By: David Fein, MD
Medical Director

You’re not who you think you are.  In fact, at least 90% of you isn’t you.

Yes, by weight, you are mostly human.  But if we count the number of cells in your body, more than 90% of them are bacteria, not human.

Until recently, we have tended to divide those bacteria into two general categories- those that are “pathogenic” and cause infectious diseases, and those that are generally “harmless” residents of certain parts of the body.  Newer research shows that these “harmless” bacteria actually can exert a tremendous amount of influence over our health, possibly even determining whether one person remains healthy while another person living on the same diet gains weight or even becomes obese .

A study recently published in the Journal of Proteome Research highlighted one mechanism by which the bacteria in our colons may affect how much weight we gain. Researchers compared mice that were germ-free with those who had a normal bacterial population in their colons.  They measured the activity of brown fat, a type of fat that burns calories more quickly than other types of fat.  In humans, brown fat is found in the neck area and in small deposits elsewhere in the body.  The brown fat in the germ-free mice was more active and burned more calories than in the regular mice, suggesting that the bacteria in the regular mice would tend to make them burn less calories and gain weight more easily.

The scientists also noted that the regular male mice were heavier and had more body fat than the female regular mice.  In the bacteria-free mice, this difference disappeared.

Scientists generally have to rely on animal models to try to determine how bacteria interact with their hosts because it’s virtually impossible to reliably alter and control the bacterial population in humans going about their normal daily activities.  Because of the difference between animal species and the bacteria they harbor compared with humans, it is possible that these results do not necessarily reflect how we are affected by our bacteria. But another recent study suggested an additional link to the role bacteria play in our weight.

An experiment was recently reported in the International Society for Microbial Ecology.  Researchers in China isolated a bacteria, Enterobacter cloacae, from the gut of a 385 pound man.  This bacteria accounted for 35% of the microbes in his gut and when he was put on a diet designed to reduce the bacteria he lost 113 lbs.  When this bacteria was given to mice that are genetically resistant to obesity, they gained weight.

Other studies have suggested that gut bacteria may produce substances that affect the hormones that modulate appetite in humans.  Since it is to the advantage of the bacteria to ensure that you continue to provide them with a constant food supply, it would not be surprising if some of them have evolved a mechanism that tricks you body into eating just a bit more to keep the bacteria fat and happy.  Just an extra 50 calories per day would translate into about an extra 5 lbs per year added to your frame.

So, while we typically think that weight gain is simply a matter of eating more calories than you burn, it could turn out to be that some of those bacterial cells that make up 90% of our bodies have effects on how many calories we choose to eat or how fast we burn those calories.  We are not yet at the point where we can treat obesity by altering gut bacteria, but that may turn out to be a useful strategy in the future.