In my long career as lunch eater, I have been flanked by thin coconut-cream pie eaters and fat carrot-stick munchers. I have heard underweight people tell me that they can eat anything they want and overweight people swear that they hardly eat a thing.

I confess here that I have not always believed them.

The way I figured it, the thin people probably skipped things . . . like supper. They probably stopped eating when they are full. Kinky stuff like that.

As for the chubbies, I assumed that they kept Hershey bars in their sock drawer and didn't count anything they ate between meals . . . even the meals.

But now scientists have proved that Mother Nature has played yet another nasty little trick on us. Some people can eat whipped cream and look like whippets. Others can eat modestly and look mountainous.

Lurking in the blood cells of each innocent newborn is the real villain of the weight-watching world, something known as ATPase. According to the latest study coming out of a group of Harvard-affiliated hospitals in Boston, there is this biochemical base to weight.

This is, of course, the ultimate proof that life isn't fair. If you have a lot of ATPase, you are going to burn more calories -- so you can eat more. If you have a little, you will use up fewer calories and add on more fat.

The good news here is that maybe people will stop judging their self-worth by the pound. Some of us apparently have no more control over our weight than our height.

The bad news is that you can't go out and buy a pack of ATPase. Yet.

There is room for fantasy. After all, the real growth industry of the decade has been in loss -- weight loss. Anybody who can get a patent on this stuff could make a fortune.

If I were king or president of Harvard University, I'd drop all those plans to go into the business of DNA development and start talking ATP.

The sales possibilities here are endless. Every Monday, millions of Americans are eager to burn up the weekend bloat-over. Every day, millions more are starting the eternal 10-pound crash diet. AN ATPase tablet could replace everything from the Scarsdale to saccharin.

Few of us, I know, actually suffer from a loss of ATPase. I myself have a different sort of biochemical problem. I was born with a defect in my genetic makeup that forces me, entirely against my will, to keep moving my hand toward my mouth. My hand is rarely empty. Also, from time to time, a metabolic switch in my brain is turned on that can only be satisfied with a bag of chocolate-covered wintergreens. A friend of mine has a smiliar problem, a deep chemical response to the sight of a full plate. He is compelled to empty it.

But the discovery of this wonder ingredient gives hope to the hopeless.

The drug companies, which have brought us all kinds of goodies, are surely inventive enough to develop a blue pill that would burn off cheesecake and a red one to gobble up a banana split. Someone will produce a main-line injection right smack into the old blood cells to work off a regular six-course-pig-out.

The market is wide open. Travel agents could include it in a package vacation trips to Rome. Restaurants would serve it with the cappuccino. The multinational peanut butter conglomerates could add it directly into the jar.

There is always the danger of overdosing, but the FDA-approved antidote would be a simple forced feeding of potatoe salad.

I could envision television ads with cartoon ATPase creatures gobbling up human hips. Billboards across America will boast, "Eat, Drink and Be Thin." The Anti-Exercise Institute will instruct: Let Enzymes Do the Jogging.

At least there is promise from the wonderful world of science. Today, eat your heart out. Tomorrow, eat to your heart's desire.