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Gut Microbes from People Can Transmit Obese or Lean Body Types to Mice

A study in mice has found that gut microbes obtained from obese or lean people, within certain dietary contexts, can transmit obesity or leanness to mice in the lab. While there is ample evidence that genetics and other factors play an important role in the development of obesity, there is also evidence that the community of bacteria living in the gut and their collective bacterial genomes, or “gut microbiome,” may affect and reflect a person’s health and nutritional status. For example, recent research raised the possibility that differences in the gut microbiome may explain why twins with identical genetic makeups can have very different disease and nutritional states.

A group of researchers were interested in exploring this idea using sets of identical twins who were “discordant” for obesity, which means one twin was obese while the other was not. Previous research with such discordant twins has shown that obesity is associated with changes in the types of bacteria in the gut; however, it is unclear whether these changes in the gut microbiome actually contribute to the development of obesity. To examine this possibility, scientists transferred the gut microbiomes from twins discordant for obesity into mice previously raised in sterile conditions and initially free of any gut microbes. Even though all mice were fed the same diet, only the mice that received the obese twin’s microbiome gained weight, while the mice that received gut microbes from the lean twin did not.

Knowing that mice often share gut microbes with their cage mates, the scientists housed the “lean” mice—those inhabited by the lean human twin’s gut microbes—together with the now “obese” mice— those with the obese twin’s gut microbes—to see if the microbes from one set of mice would spread to, and affect, the other set of mice. Under these conditions, the weights of the lean mice did not change, but after several days the obese mice lost a significant amount of weight and began to harbor the same types of bacteria that were in the lean mice. This finding suggested that the microbiome from the lean mice, along with its lean-promoting effects, was being transferred to the obese mice, but not vice versa.

When the scientists compared the microbiome from the lean mice to that of the obese mice, they found differences in genes that regulate metabolism, including the metabolism of certain amino acids (components of proteins) and effects on fats and starches, suggesting that metabolic changes are responsible for the microbiome’s effects on weight. However, the protective effects of the lean twin’s microbiome were only seen when the mice were fed a healthy diet with high amounts of fruits and vegetables and low amounts of saturated fat, meaning that changes in weight were not dependent on the microbiome alone, but were also dependent on diet.

These studies provide convincing evidence that the gut microbiome, in conjunction with diet, can strongly affect the ability to gain or lose weight in mice, and may lead to insights into the role of the gut microbiome in regulating weight in humans.