The Benefits and Risks of Probiotic Supplements

It’s been said that the only thing that stops a bad microbiome is a good microbiome. The question is how to establish that healthy gut flora. There is a multi-billion-dollar industry pushing probiotic supplements, but despite thousands of clinical trials, we, like our microbiome, are left largely in the dark. When researchers analyzed the first 150 results that Google pulled up on probiotics, commercial sites were the most common, which provided, on average, the least reliable information. Most of the claimed benefits were found to be supported by little or no scientific evidence.

A recent systematic review of randomized, controlled trials of probiotic supplements for healthy older adults found there was insufficient evidence for the improvement of health outcomes. For example, probiotics don’t appear to work for TMAO reduction or improve cognition in dementia patients. What can they do? Various probiotic preparations have been found to improve stool consistency, bowel movement frequency, and abdominal bloating, as well as reduce the incidence of antibiotic-associated diarrhea. Outside of the gut, one strain of a type of Lactobacillus was found to accelerate the healing of wrist fractures in the elderly and improve hand function recovery, though the best evidence for extra-intestinal effects involves the immune system.

Even just a few weeks to months of probiotic supplementation can improve the abilities of our immune cells to destroy pathogens and cancer cells compared to placebo. Does this translate into reduced infection rates? On average, older adults suffer between two to five infectious episodes a year, including infections of the respiratory tract, digestive tract, and urinary tract. A meta-analysis of more than a dozen randomized controlled trials involving more than 5,000 older adults (average age 75) found no overall effect on the occurrence or duration of infections from probiotic supplementation. However, by lumping all infections together, more finely-tuned effects might get washed out, and indeed, looking more closely, probiotics seem to help reduce the incidence, duration, and symptoms of the common cold, but fail to show any influence on the incidence or severity of the flu.

Similarly, how can meta-analyses make sweeping statements about the efficacy or failure of “probiotics,” when each of the studies used different types and combinations of bacteria? Perhaps some work, but the positive signal is just lost in the noise. There are even studies showing that two different strains of the exact same species of bacteria can have different effects. What’re the downsides of giving some a try? We don’t have good data on potential harms either.

Isolated accounts have been published of serious adverse effects from probiotics that appear independent of dose or formulation, but the great majority of clinical trials have not reported major hazards. Unfortunately, an analysis of hundreds of trials found harms reporting was often missing, insufficient, or inadequate, which undermines our confidence in the safety of these supplements. For example, there are concerns about antibiotic resistance.

Probiotics are often intentionally selected to be antibiotic resistant, so they can be co-administered with antibiotics to reduce diarrhea rates, but may transfer that resistance to pathogens in our gut. A study of commercial probiotic supplements from around the world, including the United States, found all batches were resistant to vancomycin, one of our “Hail Mary” antibiotics of last resort. The irony is that probiotics can actually interfere with microbiome recovery after antibiotics rather than facilitate it. Without probiotics, spontaneous post-antibiotic recovery back to baseline was found to occur within three weeks. In contrast, the microbiomes of those randomized to take probiotics remained off-kilter even six months later. The most rapid reconstitution occurred in those given auto-fecal transplants. Just as it may be wise to bank your own blood before an elective procedure, should you need a transfusion, those saving their own stool before a course of antibiotics were able restore their microbiomes back to normal within a matter of days.