So, we all know (or if you didn't, you do now) that we have a sizable population of bacteria nestled in our guts with which we have a symbiotic relationship. In fact, only about 10% of the total number of cells in and on our bodies are natively human; the rest are microbes we pick up along the way. (We're talking number here, not mass; our cells are quite a bit bigger, generally speaking). Just how many different species are in there and precisely what each does is just beginning to be explored.
Your digestive tract, the stomach especially, is a pretty harsh place; of any bacteria you happen to consume the vast majority will be destroyed before they reach your intestines. However, bacteria reproduce pretty fast... once every 20 minutes or so, which means that a single surviving cell could produce a population of 1 million in about 7 hours.
You may also have heard of probiotic foods and supplements (and if you haven't, you're about to). These are usually fermented foods (such as kefir and kombucha), or pill-form supplements which carry a population of "good" bacteria... the kind of stuff we want along for the ride. These guys may be aiding us in more ways than we think... check out the wiki on probiotics. Yes, I know, it's wikipedia... but, there are many good references and links at the bottom of the article.
I'm pretty convinced of the value of probiotics, as far as having them in your gut. The trick is, keeping them there.
Since the vast majority of whatever you're taking in is going to die in your stomach, you want the environment that the survivors are going to try to set up shop in to be as conducive to their proliferation as is possible. It does no good to take loads of probiotic supplements, and then eat in such a way as to feed the less desirable bacteria populations which are living in there... the bacteria you're trying so hard to introduce will never out-compete the established population.
Conversely, if you're eating to support the types of bacteria you want, you won't have to take probiotic supplements as often, because the good guys will flourish in your gut, and out-compete the stuff you're trying to get rid of. A fresh introduction of desirable bacteria on a regular basis certainly isn't harmful, but you don't need to take the pills and drink kefir every single day to keep your gut in balance if you're otherwise eating to sustain the population you've already introduced.
The question now is... what constitutes proper nutrition for the bacteria you want hanging out in your intestine? Well, you can probably guess where this is headed... those species of bacteria considered to be health-promoting little symbionts have much in common; they thrive when presented with plenty of soluble fiber, and when their environment is a little on the alkaline side. I know what you're going to say; your intestines are always alkaline, they strive to maintain that state. But, food moves through in a big mass, so if you're eating a lot of stuff with an acid pH (read, lots of sugars and carbohydrates, which are also sugar) not only will your intestines have to work a lot harder, but there will be regions of that digesting food mass which will still carry an acidic pH for a good portion of the trip through your digestive tract, and all that time it'll be feeding the bacteria you don't want, rather than the bacteria you do want.
In other words, eating a diet of moderate protein intake, getting most of your energy calories from healthy fats, and restricting your "carbohydrate" intake to fresh veggies and a moderate amount of fresh fruit ( quotation marks in deference to the fact that you'll be getting almost no energy calories from those veggies... they're mostly fiber, micronutrients, and water), will keep your gut flora happy and healthy. So, The Paleo Diet, or the Primal Diet, or the Zone Diet... any of these will do a good job of this. For my part, I think Paleo or Primal is just way easier to manage, but you can take whatever route to that you want.
Maintain those kinds of dietary habits, and all those probiotics you're taking will be of much greater benefit.
