If you ever wondered just how a high-fiber diet helps keep you, well, "regular," scientists may have the answer.
Their results suggest that as these bulky foods make their way down the gastrointestinal tract, they run into cells, tearing them and freeing lubricating mucus within.
More mucus is good, says Dr. Paul L. McNeil, cell biologist at the Medical College of Georgia and corresponding author on the study published online Aug. 21 and scheduled for the September print issue of PloS Biology. "When you eat high-fiber foods, they bang up against the cells lining the gastrointestinal tract, rupturing their outer covering. What we are saying is this banging and tearing increases the level of lubricating mucus. It's a good thing."
The fact that consuming roughage increases mucus production was known, and years ago, Dr. McNeil discovered frequent cell injury and repair occur when we eat.
The new research ties the two together.
"It's a bit of a paradox, but what we are saying is an injury at the cell level can promote health of the GI tract as a whole," says Dr. McNeil. Even though epithelial cells usually live less than a week, they are regularly bombarded, in most of us at least three times a day as food passes by. "These cells are a biological boundary that separates the inside world, if you will, from this nasty outside world. On the cellular scale, roughage, such as grains and fibers that can't be completely digested, are a mechanical challenge for these cells," says Dr. McNeil.
But in what he and colleague Dr. Katsuya Miyake view as an adaptive response, most of these cells rapidly repair damage and, in the process, excrete even more mucus, which provides a bit of cell protection as it eases food down the GI tract.
In research published in 2003 in Proceedings of the National Academy of Sciences, Dr. McNeil showed proof of his then decade-old hypothesis that cells with internal membranes use those membranes to repair potentially lethal outer-membrane injuries. A recent paper published in Nature in collaboration with Dr. Kevin Campbell's laboratory at the University of Iowa showed how human disease, including certain forms of muscular dystrophy, can result from a failure of this mechanism.
An outer membrane tear is like an open door through which calcium just outside the cell rushes in. Too much calcium is lethal but that first taste signals the vulnerable cell it better do something quick. With epithelial cells, several of the internal mucus-filled compartments fuse together within about three seconds, forming a patch to fix the tear. In the process the compartments expel their contents so, almost like a bonus, extra mucus becomes available to lubricate the GI tract.
"We have found a very natural way we can enhance mucus production," says Dr. Miyake, cell biologist and the study's first author. He and Dr. McNeil suspected for years that mucus escaped cells as a result of injury. "You might have predicted it, but science is about testing predictions," says Dr. McNeil.
To test their theory, Dr. Miyake, co-director of MCG Cell Imaging Core Laboratory, began working on a method to reproduce cell injuries. "Dr. Miyake developed a very potent cutting edge technology involving the two photon laser that allowed us to blast small holes in cells, mimicking what happens in the living animal. It also allowed us to assess in those living cells whether they could reseal, repair the damage and how they might respond biologically, namely in this case, whether they responded by secreting mucus as part of the healing process," Dr. McNeil says.
They found time and again that most cells did just that, including intact cells in a section of the GI tract. "Epithelial cells are high-turnover cells but they have a built-in survivability," Dr. McNeil says.
The scientists aren't certain how many times cells can take a hit, but they suspect turnover is so high because of the constant injury. Potentially caustic substances, such as alcohol and aspirin, can produce so much damage that natural recovery mechanisms can't keep up. But they doubt a roughage overdose is possible.
The research was funded by NASA.
Dr. Toru Tanaka, a pharmacologist at the Josai University in Japan and an expert in mucus measurement, is a study co-author.
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