Servicio de Información Comunitario sobre Investigación y Desarrollo - CORDIS

Knowledge about the digestive characteristics and postprandial glucose kinetics of whole-meal wheat bread

Starchy foods differ considerable in their physiological/metabolic response and consequently also in their potential health benefits. Several studies have shown that diets that contain large amounts of slowly digestible starch - which elicits relatively low postprandial blood glucose and insulin responses - may protect against chronic disease like diabetes mellitus type 2 and cardiovascular disease. To be able to distinguish starchy foods according to their hyperglycemic potential the glycaemic index (GI) is used. Also, in vitro techniques are available that aim to characterize carbohydrate digestion in the gut. With these techniques carbohydrates in food are divided in sugars and starch fractions and are grouped into rapidly available glucose (RAG), slowly available glucose (SAG) and resistant starch.

It is generally assumed that the postprandial blood glucose response to starchy foods is mainly determined by the rate of digestion of starch - thus products with slow digestible starch features are considered to be beneficial. However postprandial blood glucose is the net result of various postabsorptive processes and thus - besides absorption of starch derived glucose - also endogenous glucose production and glucose disposal in tissue determine blood glucose response. The contribution of these processes to postprandial blood glucose has so far received little attention in nutritional studies.

In many societies bread is the main source of starch. But bread made from refined flour and even whole-meal bread - if made from finely ground whole grains - has been shown to have rapidly digestible starch features. The content of RAG as measured in vitro is high (90 % of total starch) and so is the GI (71 2 mean of 13 studies; glucose = 100) - which implies that it is a less desirable food product. Yet no information is available about the digestive starch characteristics of this important diet component in vivo. Thus, the aim of this study was to characterize starch digestion of wholemeal wheat bread and the underlying glucose kinetics which determine postprandial glucose response in healthy volunteers.

In a crossover study 4 healthy men ingested either 13C-enriched wholemeal wheat bread (WB) (133 g) or glucose (55 g) in water. Plasma glucose and insulin concentrations and 13CO2 excretion in breath were monitored during 6 h postprandially. Using a primed continuous infusion of D-[6,6-2H2] glucose, the rate of appearance of glucose was estimated (reflecting glucose influx) and the endogenous glucose production calculated. All test meals were naturally labelled with 13C. Wheat was grown in an atmosphere artificially enriched in 13CO2. Wheat grains were finely milled and whole-meal bread produced.

We found that the glucose influx rate after WB was comparable with that after glucose in the early postprandial phase (0-2 h) and higher in the late postprandial phase (2-4 h). Despite the same initial glucose influx rate the 0-2 h AUC of insulin after WB was 41% lower than after glucose (P = 0.037). Paradoxically endogenous glucose production after WB was significantly more suppressed than after glucose (0-2 h AUC: p = 0.015, 2-4 h AUC: p = 0.028).

Our data show that starch in WB is partly rapidly and partly slowly digestible which could not be derived from in vitro determination. Postprandial insulin response and endogenous glucose production after WB are not solely determined by the digestive characteristics of starch. Other components of WB seem to be involved and need to be identified. Suppression of endogenous glucose production seems to be largely insulin independent after WB consumption. This is an interesting phenomenon which might be especially relevant in the insulin resistant state.

A manuscript is currently prepared to be published in a peer-reviewed international journal.

By use of wheat labelled with the stable isotope 13C, which has been produced during the EUROSTARCH project, it was for the first time possible to investigate in vivo the digestive characteristics and postprandial glucose kinetics of wholemeal wheat bread. The study has shown that prediction of in vivo digestibility with in vitro tests needs further evaluation. The use of stable isotopes also made it possible to observe that not only the digestive characteristics of starch are involved in determining postprandial metabolic responses but also other components of the bread, which need to be identified. If this can be confirmed in other starchy products this will have implications for the classification of food products which is currently focused on starch characteristics and will emphasis the need to monitor more metabolic factors than the postprandial glucose to determine the health effect of a given product.

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Reported by

University Medical Center Groningen
Antonius Deusinglaan 1
9713 AV Groningen
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