Our bodies—and guts, specifically—depend on a balance of bacteria to “maintain healthy blood sugar and cholesterol levels,” but “you gotta feed the bacteria.” So said Dr. Mehmet Oz—heart surgeon turned daytime TV host, ardent RFK Jr. supporter, believer in disproven COVID treatment hydroxychloroquine, and now possible head of Medicaid and Medicare for the Trump administration—who began his Senate confirmation process on Friday.
To aid in that gut-balancing process, Oz has pushed the benefits of both prebiotics and probiotics, including in his role as global advisor for the iHerb brand of supplements.
Both have come under scrutiny recently, including through this week’s Washington Post opinion piece by Harvard medical school instructor and physician Trisha Pasricha, who called probiotics “a waste of money,” instead recommending a high-fiber diet.
So which doctor is right? Here’s what science tells us.
What are probiotics?
The human gastrointestinal tract is colonized by a range of microorganisms, including bacteria, archaea, viruses, fungi, and protozoa, explains the National Institutes of Health (NIH) Office of Dietary Supplements. And the activity and composition of those microorganisms (often known as the gut microbiome) can affect human health and disease.
Probiotics, according to the International Scientific Association for Probiotics and Prebiotics, are “live microorganisms that, when administered in adequate amounts,” may benefit that gut microbiome composition.
While they are naturally present in fermented foods—including the homemade turmeric sauerkraut Dr. Oz mentions in his Instagram post (above) about probiotics—they can also be added to food products, and are available as dietary supplements.
“However,” notes the NIH, “not all foods and dietary supplements labeled as probiotics on the market have proven health benefits.”
That’s where a range of varied opinions come into play.
Who says what about probiotic supplements?
As Pasricha points out, of the over 1,000 clinical trials of probiotic supplements, there have been too many different strains tested and results found to reliably say they can be universally helpful.
A 2024 review of existing evidence, published in the Advances in Nutrition journal, concluded that, on one hand, “there is sufficient evidence of efficacy and safety for clinicians and consumers to consider using specific probiotics for some indications—such as the use of probiotics to support gut function during antibiotic use or to reduce the risk of respiratory tract infections—for certain people.”
However, those researchers concluded, “we did not find a sufficiently high level of evidence to support unconditional, population-wide recommendations for other preventive endpoints we reviewed for healthy people. Although evidence for some indications is suggestive of the preventive benefits of probiotics, additional research is needed.”
When looking at the body of scientific evidence regarding effect of probiotics on seven different health issues, the NIH reports the following:
Atopic dermatitis
Numerous studies have looked at the effect of probiotics on this most common form of eczema. Overall, the evidence suggests that the use of probiotics might reduce the risk of developing atopic dermatitis, but also might provide only limited relief. The effects also depend on the strain used, the timing of administration, and the patient’s age.
Pediatric acute diarrhea
While one large review found that single- and multi-strain probiotics significantly shortened the duration of symptoms, another found it was no better than a placebo.
Antibiotic-associated diarrhea
Overall, the available evidence suggests that starting probiotic treatment with strains LGG (Lactobacillus) or Saccharomyces boulardii within 2 days of the first antibiotic dose helps reduce the risk of diarrhea in patients between 18 and 64, but not in elderly adults.
Inflammatory bowel disease
IBD is a chronic inflammatory disease that includes ulcerative colitis and Crohn’s disease, for which no cure exists. In the many reviews that have looked at the effects of probiotics, researchers reached similar conclusions—that certain probiotics might have modestly beneficial effects on ulcerative colitis but not on Crohn’s disease.
Irritable bowel syndrome
IBS is a common functional disorder of the gastrointestinal tract that’s been linked to both stress and gut microbiomes. Overall, the available evidence shows that probiotics might reduce some symptoms, but stresses that additional clinical trials are needed to confirm the specifics of strain, dose, and duration of treatment.
High cholesterol
Researchers have studied the use of probiotics to improve lipid profiles. And while, overall, research suggests that using multiple probiotic strains might reduce total and LDL (bad) cholesterol levels, more research is needed.
Obesity
Again: More research is needed. The results, the NIH concludes, “indicate that the effects of probiotics on body weight and obesity might depend on several factors, including the probiotic strain, dose, and duration as well as certain characteristics of the user, including age, sex, and baseline body weight.”
Bottom line: The jury is still out. Whether you opt to try the supplements or not (as they are generally believed to be harmless, though long-term safety studies are still needed), make sure to eat plenty of fiber as well as fermented foods. That includes yogurt, kefir, fermented cottage cheese, kimchi and other fermented vegetables (as endorsed by Oz), and kombucha tea, which were shown by Stanford University researchers to increase microbial diversity and lower inflammation.
More on supplements:
This story was originally featured on Fortune.com
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