Probiotics' side effects, if they occur, tend to be mild and digestive (such as gas or bloating). More serious effects have been seen in some people. Probiotics might theoretically cause infections that need to be treated with antibiotics, especially in people with underlying health conditions. They could also cause unhealthy metabolic activities, too much stimulation of the immune system, or gene transfer (insertion of genetic material into a cell).

Selecting a multi-strain probiotic is also consistent with the theory that filling your gut with enough good bacteria outcompetes any bad bacteria for the same space and resources. In your gut, the more diversified the good bacteria, the harder it is for the baddies to gain a foothold. With all this in mind, we only looked at supplements containing multiple strains of bacteria. 

Depression often goes hand in hand with anxiety. Both conditions can have origins in the gastrointestinal (GI) tract, which typical antidepressants and benzodiazepine medications do not address. Since there is a connection between the health of the GI tract and the health of the nervous system/brain, directly through the vagus nerve and also via indirect chemical effects, improving the former can positively affect the latter.

According to a 2014 meta-analysis, probiotics benefit diabetics by improving insulin sensitivity and decreasing the autoimmune response found in diabetes. The authors suggest that the results were significant enough to conduct large, randomized, controlled trials (the “gold standard” of scientific studies) to find if probiotics may actually be used to prevent or manage diabetes symptoms. Combining probiotics with prebiotics may also help manage blood sugar, particularly when blood sugar levels are already elevated. (87)
Even if some of the bacteria in a probiotic managed to survive and propagate in the intestine, there would likely be far too few of them to dramatically alter the overall composition of one's internal ecosystem. Whereas the human gut contains tens of trillions of bacteria, there are only between 100 million and a few hundred billion bacteria in a typical serving of yogurt or a microbe-filled pill. Last year a team of scientists at the University of Copenhagen published a review of seven randomized, placebo-controlled trials (the most scientifically rigorous types of studies researchers know how to conduct) investigating whether probiotic supplements—including biscuits, milk-based drinks and capsules—change the diversity of bacteria in fecal samples. Only one study—of 34 healthy volunteers—found a statistically significant change, and there was no indication that it provided a clinical benefit. “A probiotic is still just a drop in a bucket,” says Shira Doron, an infectious disease expert at Tufts Medical Center. “The gut always has orders of magnitude more microbes.”

A Seal of Approval When looking for a good probiotic, Gans advises consumers to buy from a trusted retailer and look for a seal from a third-party certifier. “Reputable probiotic supplements should list the genus, species, and strain, in addition to the number of organisms that will be alive by the use-by date and the dose on the label,” says Gans. This transparency will help consumers understand what’s in their supplement and how to best use it.


There are four significant takeaways from this study: (1) murine gut mucosal probiotic colonization is only mildly enhanced by antibiotics, (2) Human gut mucosal probiotic colonization is significantly enhanced by antibiotics, (3) After taking antibiotics, probiotics delay gut microbiome and transcriptome reconstitution in mice and humans, (4) Autologous fecal microbiome transplant (aFMT) restores mucosal microbiome and gut transcriptome reconstitution. 
Sugar is so easily accessible in Westernized cultures that it is easy to overindulge in it. But why do you crave it? There are several reasons. One, sugar usually means sucrose, which is composed of glucose and fructose. Glucose is the sugar in your blood, so consumption of sugar results in a temporary boost of energy. However, that blood-sugar spike causes insulin to be released to usher the sugar into cells for energy or fat storage, and shortly afterward your blood sugar drops. So then you feel a slump and reach for something to bring your blood sugar back up: more sugar.
Probiotics may also be of use in maintaining urogenital health. Like the intestinal tract, the vagina is a finely balanced ecosystem. The dominant Lactobacilli strains normally make it too acidic for harmful microorganisms to survive. But the system can be thrown out of balance by a number of factors, including antibiotics, spermicides, and birth control pills. Probiotic treatment that restores the balance of microflora may be helpful for such common female urogenital problems as bacterial vaginosis, yeast infection, and urinary tract infection.
"Many chronic health problems and inflammatory diseases are caused by a damaged microbiome where the good bacteria is overrun by the harmful bacteria and can't reinforce the gut walls, which results in increased intestinal permeability where bacteria and toxins leak through the intestines into the bloodstream triggering a system-wide immune response of inflammation that is a part of our body's natural healing cycle."

As with other types of supplements, scientists are still discovering the full potential of probiotics. However, when taken regularly, it is believed that they help support digestive health by helping maintain the balance of beneficial and non-beneficial bacteria in your digestive tract. Having a natural balance of good bacteria in the digestive tract can help defend against occasional digestive issues. In addition, probiotics play an important role in supporting immune health, since 70% of the body's immune system is found in the digestive tract.∗
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