Mitochondria and Metabolism Journal Club – Presented by Julia Ritterhoff, PhD – February 2016:
The Gut Microbiota Modulates Energy Metabolism in the Hibernating Brown Bear Ursus arctos. By Sommer et al. Cell Reports, 2016 Feb 23
Brown bears and energy metabolism: A model for healthy obesity?
Hibernation. Hibernation is an adaptation that helps many animals to conserve energy during food shortage in winter. Brown bears hibernate for 5-7 month each year. In order to prepare for the hibernation, brown bears can gain 180 kg (which is about 400 lb), during a period of hyperphagia, before going into hibernation. This transition is also called healthy obesity, since brown bears remain metabolically healthy, which contrasts with the strong association between obesity and insulin resistance in humans.
Are we vastly outnumbered? This study by Sommer et al. goes in line with recent publications about the emerging role of gut microbiota on orchestrating energy homeostasis. Recently, it has been revised that the number of gut microbiota in the human body equals the number of endogenous cells (which is about 3*10^13 cells) [Sender et al., bioRxiv, 2016]. Earlier studies often presented that bacteria outnumber human cells by at least 10:1 as common knowledge [Luckey et al., Am J Clin Nutr, 1972].
Summer versus winter microbiota. Sommer et al. identified 24 different bacterial phyla in the bear fecal microbiota, which showed clear separation depending on the seasonal origin. The dominant bacterial phyla in the summer microbiota were Proteobacteria, Firmicutes and Actionobacteria. In the winter microbiota, Bacteroidetes increased in abundance. An enrichment of Bacteroidetes and a lower abundance of Firmicutes in the gut microbiota of hibernating animals have been reported in other studies as well. Similarly, an increase in Bacteroidetes most likely reflects a switch of the metabolism toward degradation of host glycans in the absence of dietary polysaccharides. Likewise, Firmicutes taxa require dietary fibers. Thus, changes in the gut microbiota reflect the nutritional status of active and hibernating brown bears.
Metabolic reprogramming of mice by seasonal bear microbiota. Finally, Sommer et al. colonized germ-free mice with summer or winter microbiota. Mice colonized with summer bear microbiota demonstrated increased weight/fat gain than mice colonized with winter microbiota. However, mice colonized with summer bear microbiota showed an improvement in glucose metabolism compared to mice colonized with winter microbiota, just as brown bears in the summer. (Indeed, brown bears become insulin resistant and hyperglycemic during hibernation.)
We are what we eat. The interplay between gut microbiota and energy metabolism has long been overlooked. Sommer et al. demonstrate that the seasonal lifestyle of brown bears with phases of severe hyperphagia and prolonged fasting is accompanied by distinct changes in the gut microbiota. This phenotype can even be transferred into other species. Thus, the microbiota does not only link to the healthy/ obese phenotype in brown bears but further supports targeting the gut microbiota as potential treatment for obesity in humans. However, long-term effects of microbiota transplantation (which, of note, are already performed for the treatment of recurrent Clostridium difficile infections) are unknown.