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and Interpretation of Clinical Trials
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Biased Meta-analysis

Critique of the meta-analysis subsequently written by the senior author of initial flawed oat bran study

A subsequent meta-analysis1 concerning the effects of oat bran on cholesterol levels was written by the senior author of the widely publicized flawed study2 that incorrectly suggested oat bran was without significant effect on cholesterol levels*.

There is a tendency for any person to desire to confirm prior opinions voiced in the literature rather than refuting their own prior results. The conclusions of a meta-analysis, like any other study, can be presented with a particular bias. The conclusions of this meta-analysis were presented in such a way as to minimize any contradictions with the initial flawed oat bran study published in 1990.

Differing conclusions for the data from the same meta-analysis:

The results of the meta-analysis1 by the author of the flawed oat bran study2 can be viewed as a cup half full or half empty, depending on the bias of the authors.

     Negative conclusions of the original authors:

The authors of this meta-analysis1 conclude that "increasing soluble fiber can make only a small contribution to dietary therapy to lower cholesterol." 
(Ingestion of 3g of soluble oat fiber resulted in a decrease of .13mmol/L in total cholesterol LDL cholesterol.) They note that soluble fiber from a total of three bowls (28g/bowl) of oatmeal is required to achieve a total of 3 g of soluble fiber.

     Alternative positive conclusions for same data:

An alternative positive statement of the conclusions for this same data would be the following: 
This meta-analysis indicates that an intake of 3g of oat soluble fiber can result in a 2% reduction in  cholesterol, which has been estimated to correlate to a 4% reduction in cardiovascular disease.3 
Similarly, an intake of 6g of fiber can result in a 4% reduction in cholesterol which has been estimated to result in an 8% reduction in cardiovascular disease. This would be a significant benefit to public health.

Additionally, it would be noted that a single standard 40g serving of oatmeal of Quaker Oats Old Fashioned Oatmeal contains 2g of soluble fiber and a single 40g standard serving of Quaker Oat Bran hot cereal contains 3g of soluble fiber per serving.

  Specifically, this meta-analysis indicated that 3g of soluble oat fiber  can result in a decrease cholesterol of .13mmol/L (5mg/dL),  while 6 g of soluble fiber can result in a decrease of .26mmol/L (10mg/dL) decrease in cholesterol. 

Both versions of these conclusions are slanted, but in opposite directions.

*The authors2 initially suggested oat bran was without significant effect on cholesterol beyond non soluble fibers.  The subsequent meta-analysis1 by the same authors was presented in a fashion to be as consistent as possible with the initial incorrect conclusions.

    1. Brown L, Rosner B, Lillett W, Sacks F. Cholesterol-lowering effects of dietary fiber: a meta-analysis. Am J Clin Nutr 1999; 69:30-42

    2. Swain JF, Rouse,IL, Curley CB, Sacks FM.  Comparison of the effects of oat bran and low-fiber wheat on serum lipoprotein levels and blood pressure. N Engl J Med 1990; 322:147-52.

    3. National Cholesterol Education Program Expert Panel on Detection, Evaluation and Treatment on High Blood Cholesterol in Adults. Report of the National Cholesterol Education Program Expert Panel.  Arch Intern Med 1988;148:36-9

 

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