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Letters to the Editor |
Human Nutrition Unit, University of Sydney, New South Wales 2006, Australia
Dear Sir:
In a search of MEDLINE (National Library of Medicine, Bethesda, MD) for articles published from 1966 to June 1996, Brown et al (1) found 67 controlled trials of pectin, oat bran, guar gum, or psyllium that met their 6 rather arbitrary criteria. This appeared to be an adequate number of studies at first, but in retrospect was an incomplete systematic review because the literature was not searched thoroughly and the rejection criteria were too exclusive. Four years ago (2), I found 22 human trials with pectin, 50 with guar gum, 39 with oatmeal, and 28 with psyllium that reported effects on plasma lipids—a total of 139 controlled trials with viscous fibers!
Concerning pectin, the first clear finding of a plasma cholesterol–lowering effect was by Keys et al (3). Their study was well designed, but was not included in Brown et al's meta-analysis, presumably because it was published in 1961 (although another paper of Keys et al published in 1957 was cited) or because it measured only total cholesterol. Kay and I (4) published our first controlled trial with pectin (21 d of pectin feeding preceded and followed by 14-d control periods) in this Journal in 1977 and Judd and I (5) published another human study with pectin (a crossover design comparing diets high and low in methoxyl pectins) in the British Journal of Nutrition in 1982. Neither of these carefully conducted studies appeared in Brown et al's meta-analysis.
In addition, the findings of our metabolically controlled trial of oat fiber published in this Journal (6) was also missing from Brown et al's meta-analysis. In this trial, subjects were fed rolled oats (125 g) for 21 d, preceded and followed by 14-d control periods. Blood samples were taken for plasma lipid measurement on the last 3 d of each period. Furthermore, we controlled for the content of polyunsaturated fatty acids in the rolled oats by adding simulated oat oil to the control diets.
A meta-analysis can only be as good as the efficiency of the literature search and the usefulness of the rejection criteria (7).
The mean plasma cholesterol–lowering effect of pectin (based on only 7 studies) calculated by Brown et al was twice as high per gram of fiber as the effect of the other 3 viscous fibers studied. The value I (2) calculated from a larger number of studies was lower than this and, not surprisingly, Brown et al did not find the pectin value to be significantly different from their mean estimates for the other 3 viscous fibers.
As for the mechanism of action of viscous fibers, Kay and I (4) reported significant increases in fecal neutral steroid and bile acid excretion in subjects who consumed citrus pectin (15 g/d) as part of a metabolically controlled diet for 3 wk. Subsequently, in another study, Judd and I (5) found little difference between the effect on plasma cholesterol of high and low methoxyl pectin consumption. In addition, it was reported that pectin was fully effective in the treatment of patients with familial hypercholesterolemia who were already taking the bile acid–binding resin cholestyramine (8). Therefore, it appears that one mechanism of action of viscous fibers is dependent on viscosity, but not on chemical binding, in the small intestine. Zhang et al (9) in Gothenburg found increased excretion of bile acids in ileostomy subjects fed oat bran. The alternative feedback effect of increased volatile fatty acid production has not yet been shown directly. Lactulose has been shown to increase colonic fermentation but not to lower plasma cholesterol (10) and oral propionate also did not change plasma cholesterol concentrations (11).
REFERENCES
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