METABOLIC PROPERTIES OF RYE PRODUCTS Focusing on insulinaemia, glycaemic profile and appetite regulation in healthy subjects

Abstract: The prevalence of metabolic disorders, such as type 2 diabetes, cardiovascular diseases and the insulin resistance syndrome (IRS) are increasing worldwide. However, disturbances in the metabolic status can be prevented by changing the daily diet towards more whole grains, vegetables, legumes and dairy products. Also the dietary glycaemic- and insulinaemic indices of foods may play a role. Rye products are interesting in this context as they are usually consumed in wholegrain form and have been demonstrated to induce low insulin responses, with or without a simultaneous lowering of the glycaemic index (GI). The objective of this thesis was to evaluate the possible cause of low postprandial insulin response to rye, and to elucidate potential effects of processing condition, extraction rate and rye variety. Insulin response as well as glycaemic response and course of glycaemia were evaluated in the postprandial phase. Additionally a marker of colonic fermentation was analysed in the postprandial phase (breath hydrogen) and appetite regulating properties were investigated using subjective ratings, analysis of plasma ghrelin, and quantification of voluntary food intake at a subsequent meal. Wholegrain rye products, ingested as boiled kernels and breads as well as endosperm rye (sifted rye) bread and porridge, induced low insulin responses and also a well regulated course of glycaemia, noted as blood glucose curves with lower incremental peaks, remaining above fasting for a longer time. However, some rye varieties were devoid of benefits on course of glycaemia and insulin economy. Two measurements of the course of glycaemia were introduced, the GP and GP2, defined as the duration for incremental postprandial glycaemic response divided by the glucose incremental peak or squared glucose incremental peak, respectively. The GP and GP2 of the products were correlated to the insulin response, as well as to late subjective satiety, suggesting that they are good predictors of postprandial events. Suggested mechanism for the lowered glycaemic and insulinaemic responses were a high content of viscous fibres, bioactive components, e.g. phenolic acids and a dense food structure, contributing to a lowered digestion and uptake of carbohydrates in the small intestines. Furthermore, rye products induced early colonic fermentation, already in the postprandial phase, possibly explained by the presence of arabinoxylans, fructans, and other dietary fibre compounds of low molecular weight. The increase in colonic fermentation, measured as increase in breath H2, correlated with lower late postprandial concentration of FFA and the GP and GP2 of the products, suggesting increased glucose tolerance already in the postprandial phase after rye products. Rye products, in particular boiled rye kernels induced high postprandial subjective satiety and promoted satiety also at a subsequent voluntary meal. The rye kernel breakfast lowered the voluntary energy intake at a second meal with 16%. The mechanism behind this satiating effect of rye was suggested to be the high content of dietary fibres (DF) and high water content introducing a bulking effect. Also, high content of viscous and fermentable DF can lower gastric emptying rate. The low postprandial glycaemia and insulinaemia seen with several rye products appears to contribute to a lowered rebound of the hunger peptide ghrelin prior to the second meal. Rye products made from whole kernels and wholegrain- and endosperm rye bread induced lower postprandial insulinaemia and glycaemia than wholegrain- and endosperm rye porridges, while porridges and whole kernels induced higher subjective satiety. Boiled rye kernels also supressed the desire to eat to a greater extent than boiled wheat kernels in the later postprandial phase (tAUC 210–270 min). Several components are suggested to contribute to the observed lowered insulinaemia, well regulated glycaemia and improved satiating effects of certain rye products.