How meta-analysis conclusions are subject to bias, ost bran example

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

The conclusions of a meta-analysis (a summation of similar clinical trials), like any other study, can be presented with a particular slant. There is a tendency for any person to desire to confirm prior opinions voiced in the literature rather than refuting their own prior results.

A subsequent meta-analysis¹ concerning the effects of oat bran on cholesterol levels was written by the senior author of the widely publicized study² that incorrectly suggested oat bran and wheat bran supplements "reduce serum cholesterol about equally" and that "oat bran has little cholesterol lowering effect." The conclusions of this subsequent meta-analysis were presented in such a way as to minimize any contradictions with the initial flawed study published in 1990.

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.

The negatively slanted conclusions of the original authors are followed by an alternative version with positively slanted conclusions for the same meta-analysis evaluating soluble fiber.
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The results of the meta-analysis¹ by the authors of the flawed oat bran study can be viewed as a cup half full or half empty, depending on the bias of the authors.

The authors of this meta-analysis¹ conclude that "increasing soluble fiber can make only a small contribution to dietary therapy to lower cholesterol."

They note that soluble fiber from three bowls (28g) of oatmeal is required to achieve even a 3 g soluble fiber intake. (Ingestion of 3g of soluble oat fiber resulted in a decrease of .13mmol/L in total cholesterol.)

An alternative positive statement of the conclusions for the same data from the same meta-analysis would be the following:

This meta-analysis suggests that an intake of 3g of oat soluble fiber daily can result in a 2% reduction in cholesterol which has been estimated to correlate to a 4% reduction in cardiovascular disease². Similarly, an intake of 6g of fiber would result in a 4% reduction in cholesterol which is estimated to result in an 8% reduction in cardiovascular disease². This would be a significant public health benefit.

(Additional details in the alternative positive viewpoint would include the following. 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/serving.)
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Both versions of these conclusions are slanted, but in opposite directions.

The positive and negatively slanted viewpoint presented the following findings of the meta-analysis in very different ways. The results of the meta-analysis indicated that 3g of soluble fiber can result in a decrease cholesterol of 13mmol/L (5mg/dL), while 6 g of soluble fiber can result in a decrease of cholesterol of 26mmol/L (10mg/dL). The positively and negatively framed conclusions had very different ways of interpreting this same data.

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. 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