My nutritionist friends are happy.  For years, they have been trying to tell people that the gut is the most important organ in the body for overall health.  The benefits of a healthy gastrointestinal (GI) system go well beyond better digestion, and include better immune function, reduced chronic inflammation (a risk for heart disease and diabetes), and even less fatigue and muscle aches in some people.  Now, the public is finally starting to pay attention to this neglected organ because research has shown that bacterial colonies living in our intestinal tract may play a role in obesity and weight gain.

A healthy adult human gut contains up to 100 trillion bacteria.  These are “good” bacteria, but unlike the few types of good bacteria found in yogurt or probiotic supplements, the gut contains 500-1000 distinct species.  Each one of us has an individual “signature” of bacterial species we carry around with us.  These bugs play a critical role not only in digesting the food we eat, but also in regulating our immune function.

Several years ago, researchers discovered that the types of bacteria living in the gut of obese people differed from the types living in the gut of non-obese people.  Now, new research suggests that it’s possible the different bacterial populations may be a causative factor in excess weight.

We know that weight gain is due to taking in more calories than you burn.  The bacteria in the gut are responsible, in part, for how many calories you extract from the food you eat.  Some bacterial species are more efficient at extracting calories from food, and therefore people who have more of these species may experience greater weight gain.

Two recent studies show that weight loss produces changes in gut bacterial populations.  In the first study, researchers found that patients who had lost weight from gastric bypass surgery had gut bacterial populations that resembled the “lean pattern” rather than the “obese pattern." Similarly, a study by Spanish researchers found that obese adolescents who lost weight through diet and exercise had changes in gut bacteria causing them to look more like typical lean people (I Nadal et al., Int J Obes (Lond). 2008 Dec 9 – Epub ahead of print).

So, does this mean we should all run out and start eating yogurt to lose weight?  Unfortunately, there is no evidence to date that boosting your intake of “good bacteria” will take off the pounds.  In addition, the kinds of bacteria found in yogurt are not necessarily the ones associated with “thinness.”  There is promise in this area, though, and research is on-going to see if there are ways to alter the pattern of bacterial species in the gut to promote weight loss.

In the meantime, though, there are plenty of health reasons to make low-fat yogurt and other fermented foods part of your regular diet.  Research shows that probiotic-containing foods or supplements do help control inflammation and improve overall health of the digestive tract.  Studies in lab animals show that probiotic supplements lower cholesterol, although this hasn’t been confirmed in humans.  Finally, overuse of antibiotics and acid-blockers, which kill off healthy bacteria in the gut, means that many of us have less-than-optimal amounts of good bacteria.  Remember, the bacteria don’t just extract calories from our food; they extract lots of important nutrients as well. So when our gut bacteria are “unhappy,” we are probably unhappy as well!  It really does all come back to the gut…