Sunday, 30 March 2014

Gut microbiota manipulates fatty acid uptake in fish

It is clear that the regulation of intestinal fat absorption is critical to the energy balance of any animal. It is also clear from previous research that the intestinal microbiota of an animal impacts its energy balance. However, what is less clear is the role of the gut microbiota in metabolism of dietary fat. Understanding this link may have advantageous implications for fish health and nutrition and for aquaculture. Understanding the link between microbiota and fat absorption may also have implications for human health, in particular obesity and malnutrition.
In humans, dietary lipids supply 40-55% of the energy requirements in the typical western diet. For vertebrates in general, dietary fats come in the form of triglycerides. These are broken down into monoglycerides by lipases in the GI tract and absorbed by enterocytes in the intestinal epithelium. They are temporarily stored in the epithelium as Lipid Droplets before progressing to other tissues such as the liver.
The majority of previous work linking gut microbiota and metabolism focuses on microbial fermentation of otherwise indigestible carbohydrates and lipids that allows the (usually mammalian) host to make use of alternative energy sources, such as my post last term on microbe-assisted seaweed digestion. However, this study uses zebrafish to investigate how microbiota regulates lipid absorption. Specifically, they used fluorescently labelled fatty acids to view how epithelial Lipid Droplet abundance differed between zebrafish with different known microbiota, known as ‘gnotobiotic’ zebrafish.
Two fluorescent fatty acids were used, quantification of epithelial fluorescence showed that both fatty acids were absorbed more successfully in zebrafish containing a normal zebrafish microbiota compared to zebrafish that were raised ‘germ-free’.  Higher lumenal fluorescence in germ-free fish suggests that the microbiota plays a crucial role in absorption. Lipid Droplet number and size was also shown to increase in zebrafish with a conventional microbiota. However, these increases may not be due to increase absorption, but decreased ability to subsequently shift fatty acids from the epithelium to other tissues, such as the liver. They tested this and showed that the increase in epithelial Lipid Droplet size and number was not due to impaired intestinal lipid export, nor was it associated with ingestion rates, digestive organ size or differences in digestive enzyme activity. They isolated microbiota as the one variable responsible for increased fatty acid absorption and also increased export to other tissues.

This study differs from previous studies in that it uses a method that distinguishes between dietary and microbial produced fatty acids. They can therefore say with certainty that microbiota composition affects the absorption of dietary lipids, regardless of the fatty acids produced by the microbiota. The mechanistic understanding of this interaction remains incomplete, this study has only shown that it does happen, not how it happens. The implications of this study could potentially be applied to aquaculture, manipulation of fish gut microbiota via probiotics may allow increased efficiency of fatty acid uptake, producing fish that are more nutritionally valuable for human consumption. If applied to humans, greater understanding of our own microbiota could lead to probiotics that manipulate fatty acid uptake for treatment of disorders such as obesity and malnutrition.

Semova, I., Carten, J. D., Stombaugh, J., Mackey, L. C., Knight, R., Farber, S. A., & Rawls, J. F. (2012). Microbiota regulate intestinal absorption and metabolism of fatty acids in the zebrafish. Cell host & microbe12(3), 277-288.

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