Misleading conclusions on effects of resistant starch due to inappropriate formulation of controls, inadequate statistical power, and anomalies in the in vitro methods.

In a recent article,Wolever et al. (1) conclude that bars formulated with resistant starch (RS) did not lower glycemic responses relative to control bars formulated with highly digestible carbohydrates. This is a surprising conclusion given the substantial database on the glycemic-lowering effects of RS. We have contributed to this body of knowledge and published reviews (2, 3) as well as numerous animal-model and clinical studies on the glycemic effects of RS and, in particular, RS from highamylose maize, the ingredient used in the study by Wolever et al. (1). A major concern in the study is the use of sucrose in the control foods. The investigators formulated 2 control bars: 1 with the same amount of available carbohydrate (40 g) and 1 with the same amount of total carbohydrate (54.6) as a test bar formulated with 15% highamylose-maize starch. Unfortunately, the 2 control bars contained disproportionately more sucrose: 23.5 and 30.3 g in the control bars compared with 15–16 g in the high-RS bars. The glycemic response to sucrose is ;40% lower than that for rapidly digested starches due to its fructose content (4). In fact, the scientific panel of the European Food Safety Authority authorized a health claim for the postprandial glycemic-lowering effects of fructose (5). The inclusion of fructose in the study by Wolever et al. (in the form of sucrose) lowers the glycemic impact of the control bars and invalidates the conclusion that RS fails to lower postprandial blood glucose. An appropriate control would have been a rapidly digestible glycan, such as highly digestible starch or flour, glucose, or maltodextrin. We have extensive experience formulating RS-enriched foods for clinical trials and diets for animal studies with appropriate controls matched for glycan content (and low in fructose). The lowering of postprandial glycemia by replacing highly digestible starch with RS is the subject of an approved European Food Safety Authority health claim (6) and has been used in numerous food applications. A well-designed study by MacNeil et al. (7) showed that the replacement of high-amylose maize starch for refined flour in a bagel lowered the postprandial glycemic response. The study by MacNeil et al. further shows that the processing methods used to manufacture bagels (boiling and baking) do not gelatinize the RS portion of high-amylose maize starch. Wolever et al. suggest that gelatinization may have affected their results; however, no information is given on the processing and cooking methods used to manufacture the test bars in their study. The process stability of high-amylose maize RS is well established in the food science literature, and processes such as baking are unlikely to gelatinize the starch granule (8). We recently showed that incorporating highamylose maize starch in yogurt that is subsequently pasteurized did not disrupt the starch granule or reduce the RS content (9). A second concern is the small sample size used. Statistical power calculations are given, and a sample size of 10 is cited as providing adequate power to detect a difference of 35% in the blood glucose incremental AUC between 2 test conditions. The study tested 7 conditions, and it appears that comparisons were made among all treatments for the incremental AUC with the use of a Tukey post hoc test. Thus, the power calculation should be based on a family-wise a of 0.05 taking into account the multiple comparisons being made with the Tukey procedure. In addition to the principal issues with regard to product formulations and statistical power, there were also anomalies with the in vitro digestibility aspect of the study and how it was expressed and interpreted. First, the assay tested 500-mg portions, which varied in their available carbohydrate content as indicated by their differing release plateau amounts, which should have had identical plateaus if expressed on an equal available-carbohydrate basis. It does not make sense, then, to attempt to correlate the in vitro data with the in vivo glycemic response data in which portion sizes were calculated to provide equal available carbohydrates. A second point relates to the enzyme activity of the assay, which, with only 10 mg pancreatin per assay, was low for the amount of carbohydrate present. No indication was given on how the profile of a rapidly digestible carbohydrate would appear, and the concern is that the test substrates could have been the rate-limiting factor in the assay. We believe the inappropriate design of the controls, the low statistical power, and the anomalies in the in vitro methods of this study seriously undermine the validity of the results and provide an explanation for the atypical findings on the glycemic effects of RS.