December 05th 2018
Slowing starch digestibility in foods: formulation, substantiation and metabolic effects related to health.
Worldwide, the number of people with metabolic diseases such as diabetes mellitus type 2 (T2DM) is rapidly increasing, especially in South Asia. Development of T2DM is influenced by lifestyle-related factors such as diet and physical activity. Sustained exposure to higher post-prandial glucose (PPG) and insulin (PPI) responses increases the risk of development of (pre-)diabetes. Data from studies with drugs inhibiting alpha-glucosidase (an enzyme involved in starch digestion) and low glycaemic index/glycaemic load diets have shown that reducing PPG levels in blood by reducing the amount of absorbed carbohydrates or slowing the rates of carbohydrate digestion reduces the risk of T2DM.
However, it is not yet clear whether a reduced (total) PPG itself directly contributes to health benefits or whether related effects of changing carbohydrate digestion and/or changed metabolism play a major role. This thesis has investigated how starchy foods can be made more slowly digestible, how this is substantiated, what is their impact on PPG and PPI as well as on the associated metabolic pathways. The hypothesis is that slowly-digestible carbohydrates cause further post-absorptive effects, which manifest themselves as changes in postprandial glucose flux parameters and levels of gastrointestinal and pancreatic hormones, as well as glucose metabolites. Carbohydrate-rich staple foods are key candidates for reducing PPG and PPI exposures, because of their frequent use. Wheat-based flatbreads and rice are two of the most common carbohydrate-rich staple foods in Southeast Asia, making them important contributors to the daily glycaemic load. The first part of this thesis is dedicated to the question of which characteristics of starchy foods contribute to PPG (and PPI) and to what extent. Storage of starch in grains occurs mainly in the form of amylose and amylopectin. A systematic review of rice studies showed that rice types with a higher amylose percentage give a lower PPG response. Post-harvest processing (such as parboiling) and consumer processing (such as cooling/reheating) further contribute to a lower PPG and PPI response after consumption due to their effects on gelatinization and retrogradation. Gelatinization is the hydration and swelling of starch granules, leading to a higher starch availability to human digestive enzymes, while retrogradation is the recrystallisation of starch resulting in a more resistant type of starch.
Many food hydrocolloids, a group of long chain polymers including many dietary fibres, are able to lower blood glucose response due to their viscous or gelling nature under gastrointestinal conditions. This can delay gastric emptying and inhibit the propulsive and mixing effects in the intestine, leading to a slower digestion and a greater release of incretin hormones. In addition, these hydrocolloids may also act by direct digestive enzyme inhibition in the gastrointestinal tract. In the food product hydrocolloids can
coat the starch granules resulting in a decrease in swelling and gelatinization of starch and the formation of a physical barrier to alpha-amylase. These insights for slowing the rate of starch digestion by food hydrocolloids were applied in consumer-relevant food products, which were subsequently tested for effects on PPG and PPI in Caucasian and Indian subjects. Southeast Asian flatbreads are usually prepared at home from a commercially-made whole-wheat flour mix (“atta”). We showed that inclusion of specific viscous fibres (guar gum and konjac mannan) produced a significantly lower PPG and PPI. Legume flours turned out to be especially valuable in lowering PPG when combined with low amounts of guar gum in flatbreads. Several studies underpin the conclusion that it is possible to modulate the PPG response to staple foods, and that this is influenced by factors affecting carbohydrate digestibility. Therefore, it might be useful for the food industry to have a reliable in vitro model to predict the rate of digestion of existing and newly developed staple foods. The method to assess this was via an in vitro digestion assay based on the widelyused Englyst method, extended by an oral digestion step, and optimization of the pH and the amount of digestive enzymes. The impact of gastrointestinal hormones, gastric emptying and metabolic feedback mechanisms are not taken into account in these kinds of models. This in vitro digestion model was validated against in vivo PPG measurements in flatbreads. A regression model with four in vitro variables (rate of starch digestion, AUC of glucose release over 120 min, the carbohydrate level and % rapidly digestible starch, as independent variables), was highly predictive of in vivo plasma glucose responses. In vivo, PPG is not just a reflection of starch digestibility, but determined by different underlying glucose fluxes: rate of appearance of exogenous glucose (RaE), endogenous glucose production (EGP) and rate of disposal of total glucose (RdT; the uptake of all glucose by tissues). Therefore, the “golden standard” to measure the rate of influx of glucose from the intestine (i.e. confirm slower digestion) is the dual stable isotope technique, in which 13C-labelled starch in combination with a deuterium glucose infusion is used to differentiate between the different glucose fluxes. In a study using 13C-labelled starch in flat breads with guar gum (2 and 4%) and chickpea flour (15%), these formulations slightly reduced the RaE, but more substantially affected RdT, as well as EGP compared to a flatbread without these additions. From a systematic review of studies comparing RaE of different carbohydrates via 13Clabelled carbohydrates we conclude that changing the RaE by diet is associated with substantial changes in PPG and PPI but also RdT. To get further insight to these postabsorptive effects metabolomics analyses of the 13C-glucose metabolites of the 13Clabelled starch was performed. Guar gum dose-dependently delayed the formation of the glycolysis-derived metabolites lactate and alanine.
In conclusion, we found that modifying rates of carbohydrate digestion, e.g. by the addition of fibres and chickpea flour, results in effects beyond just PPG lowering, including changes in the underlying glucose fluxes, hormonal levels and metabolites. Our findings can be applied to dietary approaches to achieve a more desired cascade of metabolic effects, which can contribute toward a more favourable metabolic profile and reduced risk of T2DM. Consumers also can prepare healthier carbohydrate-rich products (e.g. pasta or rice) at home by introducing shorter cooking times and cooling/reheating cycles.