for the rest of your life. You should exercise moderately for the rest of your life.

If you want to have and keep a healthy body weight, you just have to realize that you have to commit to eating properly and exercising moderately.