Why can some people lose weight easily, while others just seem to gain weight no matter what they do? The standard explanation has been genetic differences in metabolism that affect how you store and use the energy in the food you eat.
But genetics doesn’t explain differences in weight between identical twins. One hypothesis is that in identical twins, differences in the ability to gain or lose weight may be due to different populations of gut bacteria. We know that the human body is home to trillions of bacteria, many of them in the gut. Gut bacteria are involved in metabolizing what you eat, largely for their own purposes of course, but possibly also affecting your metabolism.
In a recent experiment designed to find out whether gut bacteria might be affecting metabolism enough to alter a person’s overall body weight, scientists transplanted gut bacteria from pairs of twins “discordant for obesity” (one heavy and one not) into germ-free mice. They found that gut bacteria from the obese twin caused the mice to gain more weight than gut bacteria from the lean twin.
The obvious conclusion is that the type of bacteria that inhabit your gut might indeed have an effect on a your body weight. The next step would be to find out which gut bacteria might cause people to gain too much weight or to stay lean, and then to culture the lean ones or learn how to inhibit the weight-gaining ones. Perhaps someday, cultures of specific gut bacteria will become components of diet pills. But don’t hold your breath - it could take decades to tease out which bacteria would work the best.
Call me crazy, but my guess is that some unscrupulous diet clinics may seize on this research and begin to promote an unproven "therapy" that involves transferring cultures of gut bacteria from lean people into obese ones, with the promise that it'll cure obesity. I hope I'm wrong.