Probiotics are small, single-cell microorganisms (bacteria) that, when taken in adequate amounts, create positive effects in our digestive system. Problems such as traveler's stomach, diarrhea, Irritable Bowel Syndrome and even cancer can be prevented by taking probiotics. It also boosts the immune system, lowers blood cholesterol and not to mention, creates a healthy digestive system.
As an athlete that travels quite a bit, from time to time takes antibiotics and generally lives a life that can irritate my digestive system, I've started to put my faith in these great, little creatures. They can prevent many problems that would otherwise negatively effect one's performance and general well-being.
So, where do we find these bacteria? Well, certain dairy products can be a good place to start. Good quality yogurt and kefir / buttermilk and other fermented products could contain live bacteria cultures. I recommend doing some research before running out to purchase yogurt, milk or cheese, because some production methods would effectively kill any active bacteria culture and therefore be of no good use to you.
You can also, very effectively, take probiotics through a dietary supplement. If you choose this route, make sure you take a product containing Lactobacillus GG and Lactobacillus acidophilus, as these two have been proven to survive the trip through your stomach. Some probiotics, if taken as dietary supplements, will not survive the acidic environment in the stomach and therefore not reach their goal, your intestines.
In a perfect world, I would recommend taking probiotics both as a supplement as through good, high quality dairy products. Taken over regulary and over time, it should yield great results. For a good, English language website - visit http://www.usprobiotics.org/