Fiber. It's good for what ails you.

You name it --heart disease, high blood pressure, diabetes or various types of cancer -- and fiber is probably good for it. It can help with cholesterol levels, obesity, fatigue, general malaise, hemorrhoids and, of course, regularity.

We've been hearing about the virtues of fibrous foods for some time, but now mounting scientific evidence is beginning to suggest that people who eat diets rich in fiber, like vegetables, fruits and whole grains are significantly healthier -- and longer lived.

Latest in the annals of fiber is a report -- published last month in the respected British medical journal The Lancet -- of a 10-year follow-up study of 871 middle-aged men (initially between the ages of 40 and 59) from the town of Zutphen in the Netherlands.

The Zutphen men were examined in 1960 and, over a three-month period, they and their wives were interviewed extensively by specially trained dieticians about their eating habits both at home and away.

Ten years later 107 men had died, 37 from coronary heart disease and 44 from cancer, the rest for a variety of reasons.

The data about the Zutphen men recently were subjected to new computer-assisted statistical analyses with a finding of strong inverse relationships between the deaths and the intake of dietary fiber. In other words, those who had died during the 10-year period had generally consumed significantly less fiber, vegetable protein and other complex carbohydrates.

Lung cancer victims, especially, had considerably lower fiber intakes than survivors. The study's authors speculate that this may confirm other recent findings that the substance beta-carotene present -- along with fiber -- in orange and dark green vegetables, had a protective effect against lung cancer.

But in general, "These findings may imply that a diet rich in dietary fibre," they write, "may be protective against death from chronic diseases in Western societies."

This news -- confirming some of his own pet theories -- is no surprise to Dr. James K. Anderson of the University of Kentucky and the Lexington Veterans Administration Hospital.

"The figures are striking," he says,"confirming that high-fiber intake is protective for heart disease, longevity, cancer . . . the whole ball of wax. I couldn't agree more."

Anderson, a specialist in the treatment of diabetes, has seen the beneficial effects of fiber on his patients, with some 85 percent of them able to reduce or actually stop insulin injections when they follow a high-fiber regimen devised under his direction some years ago. The diets are about 70 percent carbohydrates, moderate in protein, low in fats and virtually free of simple sugars -- including honey. He also believes in the benefits of reduced salt intake.

Anderson also discovered recently that blood pressures in patients -- even lean ones -- on high-fiber diets began to drop, as did the fatty substances in the blood--triglycerides and cholesterol, which are linked to cardiovascular disease. (Diabetics are at special risk for these diseases.)

The author of Diabetes, A Practical New Guide to Healthy Living (Arco, $7.95) is "interested," he says, in risk reduction for people in general." His book contains a series of high-carbohydrate/high-fiber diets designed for diabetics but beneficial, Anderson is convinced, to everyone.

Reports like the Zutphen Study confirm those beliefs.

So what are fibers anyway, and how do you get them?

Fibers, say the authors of the Zutphen study, are "the remnants of plant cells" resistant to action by human enzymes. In other words, most go through the system mostly undigested. They are a form of carbohydrate and some complex carbohydrates which may be up to 50 percent fiber.

Nutritionists used to believe that fiber ("crude fiber") was principally cellulose, or a substance called lignan, which makes wood hard.

But the fiber story is considerably more complicated. Nutritionists, physicians and research scientists now know there are other significant fibers that may perform even more important functions. Some are water-soluble; others are not. Some are found in fruits and vegetables and others in grains. Some foods have several types.

The fibers that seem to advantageously affect blood pressure and cholesterol levels: pectins (the stuff that makes jams and jellies jell, found in fruits, mainly citrus and apples) and gums (the stuff that makes oatmeal and beans sticky, mostly water-soluble, found in vegetables and beans).

Even in grains, not all fibers do the same thing. Anderson has found, for example, that oat bran, which is water-soluble, is useful in lowering cholesterol. Wheat bran, which is not water soluble, is more useful in regularizing intestinal functions.

Scientists also are discovering that the way fibers work is more complicated than believed previously. Indeed, some fibers -- those which are not water-soluble -- may work by, quite simply, cleaning out the intestinal tract. They speed up intestinal activity and may carry away potentially toxic, even cancer-causing residues.

The water-soluble fibers move much more slowly, becoming gelatinous, slowing down the absorption of sugars and starches. They may work by affecting hormonal balances, the excretion of particular salts or by influencing some of the exquisite, complicated physiological functions of the human body in ways not yet comprehended.

The data on which foods contain what kinds of fiber are still sketchy and scattered. Oat Bran, manufactured by Quaker, is sold in only a few health food stores throughout the country, although it may receive wider marketing soon.

The Department of Agriculture is just beginning to include fiber content of food -- limited mainly to cereals -- in its nutritional handbooks.

Indeed, not all traditional nutritionists or doctors are fully persuaded of the benefits of a diet as high in fiber as Anderson advocates -- at least 50 grams of varied fiber a day. (The Zutphen authors suggest at least 37 grams a day.) And Anderson rejects the idea of running to the health food stores for so-called purified fiber supplements.

"It is important to stress," he says, "that (in the Zutphen study) they're talking about real foods, not sprinkling health food additives -- purified junk -- on their food."

He recommends a broad spectrum of high-fiber fruits, vegetables and cereals encompassing the various fibers "because," he says, "we just don't know yet which ones are best."