Effects of Indigestible Carbohydrates and GI of Cereal Products on Glucose Metabolism, Satiety and Cognitive Function in Healthy Subjects; Emphasising mechanisms for glycaemic regulation at the acute, second and third meal
Sammanfattning: The metabolic syndrome includes a cluster of dysfunctions that identifies subjects at high risk of developing type 2 diabetes and cardiovascular disease. The prevalence of the metabolic syndrome has increased markedly over the last two decades. Central to this syndrome is insulin resistance and hyperglycaemia, but also other manifestations, e.g. central obesity, dyslipidaemia, elevated blood pressure, imbalance in lipoproteins and pro-thrombotic factors, or sub-clinical inflammation, are involved. Diets with a low glycaemic index (GI) have proven beneficial in prevention and treatment of diabetes, cardiovascular disease, and the metabolic syndrome. Furthermore, a whole grain diet has shown similar benefits adjunct to these diseases. Less is known regarding the effect of GI or other food characteristics on cognitive function. However, due to the significant differences in postprandial blood glucose profiles after a high- compared with a low-GI meal, cognitive functions during the postprandial phase could be expected to differ depending on the choice of carbohydrate food and time point in the postprandial phase. The purpose of the present thesis was to evaluate the importance of GI features and/or contents of indigestible carbohydrates of cereal products on glycaemic regulation, metabolic risk markers, and cognitive function in healthy subjects. The test products were either evaluated in the acute postprandial phase, or following one or two subsequent standardised meals. The results of the present thesis show that certain cereal products, with low GI and/or with a specific mixture of resistant starch (RS) and dietary fibre (DF) have the capacity to reduce glucose response not only in the acute phase, but also the incremental blood glucose responses following two consecutive meals during the course of a whole day, and in the perspective from a late evening meal to a subsequent breakfast. Boiled barley- and rye kernels, or a white wheat bread (WWB) added with RS and barley DF in an amount similar to that in barley kernels, possessed such properties. It was concluded that the GI features of the cereal based breakfast was the major determinant of glycaemia at the subsequent standardised lunch. Instead, improved glycaemic regulation within a 10.5 ? 12.5 h perspective post the cereal test meal was associated with increased levels of colonically derived metabolites (e.g. plasma short chain fatty acids (SCFAs)), suggesting the involvement of colonic fermentation of indigestible carbohydrates present in the test meal. A lowering of circulating FFAs was identified as a major factor for the improvements seen in glucose tolerance in the perspective from an evening test meal to breakfast. In addition, serum IL-6 was reduced and plasma adiponectin increased at breakfast following a barley kernel based evening meal, compared with an evening with white wheat bread (WWB). The barley kernel evening meal thus improved glucose tolerance and improved markers of inflammation and insulin sensitivity. Following the evening test meals, markers of colonic fermentation (breath H2) at breakfast were negatively correlated to the gastric emptying rate (GER) and positively correlated to satiety. A negative correlation was seen between GER and satiety post the breakfast. Further, the concentrations of SCFAs and GLP-1 were negatively correlated to the blood glucose response at breakfast. It is suggested that cereal products which promote colonic fermentation and GLP-1 release improve glucose tolerance and satiety in an overnight perspective. The findings in the thesis provide evidence for a link between the gut microbial metabolism and key factors associated with improved insulin sensitivity. Further, the data obtained indicates that impaired cognitive function may be an early manifestation of the metabolic syndrome, since a significant decline in cognitive performance was displayed in middle-aged subjects with lower glucose tolerance, albeit still within a normal range. In addition, it was shown than a breakfast with low-GI features i.e. avoidance of a rapid decline in glycaemia (at 90 min), and maintenance of a net blood glucose increment in the late postprandial phase (at 170 min), was associated with improvements in measures of cognitive function (working memory and selective attention, respectively) at these time points. The results in the thesis provide information to be used for tailoring of low-GI whole grain products, which facilitate glycaemic regulation and related parameters over the course of several meals, with beneficial implications on metabolic risk factors, weight control, and cognitive function.
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