Of microbes and meals: the health consequences of dietary endotoxemia

Abstract

The human intestinal tract comprises a rich and complex microbial ecosystem. This intestinal microbota provides a large reservoir of potentially toxic molecules, including bacterial endotoxin (ie, lipopolysaccharide [LPS]). This potent inflammatory molecule is detectable in the circulation of healthy individuals, and levels transiently increase following ingestion of energy-rich meals. Chronic exposure to circulating endotoxin has been associated with obesity, diabetes, and cardiovascular disease. Western-style meals augment LPS translocation and by this mechanism may contribute to the pathogenesis of these diseases. By contrast, the gut and other organs have evolved mechanisms to detoxify endotoxin and neutralize the potentially inflammatory qualities of circulating endotoxin. Of specific interest to clinicians is evidence that acute postprandial elevation of circulating endotoxin is dependent on meal composition. In this review, the authors present an overview of the biochemical and cellular mechanisms that lead to endotoxemia, with emphasis on the interplay between microbial and nutrition determinants of this condition. The link between endotoxemia, diet, and changes in the intestinal microbiota raise the possibility that dietary interventions can, at least in part, ameliorate the detrimental outcomes of endotoxemia.

Figure 1

Transcellular and paracellular transport represent non-exclusive pathways for LPS movement from the enteric lumen into circulation. In the transcellular model, lipid absorption serves as a vehicle for LPS, which is included in micelles and later incorporated into chylomicrons through interaction with LPS-binding protein (LBP). In the paracellular model, fat-rich chyme results in internalization of tight junction proteins by mechanisms that remain unclear. The impaired epithelial barrier then permits LPS to pass between epithelial cells.

Figure 2

Intestinal alkalkine phosphatase (ALPI) is an important enzyme in LPS detoxification. This highly expressed brush border enzyme is secreted into the lumen and co-localizes with intracellular lipid, maximizing contact with LPS. Under physiologic conditions, this enzyme converts the “toxic” LPS moiety (diphosphoryl lipid-A) to a less inflammatory form (monophosphoryl lipid-A). Of clinical importance, expression of ALPI decreases during fasting, but is maintained by enteral nutrition.