Soon after birth, the human body is colonised by bacteria. Trillions of bacteria take up residence in the gut and perform a range of useful functions such as helping with digestion and absorption of nutrients, producing vitamins, preventing growth of pathogenic bacteria, and developing the immune system. In 2006, it was shown that the proportion of Bacteroidetes relative to Firmicutes was reduced in the guts of obese people compared with lean individuals and also in the guts of genetically obese mice compared with lean littermates. Researchers at Emory University have now shown that mice engineered to lack toll-like receptor 5 (TLR5) – a component of the innate immune system that is expressed in the gut mucosa and that helps defend against infection – are 20% heavier than normal mice and have elevated triglycerides, cholesterol and blood pressure as well as slightly elevated blood sugar and a decreased response to insulin. TLR5-deficient mice consume about 10% more food than wild type mice and, although they lose weight when food is restricted, they still show insulin resistance. On a high fat diet, TLR5-deficient mice gain more weight than normal mice and develop full-blown diabetes and fatty liver disease, mimicking “metabolic syndrome” which increases the risk of developing heart disease and diabetes in humans.
Treating TLR5-deficient mice with antibiotics to kill most of the bacteria in the intestine reduced their metabolic abnormalities and, conversely, transfer of intestinal bacteria from TLR5-deficient mice to germ-free wild type mice transferred many of the characteristics of metabolic syndrome, including increased appetite, obesity, elevated blood sugar, and insulin resistance. Although earlier studies had shown that greater numbers of Firmicutes bacteria lead to more calories being extracted from the diet, the TLR5-deficient mice had normal proportions of Firmicutes and Bacteroidetes but differed in the composition of bacterial species in the two families. The new study shows that, as well as influencing how well energy is absorbed from food, gut flora can also influence appetite and may contribute to human obesity and metabolic disease.
The study is published in Science Express.