Fiber: Friend or Foe? Researcher Says
by Lucinda Weiss ncluding fiber in your diet reduces your cholesterol level. That belief is now widely accepted in the fight against coronary heart disease, the number one killer in the U.S. of men and women.
But for post-menopausal women, dietary fiber may actually increase their triglyceride levels - another risk factor for cardiovascular disease - depending on the type of fiber they choose.
That is the surprising finding of research directed by Maria-Luz Fernandez, a professor of nutritional sciences.
In a study conducted by Fernandez and her former graduate student, Sonia Vega-López, with funding from the U.S. Department of Agriculture,
all 68 participants - men, pre-menopausal women, and post-menopausal women - lowered their low-density lipoprotein (LDL), the so-called "bad cholesterol," by 7 to 9 percent, and kept their high-density lipoprotein (HDL) levels, the "good cholesterol," about the same.
They did it by eating cookies. Not the fat-laden, frosted kind, but special cookies baked in the foods laboratory at the Department of Nutritional Sciences and laced with psyllium powder. Psyllium is the plant that yields the fiber used in Metamucil and cholesterol-lo wering dietary supplements sold at health food stores.
The powerful effect of psyllium in decreasing low-density lipoprotein levels had an unexpected effect on triglyceride levels in older women, however. The 24 men in the study lowered their triglyceride levels by 17 percent, and the triglyceride levels of the 23 younger women in the study stayed about the same. But the 21 post-menopausal women increased their triglyceride levels by 16 percent.
While low-density lipoprotein has a major role in plaque formation in the arteries, triglyceride is considered a marker for cardiovascular disease, or an indicator that cholesterol is being carried in the blood.
"If we had done the study only on men, we would have said that psyllium lowers plasma triglycerides," says Fernandez, whose research focuses on how lifestyle and diet affect cholesterol levels.
The importance of this study, she notes, is as a demonstration that "when we make recommendations, we have to realize who we're making recommendations for." And that must include awareness of the subjects' gender and hormonal status.
That hasn't always been the case. Only since 1993 have the National Institutes of Health required researchers to include women and minorities in clinical research studies. Physicians' recommendations on cholesterol control tend to generalize, not taking into consideratio n gender differences, Fernandez says.
An alternative source of fiber for older women might be fruits, vegetables, and grains, rather than soluble fiber such as psyllium.
Fernandez's interest is not in developing dietary recommendations, but in determining how the body responds to changes in diet - how metabolism is affected.
The full picture of how fiber lowers cholesterol in humans is still unknown. The small study that she and Vega-López conducted would have to be enlarged to a clinical trial to be relevant as a basis for recommending diet change, she noted.
From blood samples taken from the 68 participants, Fernandez and Vega-López isolated monocytes, a type of white blood cell, to measure the expression of certain genes. This gave them a fine tool to study how diet was affecting enzyme activity, she explains.
To learn why post-menopausal women had an increase in triglyceride levels, one can look at what was going on in the liver, Fernandez said. One possibility is that the women may have responded to the psyllium by taking up more cholesterol through the liver and increasing the production of very-low-density lipoproteins (VLDL) in the blood plasma.
That, in turn, may be linked to greater triglyceride levels, since very-low-density lipoproteins can be a major carrier of triglyceride in plasma. Another possibility is that a lack of estrogen in the post-menopausal women translated to less activation of the lipoprotein lipase enzyme that normally breaks up triglycerides.
Some of the men, on the other hand, had high triglyceride levels going into the study, and psyllium appeared to have a major role in lowering triglyceride in men with this condition.
Fernandez and her research group, which currently includes seven doctoral students and two master's degree candidates, have a number of dietary studies underway, ranging from the effects of egg consumption on health to heart disease factors affecting children in Mexico.
Fernandez, who is a native of Mexico, received her doctorate from the University of Arizona. Sonia Vega-López earned her Ph.D. from UConn and is now a post-doctoral researcher at the University of California, Davis.
One essential her research group never lacks is subjects, Fernandez says. They are recruited from the campus community.
"You'd be surprised how many people are interested in doing these studies," she says. She attributes this to a broad interest in health and nutrition: "It's a complicated science, I think, and it attracts a lot of attention. People are interested in what they eat."
But there's no accounting for taste. Some of the subjects in the cookie study detested the psyllium cookies, which stuck in the teeth, since psyllium absorbs water. They preferred the control cookies, the plain sugar cookies that all of the subjects ate half the time. Other participants asked for the recipe of the chocolaty-looking psyllium treats.
"There was no homogeneous response," comments Fernandez, with scientific understatement.