Like its fellow carbohydrates, the various types of dietary fiber are the product of sunlight, water and carbon dioxide combining in green plants. Most form part of plant cell walls. But unlike the other carbohydrates, fibers do not break down into sugars in the human digestive system and then course through the blood stream fueling muscles and nerves. Rather, when eaten they tumble intact through the stomach and small intestine and end up in the colon where billions of bacterial feed on them - in turn producing intestinal gas. No wonder, then, that dietary fiber has been unwelcome in many of history's nicer neighborhoods. Even 20th century doctors reasoned that since the bulky material provided not a single nutrient, it would only strain already troubled guts. Accordingly, they recommended low-fiber diets for patients suffering from hemorrhoids and other colon disorders often found in the West. But then, about 15 years ago, the prescription was reversed as researchers found that poor Africans, who eat lots of fiber, rarely suffer from such complaints. Fiber, the researchers learned, actually eases the bowel's burden by mixing with water and other food residues. Soon, nutritionists came to see the low-fiber diet of most North Americans as a culprit in the onset of disorders ranging from tooth decay to heart attacks. Increasing the consumption of certain kinds of fiber, they found, could slow the body's absorption of sugars to which diabetics are sensitive, and of cholesterol, which may lead to heart disease. Furthermore, fibers fight obesity. They're filling, especially the pectins in citrus fruit and the gums in some beans. They're mainly indigestible, therefore, dieters eating lots of fiber are likely to eat less of other, more fattening foodstuffs. As for why populations on high-fiber diets seem to experience fewer colon cancers, no one knows for sure. In any case, there is no doubt that fiber is nature's laxative, the dietary key to regularity. Nutritionists therefore advise you to stay away from foods containing processed and refined ingredients such as bleached flour and white sugar. Remember that meats contain little fiber and that overcooked vegetables and fried foods have lost much of theirs. Nevertheless, fiber supplements are usually unnecessary. A healthy diet should be full of fresh fruits and vegetables - and some of the most fibrous foods such as bran cereals, whole wheat breads, peas, beans and lentils.