Meta Confusion about Glucomannan

Maybe you’ve heard of glucomannan — a so-called “super fiber” that Dr. Oz hypes as “nature’s skinny sponge.” Maybe you read a systematic review and meta-analysis in Nutrition of the effect of glucomannan on body weight that concluded it “may help reduce body weight.” As it turns out, that analysis was wrong.

The study was just published in the March issue of Nutrition. Already, a letter to the editor has been published online, pointing out the error:

The effect of treatment after two weeks, that was originally reported as statistically significant in the meta-analysis published, turned out to be insignificant.

Errors sometimes flourish like weeds and it takes a sharp eye to root them out. That’s why it is so surprising to see a letter to the editor published so quickly to correct this flawed analysis.

Systematic review and meta-analysis provides a way to sort through a body of research with a range of outcomes. But it’s not a simple task. David Allison and colleagues, who noticed unusual values in the analysis, contacted the authors and worked with them to find the sources of the errors and prepare a correction. He tells us:

I think this error, which was large enough to alter conclusion, typifies a very common problem. I believe that many investigators conducting meta-analyses may not have sufficient statistical support and commonly make errors in the calculations of effect sizes because of confusion about variances.

To people who get frustrated with statistical nuances, we submit this case for why patient pursuit of the truth matters. False claims for “super fiber” or any other miracle weight loss supplement should stay out of the scientific literature.

Click here to read the correction and here to read the original publication. Click here for an example of unfortunate hype about glucomannan.

Konjac Noodle, photograph © With Wind / flickr

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