Here's an addendum:
According to comments made by Walter Coyle, MD, of the Scripps Clinic in La Jolla, Calif at the American College of Physicians' annual meeting and a report published by MedPageToday, "the science of the intestinal "microbiome" is still in its infancy and it remains unclear what changes to make, let alone how best to make them."
One of the primary reasons relatively little is known about intestinal bacteria is that the mix of gut flora varies greatly between individuals. For example, according to Coyle, a study of three members of the same household revealed that their intestinal bacterial composition differed markedly.
Also, it is believed that both environmental (outside) influences and host factors (the person's own internal activity) both play a role in how an intestinal bacterial community evolves, "host genetic influences [on the gut microbiome] remain unexplored," according to Coyle.
Host factors are considered important because during adulthood, individuals achieve a "characteristic "core" population of intestinal bacteria that remains stable even in the face of disruptions such as antibiotic treatment."
Here's some additional interesting information:
Two major categories of bacteria dominate in the intestine: Firmicutes and Bacteroidetes. Studies have found that obese people tend to have a higher ratio of the former to the latter. In fact, one clinical study of 12 people eating a calorie-restricted diet for one year found that there was no weight loss until the ratio of Firmacutes to Bacteroidetes shifted.
Unfortunately, it is not known if the ratio shifted as a result of metabolic changes or that metabolic changes resulted from a shift of the ratio. So if you are looking to lose weight, don't rush out and buy probiotics with Bacteroidetes just yet because you may be wasting your money.
There is quite a bit of additional study needed before any reasonable conclusions and therapeutic applications can be derived for safe and effective medicinal use of probiotics. In the interim, probiotics appear to remain somewhat useful for irritable and inflammatory bowel syndromes and with antibiotic usage. Probiotics can also prevent yeast infections for women on antibiotics so I highly recommend their usage while on antibiotics. Remember, you need at least a billion (yes, "b") colonies (read more below). Otherwise, avoid them as you should any other unsubstantiated dietary supplements.
Blog from March 12, 2012:
What About Probiotics?
Okay, so now you have a good broad understanding of what they are, the question remains should you actively consume them? I have studied this subject extensively, and yet I claim no definitive expertise on it because there is so much ongoing study. However, here is what I can tell you.
As far as I have come to understand after review of multiple sources, probiotics do no harm. Of course, that statement only holds true within reasonable limits. If outlandish quantities were consumed (trillions), I'm not sure what would happen. For example, even drinking too much water can be harmful as it would overwhelm the kidneys and flood the gastrointestinal system.
So if it probiotics aren't harmful, does that make them helpful? For general use, the jury is still out, but the overwhelming consensus and numerous previous studies suggest that they provide no real benefit for a person with a healthy gut. Changing the bacteria composition of one's gut is not easy under normal circumstances, and in fact, most commercially available probiotic products neither contain sufficient colonies (need at least 1 billion) nor the wide range of different types of good bacteria to make any difference.
(As a side note, carnivores (meat eaters) typically have different bacteria colonies than herbivores (plant eaters). The significance of this fact is not well understood at this time. Also, immediate family members often have the same bacterial colonies and if a family member wants to be restored to their historical composition after a bout of serious infection or illness, it can be accomplished by fecal implant through a nasogastric tube or sometimes through a fecal enema.)
So when are probiotics helpful? They are helpful when you are taking a course of antibiotics, which by definition kills all bacteria, good and bad. The probiotics help to replenish the healthy bacteria even as the unhealthy perish. Probiotics are also believed, based on several studies, to be helpful with diarrhea, dysentery, and allergic gastritis. In fact, with diarrhea related illness, they are said to reduce symptoms and duration of disease by as much as 50%.
(On a personal note, I can attest to their apparent efficacy after recently being afflicted with such a viral illness. Taking probiotics gave me some relief, even if it was only through peace of mind that I was doing something to help myself. Imodium and electrolyte replacement with pedialyte helped as well.)
Some studies report benefits from taking probiotics with Irritable Bowel Syndrome (IBS) and inflammatory bowel diseases such as Crohn's Disease (Terminal Ileitis) and Ulcerative Colitis. They are also reported to be useful for atopic eczema (a skin condition most commonly seen in infants) and with Helicobacter Pylori or H Pylori as it is often referred, infections, now known as the main cause of gastric ulcers.
The good news is that studies are continuing and we expect to have more definitive answers in the not too distant futures.
In the interim, I suggest you save your money and only use probiotics when either something has already gone wrong that may be amenable to correction with probiotics, or in the case of you are taking antibiotics where they may prove protective.
Notwithstanding, make sure that you only use products that contain at least 1 billion colonies and have at least a few different strains. VSL3, Bio-K, and Lactobacillus GG seem to be three reputable products. (No, I have no financial or other relationship whatsoever with any of these manufacturers.)
The best advice I can give you is be careful what you put in your mouth, don't eat anything if you are not sure it is fresh, and wash your hands well before eating. Also, learn how to manage your stress. These protective measures may spare you ever needing a probiotic. Stay healthy, my friends!
Update for September 2013
A very recent study shed light on the effectiveness/ineffectiveness of probiotics. This recent well designed study failed to show any benefits for probiotics in preventing Antibiotic Associated Diarrhea (AAD). This is significant because many doctors, myself included, often recommend adding probiotics when taking antibiotics for the specific reason to prevent AAD and Clostridia Dificile superinfection. This new study throws that recommendation into disrepute and knocks off yet another reason to take probiotics.