A translational investigation into the role of dietary fatty acids in modulating endotoxemia and associated inflammation
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The presence of bacterial endotoxin in the bloodstream, known as endotoxemia, may cause inflammation and play a role in the development of chronic disease. Dietary fatty acid composition has been demonstrated, albeit inconsistently in different animal species, to modulate endotoxemia potentially via altered lipopolysaccharide (LPS) absorption from the intestinal lumen. We first sought to uncover whether postprandial endotoxemia in healthy people is influenced by the consumption of meals rich in saturated or long-chain n(ω)-3 fatty acids, respectively. Considering the physiological similarity of the human and porcine gastrointestinal tracts, the effect on endotoxemia from long-term consumption of high fat diets distinct in fatty acid composition was examined in pigs. Mechanistic investigations were performed in porcine tissue in vitro and ex vivo to understand how dietary fatty acids might differently regulate LPS transport and LPS-induced inflammation at the intestinal epithelium.
Previous studies using human participants have indicated that the consumption of a high-fat meal is associated with increased postprandial endotoxemia and circulating markers of inflammation. However, it was unclear whether this effect was dependent on high or low fat content or the fatty acid composition of the meal. To address this question, healthy adult men and women were recruited for a randomized cross-over study in which participants consumed a low-fat meal, or one of three high-fat meals containing saturated, n-3, or n-6 fatty acids. Serum endotoxin concentration was lower (p<0.05) following the consumption of an n-3 fatty-acid rich meal compared to serum endotoxin concentration after the consumption of a saturated fatty acid-rich meal. Postprandial endotoxemia was not different depending on whether the participant had consumed a low-fat content or high fat content meal. Despite alterations in blood endotoxin, none of the meals were found to impact biomarkers of systemic inflammation. These results suggest the modulatory effect of a high fat meal on blood endotoxin concentration is dependent on the fatty acid composition, but not total fat content of a meal.
Next, we used a porcine jejunum epithelial cell line, IPEC-J2, to investigate in vitro a potential mechanism for common dietary fatty acid mediation of LPS transit across the small intestine epithelium where fatty acids are absorbed in vivo. However, our investigation did not find LPS to transit across the IPEC-J2 polarized monolayer regardless of the presence of saturated, n-3, or n-6 fatty acids. Since chemically-purified LPS does not exist in vivo but is instead found within bacterial membrane fragments or in live bacteria, we also sought to determine whether the inflammatory effect of purified versus in vivo-relevant forms of LPS differently elicit inflammation and if these sources of inflammation are commonly modulated, and via what pathway, by dietary fatty acids. We found purified LPS to stimulate comparatively less (p<0.05) inflammation than native forms of LPS. Moreover, long chain polyunsaturated fatty acids inhibited (p<0.05) LPS-induced production of inflammatory cytokines via a TLR4 pathway, whereas medium chain saturated fatty acids did not alter (p>0.05) normal inflammatory response. These results indicate that purified and native forms of LPS behave differently in in vitro cell culture suggesting mechanistic investigations for in vivo uptake of LPS from the gut lumen should use in vivoÃ Â¬Ã Â¬-relevant forms of LPS.
Little is known about how long-term consumption of high fat diets that differ in dietary fatty acid composition alter endotoxemia. We used a pig model to investigate how daily consumption of a low fat, high fat, or high fat diet with n-3 fatty acids over 9 weeks influences endotoxemia. An effect of the fat composition of the diet on endotoxemia was not found (p>0.05). Pig jejunum segments were harvested from these animals and mounted in Ussing chambers to assess the flux of LPS and live E. coli. It was found that while LPS did not cross the intestinal epithelium under any condition, the transit of live bacteria was influenced by diet. Significantly more (p<0.05) E. coli transited across jejunum from a high fat diet animal than from either a low fat or high fat with n-3 fatty acid animal. This effect was due to the effect of a high fat diet in increasing passage of bacteria is mitigated by consumption of n-3 fatty acids via a TLR4-dependent pathway. These results suggest the acute modulatory effect of a novel high fat meal on postprandial endotoxemia may not persist with repeated consumption of the same meal-type. Moreover, endotoxemia from the gut may be related to passage of Gram negative bacteria but not uptake of LPS.