Trying to lose weight?  It’s important to understand that while you can lose weight quickly, you cannot lose fat quickly.  Although I’ve said this many times in this column, I still regularly encounter folks who tell me how thrilled they are on a new diet and have already lost many pounds in only a week or two.  Their efforts are encouraged by weight-loss ads.  An infomercial for a new diet plan currently running on TV, for example, presents several testimonies from women who say they lost a minimum of 6 pounds the first week.

    Such testimonies are appealing.  But they are entirely bogus if, in fact, your intention is to lose body fat.  Here’s why.

The First Law of Thermodynamics

    The first law of thermo- dynamics tells us that energy can neither be created nor destroyed, but can be transformed from one form to another.  If you are going to lose body fat, you must convert the chemical energy stored in body fat to mechanical energy used in muscular contractions and other bodily functions.

   That’s a tall order, because just 1 pound of body fat contains 3,500 calories.  To put this into perspective, just 1 pound of fat contains more energy than a runner will expend during an entire marathon!  Indeed, fat is a miraculous energy-storage device, and to remove if from the human body required sustained effort over a prolonged period.

    If you lost 6 pounds in one week, and if all 6 pounds came from stored body fat, here’s what would have to happen:  The 6 pounds of fat, at 3,500 calories per pound, would equal 21,000 stored calories.  The first law of thermodynamics demands that you would have to burn off 21,000 more calories that you took in for the week, or 3,000 per day.  Is this possible?  Yes, if you ran more than 20 miles a day! 

When people tell me they have lost a large number of pounds quickly, I am always polite, and I congratulate them.  In my mind, however, I am asking, “How many marathons did you run last week that contributed to your weight loss?”  The answer is, of course, zero marathons.  What’s more, the weight is usually lost without any exercise at all.  How is this possible?

Sugar, Water & Muscle

    On any crash diet, the first weight you lose always comes from stored sugar (glucose stored as glycogen).  One pound of stored sugar accounts for 1,816 calories.  But you also lose 3 pounds of water that is stored with the sugar, for a quick loss of 4 pounds in just a matter of two or three days.  After that, you start losing muscle mass.  One pound of muscle provides only 700 usable calories. 

Your Body Loves Fat

   Why does the body do this to us?  Simple.  It’s a matter of survival.  When you go on a crash diet, your body believes it is starving, and it worries about its survival.  In response, it clings to its fat stores for all it’s worth as a means of providing energy over the long haul – as long as the period of starvation is likely to last.  In ancient times, a period of starvation could be quite prolonged, and the body has evolved to cope with such periods.

    The body will eagerly give up muscle, because muscle is hungry tissue that demands a lot of energy to keep it going.  Thus, the body reasons that the more muscle it gives up, the better chance it has to survive.  Every crash-diet scheme is based on these principles.  That’s why they can deliver on a promise of rapid weight loss.  That’s also why the weight that is lost returns so quickly.

The Bottom Line

*Bryant Stamford has a doctorate in exercise physiology and is director of the Health Promotion and Wellness Center at the University of Louisville; email him at Bryant@louisville.edu.

**This article was originally a newspaper article on a column entitled “Body Shop:  Exercise, Diet, Fitness and Health.”