Metabolic cross talk between the colon and the periphery: implications for insulin sensitivity

Abstract

Until recently, a glance at a standard undergraduate textbook would have given the impression that the colon was merely a storage organ for faeces and maybe something about the absorption of electrolytes and water. In reality, the colon is a highly-metabolically-active organ, the function of which has implications not only for the remainder of the digestive tract, but also for peripheral organs such as adipose tissue (AT), liver and skeletal muscle. The present review focuses on two distinct but complementary areas: (1) the metabolic adaptation that occurs following surgical removal of colonic tissue; (2) the effect of modulating the colonin situin terms of postprandial metabolism, insulin sensitivity and disease risk. Work in these two areas points to the colon being important in modulating normal tissue insulin sensitivity. The role of fatty acids is central to the insulin sensitivity hypothesis. AT acts as a daily ‘buffer’ for fatty acids. However, following colonic resection there is an apparent change in AT function. There is an increase in the AT lipolysis rate, resulting in the release of excess fatty acids into the circulation and consequently the take up of excess fatty acids into skeletal muscle. This resultant increase in either storage of lipid or its oxidation would result in a reduction in insulin sensitivity. The insulin-sensitising effects of high-fibre diets are also related to changes in AT function and fatty acid metabolism, but manipulating colonic tissuein situallows the mechanisms to be elucidated. This research area is an exciting one, involving the potential role of SCFA (the absorbed by-products of colonic bacterial fermentation) acting directly on peripheral tissues, following the recent identification of G-protein-coupled receptors specific for these ligands.

Microbial Degradation of Whole-Grain Complex Carbohydrates and Impact on Short-Chain Fatty Acids and Health

“…Fructans consist of 1 glucose unit linked to a varying number of fructose units (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20) present predominantly in the endosperm and germ tissues (19). Both branched and unbranched chains occur.…”

“…Most SCFAs are absorbed by the colonic epithelium or metabolized by other colonic bacteria, resulting in only 5-10% of SCFAs being excreted in feces (8). SCFAs play an important role in gut and potentially metabolic health (9,12). The main purpose of the present review is to discuss current knowledge concerning the microbial degradation of complex carbohydrates from cereals and influence on SCFA production and impact on health.…”

Factors related to colonic fermentation of nondigestible carbohydrates of a previous evening meal increase tissue glucose uptake and moderate glucose-associated inflammation

“…The molar percentage enrichment of [6,6-2 H 2 ]glucose and the 13 C atom% were calculated as previously described (17) and smoothed by using the Optimized Optimal Segments (OOPSEG) program developed by Bradley et al (20). The rate of appearance (Ra) of total glucose in plasma (RaT) from exogenous (meal) and endogenous (hepatic) sources was calculated by using the nonsteady state equation of Steele et al (21) as modified by De Bodo et al (22).…”

“…Several hypotheses as to how SCFAs mediate the glucose-lowering effect have been postulated. SCFAs may delay gastric emptying (12), have insulin-like properties (8), increase insulin sensitivity by decreasing free fatty acid (FFA) concentrations (13), have antiinflammatory effects (14), or promote insulin-independent glucose sparing.…”

A Cereal-Based Evening Meal Rich in Indigestible Carbohydrates Increases Plasma Butyrate the Next Morning ,

“…Although a substantial part of the SCFA formed is extracted during passage of the liver (19), a fraction may enter the systemic circulation and can be determined in the peripheral blood (20). It has been suggested that certain SCFA may be involved in the modulation of glucose metabolism (14,16,21) and lipid metabolism (22) and it can be hypothesized that this may in fact be one possible mechanism whereby whole grain foods exert beneficial metabolic effects as seen in observational studies. However, whole grain foods are, in addition to being rich in DF, also rich in other potentially bioactive phytochemicals such as minerals, plant stanols and sterols, phenolic antioxidants, and vitamins (10,23).…”