Ever wonder how much you'd have to eat in a single sitting to gain a pound? You'd have to down about 9,000 calories - more than three times your daily quota - to actually "store away" a pound. Studies have shown that your digestive system reaches a point of saturation beyond which additional helpings become progressively less likely to be absorbed.

It's not feasting that makes us fat so much nowadays so much as it's food out of habit - mindless munching of the between-meal snacks, no sooner gone than forgotten. We're a nation of nibblers, and the problem lies with what we eat and how often.

The calories that have been refined have a way of taking firmer hold than Mother Nature intended. There are people in Africa who eat nothing but corn, beans, and sugar cane - sometimes 6,000 calories' worth a day - and never get fat. Why? A full 90 percent of Zulu's diet comes from carbohydrates, but so much of it is in the form of indigestible plant materials that even sugar has trouble sticking to his ribs.

No such luck with the mush we're hooked on, meals included. Foods of animal origin have no fiber whatever. And even our flour, though we may manufacture it to go "crunch" in our mouths, has all the solidarity of soup by the time it gets to our intestines.

The problem? Absorption - too much, too fast. We soak up the calories from a low-fiber diet like a sponge. And along with the good, unfortunately, we also take some bad - cholesterol, for instance. As part of a low-fiber diet, it has time to enter the bloodstream through the walls of the small intestine. Fiber, by keeping things moving, won't let it. The bulky stuff is a very discriminating conductor, prejudiced against heart disease and obesity alike. It knows what should go where, and when. And by enforcing laws of digestion that refined foods have long since forgotten, it regulates the absorption of nutrients - and calories - in a manner more in tune with what nature had in mind.

Proof? A study by Drs. June Kelsay, Kay Behall, and Elizabeth Prather for the US Department of Agriculture revealed that a 26-day diet high in fruits and vegetables decreased the digestibility of calories for 12 middle- aged men by 4.8 percent. As part of a normal day's eating, that would translate to 144 calories - a baked potato smothered in butter - that you could eat and not have to account for. Fiber, in other words, had actually made whatever else the men ate less fattening.

How? By speeding digestion. Calories, mostly in the undesirable form of fat, had been shown the way out rather than the way in. Studies revealed that the men had excreted about 10 grams more fat a day when on the diet high in fiber. Fiber, evidently, had kept fat out of the blood and in the bowels, where it could do no harm. Blood pressures, which had been high in six of the men going into the experiment, also dropped markedly.

If it all sounds too good to be true, it's not. By eating more fruits, vegetables, and whole grains (bran especially) you can begin to process food as it was designed to be processed. Mother Nature didn't knock the chaff from wheat: Man did.

A number of doctors have argued the benefits of fiber, none quite as convincingly perhaps as Sanford Siegal, M.D., D.O. In a handy little pocket pamphlet put out by Dell Publishing Co., Siegal tells you not only what to eat, but when. Of the 150 foods listed, wheat bran, he says, is best. Include a teaspoon or so as part of every meal and you provide your calories just the encouragement they need. Crunchiness is no criterion when it comes to bulk as far as your stomach is concerned. Some of the foods you expect to have the least fiber have the most - coconut, for example, and strawberries, mushrooms, artichokes, and avocados. The diet includes some fairly fun stuff - some sweet, some even a little fatty. Nuts, for example, are loaded with fiber. As, of course, are Grandma's old standbys, cabbage, carrots, and spinach - the stuff you'd never eat.

The secret, says Siegal, is to fill you up as much as possible with the stuff that's going "to pass." That way anything else you happen to throw in is likely to get swept along in the process.

So, go ahead and have your feast. And go ahead and invite, if you must, some less than dietetic guests. Include enough fiber at the affair and they won't make you fat.

Jean-Marc Katzeff recently completed his M.S. in Exercise Physiology and Exercise Science (with an emphasis on nutrition) at George Washington University. He was Health and Fitness Director at the National Press Club for four years and youth program director in Reston. A competitive swimmer in high school, he is a three-time national champion triathlete