Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Availability of 13C-labelled starch sources for the investigation of starch digestion and fermentation

Starch is the most important source of energy. In spite of that, the information about the metabolic quality of starchy foods is scarce. Much of the discussions about the metabolic quality of starchy food is focused on the discussion about the glycaemic response and the from it derived glycaemic index. However, the glycaemic response is the net result of the influx rate of exogenous glucose, the glucose production of the liver and the glucose disposal in the tissue. To be able to investigate in detail physiological and metabolic effects related to various starchy foods it would be advantageous to be able to monitor the underlying glucose kinetics and to relate this to other metabolic effects. For example the influx of glucose over a long period as compared to a high and short influx not only results in lower glycemia but also in differences in insulinemia, suppression of endogenous glucose synthesis and other metabolic parameters which are likely to be relevant for the prevention of disease. Labelling of starchy food with stable isotopes provides the possibility to study postprandial glucose kinetics in more detail.

In the EUROSTARCH project the dual isotope technique was applied which comprises a continuous D-[6,6-2H2]glucose infusion and the ingestion of a starchy meal labelled with the stable isotope 13C. With this technique the rate of appearance of exogenous glucose can be estimated, reflecting the net rate of intestinal glucose absorption and the endogenous glucose synthesis can be calculated. To be able to apply this technique starchy products which are labelled with the stable isotope 13C are thus needed.

Carbohydrates, that are not digested in the small intestine, pass into the colon and are (partly) fermented to form a variety of metabolites. This process was once thought to be waste processing, but nowadays the formation of metabolites during fermentation process and subsequent metabolism in the colonocytes are processes which are linked to the occurrence of diarrhoea, constipation, energy salvage, colon cancer, and immune status. More recently evidence is emerging that colonic fermentation not only affects gut metabolism but can also influence metabolic processes in other tissues and organs. Short- and long-term consumption of starch resistant to small intestinal digestion for example has been shown to beneficially increase insulin sensitivity in healthy subjects.

The underlying mechanism of this phenomenon is not known, but short-chain fatty acids (SCFA�s), like acetate and propionate, which are products of starch fermentation, could be involved. These SCFA�s are metabolized to some extent by the colonic epithelial cells but also enter the portal circulation. Effects of SCFA s on liver metabolism have been reported and recently SCFA s have been identified to be ligands for the orphan G protein-coupled receptor GPR41 which is primarily expressed in adipose tissue. Adipose tissue is known to secrete various signaling peptides - the adipokines - influencing among others insulin sensitivity, feeding behavior and inflammation and could constitute a link between colonic fermentation and peripheral metabolic effects.

However, the scientific evidence of the interrelationship between fermentation products and systemic effects is limited, partly caused by the inaccessibility of the colon. By labelling sources of resistant starch with 13C it is possible to monitor the production of 13C-labelled SCFA�s and relate this to systemic effects or health outcomes.

Since there are only limited starchy food sources which are intrinsically labelled - corn and teff - culturing of grains or tubers under 13CO2 atmosphere is necessary. Furthermore the 13C-enrichment of intrinsically labelled starch sources has been shown to be too low to be able to follow fermentation products in vivo.

During the project 13C-enriched wheat and barley were produced. Durum wheat has been used to prepare wholemeal bread and the glucose kinetics of after ingestion of bread could be monitored for the first time in healthy volunteers. The magnitude of 13C-enrichment necessary for monitoring in vivo 13C-SCFA s has been established. Highly enriched 13C-labelled barley has then been used to follow 13C-acetate production in vivo.

The availability of 13C labelled starch sources opens new avenues for research.
The availability of 13C-wheat allows us to study the digestion of 13C-wheat bread. 13C-barley gives us the possibility to analyse in detail the second meal effect. 13C-lactose allows us to make 13C-lactulose an important substrate for colonic fermentation.

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