Ruth E. Ley …
Summary Large-scale alterations of the gut microbiota and its microbiome (gene content) are associated with obesity and are responsive to weight loss. Gut microbes can impact host metabolism via signaling pathways in the gut, with effects on inflammation, insulin resistance, and deposition of energy in fat stores. Restoration of the gut microbiota to a healthy state may ameliorate the conditions associated with obesity and help maintain a healthy weight.
Introduction
Up until the last few decades, obesity has been a rare physiological state. Now however, the number of obese or overweight humans has come to outnumber those suffering from malnutrition.[1] This is an unprecedented state for our species, resulting from a mismatch between our evolutionary biology and our modern environment. The human body is a complex system, made all the more complex through its interactions with the trillions of microorganisms that coat the body surface and densely populate the gut. Recent work has shown that the microbes of the gut may play a role in human metabolism and adiposity. Because they are environmentally acquired, microbes constitute one part of our environment that may contribute to the obese state. This review discusses the most recent findings and insights into the relationship between the human microbiota, obesity, and obesity-associated diseases. …
shotgun sequencing of the microbial community DNA, showed an enrichment in genes involved in energy extraction from food in the obese host's microbiome relative to that of the lean host's microbiome. A microbiota with greater energy extraction efficiency resulted in less energy left over in feces and greater levels of short-chain fatty acids (SCFAs) in the cecum. Furthermore, when the luminal contents from the ceca of obese or lean mice were provided to lean germ-free recipients, the mice receiving the microbes from the obese donors gained more weight over a 2-week period than recipients of the lean microbes, despite equivalent food intake.[3] In a study extending these observations to humans, 12 obese participants were randomly assigned to either carbohydrate-restricted or fat-restricted diets, and on average, the proportion of Bacteroidetes bacteria enumerated via 16S rRNA gene sequencing increased over time, mirroring reductions in host weight but not changes in diet.[4] Together these studies showed that the gut microbiota was generally altered in the obese host and could contribute to host adiposity in humans and mice.
Metagenomics and Obesity
A subsequent and much larger study of the microbiome associated with obesity conducted with humans also showed that obesity was associated with a depletion of Bacteroidetes, together with an enrichment in carbohydrate and lipid-utilizing genes in the microbiome as a whole. Turnbaugh et al.[5••] focused on twins to assess the gut microbiota's relationship to host weight. The fecal microbial communities of young adult female monozygotic (n = 31) and dizygotic (n = 23) twin pairs concordant for either leanness or obesity were compared, along with those of their mothers (n = 46), using a combination of traditional 16S rRNA gene clone libraries and high-throughput metagenomic analyses of the microbiome. Fecal samples were obtained from the majority of participants at an initial time point and then again 2 months later. Comparisons between all 154 participants showed obesity to be associated with reduced bacterial diversity and reduced representation of the Bacteroidetes. Furthermore, the microbiome differed between obese and lean hosts in much the same way it had in the obese mouse model, with obese host microbiomes enriched in gene categories involved in carbohydrate and lipid metabolism. …
Conclusion
An increasing number of studies relate imbalances in the composition of the gut microbiota to obesity and its associated diseases. The approaches used to characterize gut microbiotas vary widely, which might explain in part why the specific alterations in the microbiota associated with excess body fat, or weight loss, can also vary between studies. To compare studies will require some standardization of approaches or use of a variety of approaches within single studies. However, irrespective of the specific changes observed in microbial communities with respect to body fat, evidence suggests that microbes do indeed respond and contribute to host energy balance. They may do this using a number of different and possibly interactive signaling mechanisms involving innate immune responses, endocrine cells, and epithelial cells. An integration of mechanistically based investigations and microbial ecology studies using high-throughput sequencing will provide insights into how best to reshape host-microbial interactions to promote health. http://www.medscape.com/viewarticle/714569
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