Investigating the Purported Slow Transit and Starch Digestion of Whole Grain Foods

Epidemiological studies have strongly supported the substitution of whole grains or whole grain foods for refined grains in the diet to reduce the risk of type II diabetes and other chronic diseases. However, the mechanisms of whole grain digestion, and the properties that are responsible for the associated responses of low glycemia and slow transit time of whole grains are still not well understood. The moderated glycemic response related to consumption of whole grains is largely associated with dietary fiber, yet there are properties of other components, such as starch itself, that warrant further investigation. Gastric emptying rate, or the rate at which foods exit the stomach, is an important physiological parameter that is also associated with glycemic response. Gastric emptying rate is influenced by the physical attributes of a food, such as viscosity or particle size, as well as by feedback systems that are stimulated by the digestion and fermentation properties of food. The overall research goal of this thesis was to identify properties of whole and refined grain foods that affect glycemic response and gastric emptying rate in humans. The specific objectives of this research were to: 1.) identify properties of starch in a whole grain food and its refined counterpart (brown and white rice) that could slow starch digestion and affect gastric emptying rate in humans; 2.) investigate the purported low postprandial glycemia of whole grain wheat food when its source and physical and chemical properties are controlled, the role of particle size of whole and refined grains on digestive properties, and whether whole versus refined grains actually influence gastric emptying rate in humans; and 3.) identify a potential mechanism for slow starch digestion of whole grain wheat milled products related to entrapment of starch within cell wall structures. Findings from the first study showed that brown rice had significantly slower gastric emptying compared to white rice. This was likely due to the physical presence of its bran layer; although further increasing the slowly digestible and resistant starch fractions also trended to slower gastric emptying time of foods. In the second study, no differences in glycemic response were observed between whole and refined grain porridge meals prepared under controlled conditions and with the matched particle sizes, starch contents, viscosity, and fiber content. The highest glycemic response was observed for whole wheat flour and lowest for refined semolina. In the third study, starch remained entrapped within intact cell walls of whole grain milled fractions, having limited gelatinization properties and exposure to digestive enzymes. Overall, the thesis findings showed that whole grain foods, when carefully matched for physical and chemical properties and from the same source and milling streams, have minor to no differences in glycemic response and gastric emptying rate. This is counter to the current viewpoint and opinions that have led to dietary recommendations for increased intake of whole grain foods. Still, this is not to say that whole grain products might not be low glycemic and have an extended energy property, which could be due to other factors of whole grain foods such as product density and form