The world is dominated by microorganisms such as bacteria and yeast. In our own bodies, for example, microbial cells outnumber human cells by about 10 to 1. While we often view microorganisms as the cause of illness and disease, scientists have discovered that most are not harmful and many are beneficial. An understanding of how these ‘friendly’ microscopic guests contribute to human health should provide exciting new platforms for disease therapy.
Jeffrey I. Gordon, MD and colleagues at Washington University in St. Louis study the physiological benefits of the microbiota--the term given to the collection of trillions of microorganisms living in the gut. Most recently, as reported in two papers published in the December 21 2006 issue of Nature, Gordon and co-workers established a link between the microbiota and obesity. They propose that obesity may be, at least in part, related to the ability of the microbiota of obese individuals to extract more calories from food.
The group first demonstrated that the microbiota of obese mice differed significantly from that of their lean counterparts. In lean mice, bacteria belonging to the Bacteroidetes division make up approximately 40% of the bacterial community. In obese mice, about half of these bacteria were selectively lost (Ley et al. 2005 PNAS 102). More importantly, a microbiota from an obese mouse transplanted into a lean recipient was sufficient to cause a significant increase in total body fat, due to enhanced calorie harvest by the transplanted microbiota (Turnbaugh et al. 2006 Nature 444).
The researchers also tested whether these findings in the mouse could be extended to human obesity. As with the mouse, a study of 12 obese human volunteers revealed that the relative number of Bacteroidetes was significantly decreased when compared to lean volunteers. In the same study, obese volunteers who ate a calorie-restricted diet showed a marked recovery of Bacteriodetes that was dependent on weight loss (Ley et al. 2006 Nature 444).
These studies indicate that there may be a microbial component to obesity, and if so, probiotics (the use of bacteria to promote health) may be a future approach to weight loss. However, there remain several key issues to be resolved before such therapeutic potential is explored. For example, in humans, it is unclear whether the bacterial composition of the gut determines, or is the determined by, body weight. Furthermore, it is difficult to predict whether the modest changes in caloric extraction between obese and lean people will translate into meaningful weight loss. Nonetheless, these studies do suggest that factors other than diet, exercise and genetics may play a role in the growing epidemic of obesity.
