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Whole grain brown rice had slower gastric emptying rate, which appears to be related to the physical presence of the bran layer. Extended gastric emptying of brown rice explains in part comparably low glycemic response observed for brown rice.

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Background Epidemiological and some clinical studies support the view that whole grain foods have lower glycemic response compared to refined grain foods. However, from the perspective of food material properties, it is not clear why whole grain cereals containing mostly insoluble and non-viscous dietary fibers (e.g., wheat) would reduce postprandial glycemia. Objectives We hypothesized that glycemic response for whole grain wheat milled products would not differ from that of refined wheat when potentially confounding variables (wheat source, food form, particle size, viscosity) are matched. Our objective was to study the effect of whole grain versus refined wheat milled products on postprandial glycemia, gastric emptying, and subjective appetite. Design Using a randomized crossover design, healthy participants (n = 16) consumed six different medium-viscosity porridges made from whole grain or refined wheat milled products, all from the same grain source and mill: whole wheat flour, refined wheat flour, cracked wheat, semolina, reconstituted wheat flour with fine bran, and reconstituted wheat flour with coarse bran. Postprandial glycemia, gastric emptying, and appetitive response were measured using continuous glucose monitors, the 13C-octanoic acid breath test, and visual analog scale (VAS) ratings. Bayes factors were implemented to draw inferences about null effects. Results Little-to-no differences were observed in glycemic responses, with lower incremental area under the curve (iAUC0-120 min) glycemic response only for semolina (mean difference [MD]: –966 mg min/dL; 95% CI: –1775, –156; P = 0.02) and cracked wheat (MD: –721 mg min/dL; 95% CI: –1426, –16; P = 0.04) compared to whole wheat flour porridge. Bayes factors suggested weak-to-strong evidence for a null effect (i.e., no effect of treatment type) in glycemic response, gastric emptying, and VAS ratings. Conclusions While whole grain wheat foods provide other health benefits, they did not in their natural composition confer lower postprandial glycemia or gastric emptying compared to their refined wheat counterparts. Clinical Trial Registry number and website where it was obtained: NCT03467659, clinicaltrials.gov.

White rice is considered to have lower nutritional quality compared to brown rice due to the loss of its bran fiber during processing. However, components besides bran fiber that may improve the physiological effects of white rice, such as amylose content, slowly digestible starch (SDS) and resistant starch (RS), should be further defined. Delayed gastric emptying is associated with extended energy delivery and increased satiety, and may be influenced by these components. Temperature‐cycling was used to increase the SDS and RS fractions in moderately high amylose varieties of cooked white rice (~24 – 29% amylose) with digestion rates monitored in vitro for comparison with brown rice. Gastric emptying rates of rice in human subjects (n=12) were then measured using a 13C‐labeled octanoic acid breath test. In vitro starch digestion indicated that the SDS and RS fractions, in the moderately high amylose white rice, were increased following temperature cycling to levels similar to those in brown rice. Moderately high amylose varieties of white and brown rice also contained more SDS and RS in vitro, and trended to more delayed gastric emptying rates compared to the low amylose rice varieties. However, more delayed gastric emptying rates were still seen with brown rice. This may be a result of properties other than starch digestibility or amylose content that affect gastric emptying rate, such as those related to degradation of the physical form in the stomach.This research was supported by the Whistler Center for Carbohydrate Research and the Ingestive Behavior Research Center.