Fiber: Good for some of what ails you
Monday, October 23rd 2000, 12:00 am
By: News On 6
By Laura Beil / The Dallas Morning News
Lately, fiber has had it rough.
Headlines once touted a high-fiber diet as a way to help prevent cancer, heart disease, diabetes and a host of lesser ailments. The latest studies, though, have poured cold milk over fiber's cancer-preventing reputation. Not only does fiber not seem to do much for colon cancer, but one study this month suggested that fiber supplements might actually increase the risk of precancerous growths.
So is fiber good for you?
"It seems like a simple question," said Dr. Joanne Lupton, a nutritional scientist at Texas A&M University. Yet questions that involve the food people eat can be complicated for scientists to answer.
Don't abandon the bran, Dr. Lupton said. Even if the cancer hypothesis has gotten soggy, the evidence supporting fiber's other benefits is much crisper.
"I think the overwhelming evidence says that high-fiber foods are protective against a whole variety of diseases," Dr. Lupton said.
A few examples: Two large studies â€“ one involving 43,000 male health professionals and another involving 68,000 female health professionals â€“ found that fiber decreased the risk of coronary heart disease. One of those studies also found a lower risk of diverticular disease, a painful intestinal condition that often strikes older people. And Dallas researchers recently reported that fiber can help diabetics keep their glucose levels under control.
Fiber is a general name for the parts of edible plants that aren't digested, which is why fiber is often referred to as "roughage." Generally speaking, fiber comes in two types, water-soluble and water-insoluble. The exact mechanism behind fiber's health benefits isn't known, but many scientists believe that it affects the way other nutrients in the body are absorbed in the intestine. That's how fiber could tip cholesterol levels, or blood glucose.
The cancer-protection hypothesis emerged in the 1970s, when a British researcher observed that African populations with high-fiber diets tended to have much lower colon cancer rates. Perhaps, the theory was, fiber somehow reduces the colon's exposure to cancer-causing chemicals.
Early studies about fiber seemed to generally support the idea, although the results weren't as strong as researchers would have liked. Beginning last year, the colon cancer picture began to get even fuzzier: Researchers from Harvard Medical School reported the results of one of the largest studies of colon cancer and fiber so far, saying that fiber appeared to make little difference.
But the most damaging blow to the cancer hypothesis was reported this year. Government researchers, conducting a large study to see whether fiber could prevent the formation of polyps â€“ the growths that sometimes lead to cancer â€“ couldn't find any benefit.
The scientists who conducted the study aren't recommending cheeseburgers. There could be many reasons this particular study didn't work, said Dr. Arthur Schatzkin of the National Cancer Institute. For example, people in the study already had a tendency to develop polyps, and the study was designed only to test their recurrence. But no one can say whether fiber might prevent polyps from developing in the first place.
"I would say that the jury's still out," Dr. Schatzkin said.
Studies of diet are maddeningly complicated to conduct, in part because scientists have only a few years to try to measure the impact of a lifetime of eating.
"The ideal trial would be to follow people from birth, randomize them to different diets and follow them for 70 years," Dr. Schatzkin said. Since that kind of study won't happen â€“ the study would outlast the researchers â€“ science must often rely on studies that typically follow people for several years, ask them to fill out questionnaires about their eating habits, and watch to see who develops disease.
Usually, this works pretty well. Eight or nine studies like this have all supported the idea that fiber protects against heart disease, said Dr. Eric Rimm of the Harvard School of Public Health. "They have actually been incredibly consistent," he said. Even researchers in Finland, where diets are different from those in the United States, have found that fiber protects against heart disease.
In the case of colon cancer, though, there may be important factors that scientists haven't yet uncovered. Perhaps a high-fiber diet is protective, but only if it's eaten early in life, Dr. Schatzkin said. "Maybe it's what you eat when you're 13 years old."
Or, the benefit may be too small to show up in the three or four years that a study is under way. A small difference over a few years could add up to a big difference over decades.
The formation of cancer "is a long, drawn-out process," said Dr. Abhimanyu Garg of the University of Texas Southwestern Medical Center at Dallas. Dr. Garg studies fiber and diabetes, not cancer, which in a way makes his task a little easier. He and his colleagues can boost the amount of fiber in people's diets and shortly thereafter measure the effects on their glucose levels. They don't have to sit and wait for years to see an effect.
Even these kinds of studies have their challenges, though, because they still deal with eating habits. People don't eat just fiber, so research has to take into account how any other component of a diet might affect glucose levels. This dogged the question of glucose and fiber for years, he said, because skeptics could point to other reasons that a change in diet might have caused blood sugar to fall.
In May, however, Dr. Garg and his colleagues described a study that tried to leave no doubt that fiber was responsible for lower glucose levels. The researchers studied volunteers who ate two different diets for six weeks, taking care to make sure that the amount of fiber was the main difference between the two groups.
Dr. Garg and others say that any controversy about fiber would be better addressed if researchers knew more about how it worked. "We haven't reached this stage yet where we have started to dissect the finer properties of fiber," he said. For example, maybe the type of fiber makes a difference. Soluble fiber, for instance, appears to be more beneficial in controlling blood glucose and cholesterol.
And there are probably even more subtle differences than that, said Texas A&M's Dr. Lupton. Some types of fiber undergo fermentation in the intestine and others don't, and this may have a role in disease prevention, she said. Doctors don't just say "minerals" are good for bones, they specify calcium. "Maybe we're being too broad just asking about fiber itself," she said.
At the very least, fiber can make a person healthier by helping in weight control, she said. Since fiber isn't digested, it doesn't have calories. So high-fiber foods can be more filling without adding to body weight.
"There are still beneficial effects" to a high-fiber diet, said Dr. Garg. "Even if the benefit doesn't result in less colon cancer, at the very least you won't be constipated."