The Human Digestive Tract is Home to All Sorts of Bacteria and Microbes
You have around 100 trillion “bugs” in your stomach and intestines! In the average healthy person, most of these bugs are helpful (these are also known as probiotics), and keeping your gut flora skewed to the helpful side can have great benefits in terms of your overall health, including your immune system, mental health, memory, and even weight! 
Of Mice & Men
In a study conducted at Washington University, researchers wanted to look into the effect that different compositions of gut flora had on weight gain. To minimize the effect of lifestyle and genetic factors in their subjects, the researchers recruited pairs of identical twins who had significantly different body compositions (i.e. one twin was obese, the other lean). Samples of each twin’s gut flora were taken, and injected into sterile mice.
Researchers noted that mice who received gut flora from the obese twin gained more fat than the mice who had the leaner twin’s flora injected, despite eating the same diet. Researchers determined that the difference was caused by the gut flora affecting the mice’s metabolism. 
The researchers then put the mice into communal living and noticed a rather strange transformation! The mice that had been injected with the obese twin’s gut flora stopped gaining fat and started slimming down. Researchers found that the “lean” flora were invading the obese mice (due to shared living conditions) and were driving out the “obese” flora. And the invasion was a one-way street. 
It’s not all about the bugs, though! In this same study, researchers changed the mice’s diet from standard mouse-chow (low in fat and high in fiber) to more closely mirror the standard American diet (high in saturated fats and low in fiber). The researchers found that, in groups of mice eating the standard American diet, the invasion they’d previously observed did not happen. But when they fed mice a low fat, high-fiber diet, the “lean” flora again took over. 
What we learn from this is that, not only does the flora in your gut matter a great deal, but your diet matters, too!
Not All Probiotics Are Created Equal
So how do you cultivate trillions of helpful gut flora? As the experiment at Washington University shows, even identical twins can have vastly different microbes in their digestive tracts. But there is something quite simple that you can do to reinforce the good guys: ingest probiotics.
· Probiotics are naturally-occurring micro-organisms found in naturally fermented or cultured foods. Foods like fermented vegetables (think kim-chi, or pickles), fermented raw milk (used to make kefir), cultured raw milk (used to make yogurt), and tempeh and natto (different types of fermented soy beans) all contain naturally occurring probiotics that can improve gut flora. These products can be found in local health-food stores or co-ops.
· When looking for probiotic foods, you want to stay as far away as possible from pasteurized foods claiming to be “probiotic”. Pasteurization means that the milk used to make these products has been heat-treated, killing many of the probiotics and altering the structure of proteins in the milk into unrecognizable compounds, a can of worms in and of itself.  The probiotics that remain are a ghost of their former selves, and in addition, these products usually have gobs of excess sugar (often high fructose corn syrup) and unhealthy dyes.
Go With Your Gut!
Did you know that these fermented and cultured foods can be made at home, too? Here’s a few easy ones you can try!