The human body is a wonderful, complicated machine.

Your disgestive system, your brain and your fat stores all work together through a highly complex biochemical interaction to help you maintain a stable weight. These different parts of your body communicate with one another through various feedback mechanisms in an effort to coordinate the various activities that maintain your weight at a specific level. That level is known as the setpoint.

Think of your setpoint as a thermostat. In your home, you set your thermostat at the temperature you most enjoy and expect your heating or air conditioning system to respond to outside conditions and maintain your home at that termperature. By the same token, your setpoint raises or lowers your appetite and metabolism-the rate at which your body burns calories-in response to how much you eat.

You may now ask the obvious question: "If my body is designed to maintain a stable weight, then why did I gain weight and why is it so darn hard to lose it?

Back to our thermostat analogy. Let's say that the outside temperature is 85 degrees and you want your home's cooling system to maintain an indoor temperature of 72 degrees. No problem. Your air conditioner won't have to work too hard to cool the outside air by only 13 degrees.

But let's throw in a heat wave when the outside temperature climbs to a sweltering 110. No matter how hard your ASC struggles, it won't be able to maintain that desired 72 degree temperature. The gap is just too big. So what does it do? It maintains the lowest temperature it can-but it will still be higher than 72 degrees!

That's what happens with your setpoint. If, over a long period of time, you develop a greater gap between the calories you eat and those you use up in exercise, your body's weight regulation system will adjust upward. Your body then settles in to maintain that higher weight.

That answers the first part of your question, but what about why it's so hard to the weight back off?

When you start to lose weight, your body's metabolic alarm goes off. It alerts your body-which strives for equilibrium or that stable status quo-that you are not eating as much as usual. In turn, your body demands more food. It's a survival mechanism, built in eons ago, and not easily reprogrammed.

For years, I have reassured my patients that dieters don't fail for lack of willpower but because of cravings! As long as your setpoint remains evelvated, you will be assaulted by those blasted cravings every time your body senses that you are not eating enough to maintain your present weight.

Those physiological hunger alarms thus make it extremely hard for overweight people to lose weight, and even harder for them to keep it off. Your body is fighting to hold on to whatever excess fat it has become accustomed to, and it does its best to replace any weight you lose.

Last week, I offered a number of strategies for dealing with cravings, but here's a little framework for bringing them into play.

People who know the phrase "lead us not into temptation" grow up thinking of temptation as the first step down a slippery slope into some kind of disaster, and it often is, if you yield to it.

But you can also think of temptation as an early warning system. Sure, there are times when it just doesn't bother you to be around a lovely plate of brownies, or French fries, or some other treat that's just not on your dietary program. But sometimes you are tempted. But instead of regarding that craving as the first step toward actually eating the off-limits treats, regard it as the signal to pull in reinforcements, just in case. Even before you start to feel really compelled to go for the goodies.