Obesity has become a global epidemi

Obesity has become a global epidemic and is a major risk factor for type 2 diabetes, cardiovascular diseases and certain cancers. Evidence has accumulated that the gut microbiota is an important environmental factor contributing to obesity by altering host energy harvest and storage1,2. Gut microbiota transplantation experiments suggest a causal relationship between the gut microbiota and obesity development3, and human studies have associated gut microbial dysbiosis with obesity4,5 and metabolic disorders such as type 2 diabetes6,7. In general, obese individuals show decreased bacterial diversity5 and gene richness8,9. It was originally reported that obesity was associated with a lower ratio of Bacteroidetes to Firmicutes4, and it has been reported that this ratio increases after weight loss induced by dietary intervention4 or Roux-en-Y gastric bypass10,11. Nevertheless, this view has been challenged by subsequent studies that have provided conflicting findings12,13.