Dietary Fiber for the Treatment of Type 2 Diabetes Mellitus

A Meta-analysis

Robert E. Post, MD, MS; Arch G. Mainous III, PhD; Dana E. King, MD, MS; Kit N. Simpson, DrPH

Disclosures

J Am Board Fam Med. 2012;25(1):16-23. 

In This Article

Discussion

The results of the meta-analysis show a statistically significant improvement in fasting blood glucose and HbA1c when an increase in dietary fiber was used as an intervention in patients with type 2 diabetes mellitus. However, the effect for HbA1c was more modest than that for fasting blood glucose. A possible cause for this is that the time period for 8 of the 10 interventions for HbA1c was less than 12 weeks, and HbA1c is representative of an overall glycemic control for a 12-week period. Therefore, most of the studies evaluating HbA1c were of a length less than the recommended testing period between HbA1c measurements. More studies evaluating HbA1c for a duration of longer than 12 weeks would be warranted.

Some may suggest that the effect on HbA1c, though statistically significant, my not be clinically significant. Metformin, one of the oral medications most commonly used to treat diabetes, has shown HbA1c reductions anywhere from the range of 0.2% to 2%.[34] Comparatively, making a change in diet to reduce HbA1c by about 0.3% could be useful as an adjunctive therapy to medication as part of an overall treatment strategy for controlling diabetes. Again, this effect may be tempered by short follow-up durations of the majority of these studies, so fiber may truly have a larger affect on HbA1c than what is reported in this study.

These results suggest that increasing dietary fiber in the diet of patients with type 2 diabetes is beneficial and should be encouraged as a disease management strategy. High-fiber versions of traditionally low-fiber foods have been shown to be palatable for patients with type 2 diabetes.[35] Consequently, this disease management strategy should not be particularly onerous for patients and may be easier to comply with than some other more intense lifestyle changes. The mean increase in fiber in the included studies was approximately 18 g/d. To implement this in one's diet would include eating a bowl of high-fiber cereal and adding a few more servings of vegetables a day.

The studies in this analysis used a variety of grams of fiber per day in their interventions, comprising a large range, from as little as an additional 4 g/d to as much as 40 g/d. The most commonly used fiber intervention dose was a 15-g/d supplement to usual diet. However, the American Diabetes Association recommends fiber consumption of at least 14 g/1,000 kcal/d or a range of 25 to 30 g/d, the values recommended by the United States Department of Agriculture,[4] whereas the American Dietetic Association's position statement recommends 30 to 50 g of dietary fiber per day for patients with type 2 diabetes mellitus.[36] Dietary fiber intake from a variety of sources has been associated with a significantly decreased risk of coronary events in epidemiologic studies in patients with and without diabetes. For example, in the Nurses Health Study, women with the highest fiber intake (median, 22.9 g/d) had an age-adjusted relative risk for major coronary events that was almost 50% lower than women in the group with the lowest fiber intake (11.5 g/d).[37] Another study showed that fiber intake more than 20 g/d is associated with lower risk of coronary heart disease among women.[38] Large epidemiologic studies have demonstrated an inverse relationship between fiber intake and cardiovascular disease rates in men as well.[39,40] More studies of those specifically with type 2 diabetes mellitus with larger fiber doses as an intervention would aid in supporting or refuting recommendations for diabetics.

Limitations

The authors did not examine unpublished data, studies not in English, or any studies that were not published in peer-reviewed journals. One can argue that there is publication bias and that many negative studies are not published; however, many of the studies included in this analysis showed non-statistically significant effects. Also, the authors were trying to find studies of the highest quality, and these types of studies usually are published in English language, peer-reviewed journals. The studies exhibited a wide range of intervention amounts of fiber and lengths of time, but these differences are inherent in any meta-analysis.

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