Delayed gut colonization changes future insulin resistance and hepatic function but not adiposity in high-fat diet induced obese mice

Since the first findings that germ free mice appeared resistant to diet induced obesity, an increasing number of studies have ephasised the role of the gut microbiota as a core regulator of host weight gain and metabolism. However, several studies have not been replicable and conflicting results have left the discussion surrounding causality elusive. Specifically, the importance of early microbial dysbiosis in later development of obesity and metabolic disorders has been a subject of debate.Male Swiss Webster mice raised under germ-free conditions were introduced to a specific-pathogen-free facility for microbial colonization at 1 week and 3 weeks of age. They were compared with control animals with a complete gut microbiota from birth. At 10 weeks of age, all mice were challenged with a high fat diet for 17 weeks. Obesity and insulin resistance parameters such as weight gain, food consumption, oral glucose tolerance test and serum cytokines were measured. Fecal pellets were collected before the high-fat diet challenge for 16S rRNA gene amplicon sequencing of the gut microbiota, and RNAseq was performed on liver samples from matched groups terminated at 7 weeks of age.There were no significant differences in the measured obesity parameters weight, food consumption, epididymal fat pad weight and serum leptin. However, the oral glucose tolerance test and serum insulin analysis showed that the mice that had been germ-free the first 3 weeks of life had increased insulin resistance. At 10 weeks of age, there were no differences in the gut microbiota between the groups, indicating that all mice had a fully colonized gut microbiota when they entered the high-fat diet challenge. The RNAseq analysis demonstrated marked differences in hepatic gene expression between SPF and 3W animals, suggesting a potential predisposition to a higher risk of developing insulin resistance.