A few years ago, probiotics were off most people’s radar. But that’s changing—and fast.

The latest research on gut bacteria and probiotic supplements shows that they impact both body and mind in myriad ways. That’s pretty heady, because until recently most people “considered the contents of the bowels as mere waste products,” says Kirsten Tillisch, MD, of University of California, Los Angeles.

An estimated 1,000 species of bacteria and other beneficial microorganisms (known collectively as the microbiome) inhabit our lower digestive tract. Scientists have gleaned that a more diverse bacteria population is healthier than a narrow range. Antibiotics upset the normal balance, creating what’s called dysbiosis, a medical term for the inbalance of gut flora caused by too few beneficial bacteria and an overgrowth of bad bacteria, yeast or parasites. Food poisoning, stomach flu, poor eating habits and a lack of dietary fiber also hurt the microbiome.

“Probiotics support the body’s physiology and impart a positive influence on health,” says Gerard E. Mullin, MD, author of The Gut Balance Revolution (Rodale, 2015). “Thousands of studies show that probiotics provide harmony and balance to the gut’s immune system to improve inflammatory, allergic and autoimmune conditions.”

Not all of the probiotics described here are sold individually, so you might have to opt for a multiprobiotic supplement that includes the ones you want. Follow label directions for use, but always take probiotics immediately after a meal to buffer the bacteria-killing acid of the stomach. Probiotic quantities are measured in colony forming units (CFUs).

Immune Enhancer. Our gut bacteria program our immune system when we’re children, and a study at the Georgetown University Medical Center found that children taking supplemental Lactobacillus casei were almost 20 percent less likely to develop respiratory or gastrointestinal infections. A study in England tested daily Bifidobacterium lactis on 37 seniors and found that the supplements boosted white blood cell activity within three weeks.

Weight Loss. Overweight and thin people have vastly different gut bacteria populations, suggesting that dysbiosis may be a factor in obesity. When overweight individuals lose weight, their gut bacteria starts to resemble that of thin people. A six-month Canadian study found that L. rhamnosus supplements led to almost a 12-pound weight loss, double that of the placebo group. Another study found that Lactobacillus-rich yogurt led to an average loss of 3 to 4 percent of body weight, the equivalent of 10 pounds for a 250-pound person, after just 43 days. “Probiotics improve blood glucose and weight control by cooling down systemic inflammation and balancing gut-derived hormones that regulate appetite and insulin function,” says Mullin.

Mood Booster. Stress alters the microbiome’s composition, and the gut-brain connection is more than just a “gut feeling.” Gut cells make 90 percent of the body’s serotonin, along with appreciable amounts of other mood-enhancing neurotransmitters, including GABA, dopamine, adrenaline and noradrenaline. Recent human research has found that live-culture yogurt (compared with a placebo) improved stress resistance and reduced anxiety, and that a multispecies probiotic reduced negative thoughts. In other research, a combination of L. helveticus and B. longum eased stress, anxiety and obsessive-compulsive behavior and improved problem solving.

Antibiotic diarrhea. Antibiotics indiscriminately kill both harmful and beneficial gut bacteria, resulting in diarrhea and a greater risk of gastrointestinal infections. Researchers used a combination of L. acidophilus, L. paracasei and B. lactis to treat patients taking antibiotics for three weeks. The probiotics halved the risk of developing diarrhea and a subsequent C. difficile infection.

Allergies. More than 50 million Americans have allergies, but supplemental L. paracasei might ease some symptoms of allergic rhinitis, also know as hay fever. After five weeks, people taking the probiotics had greater overall improvements in their symptoms. Three other promising strains: L. paracasei, L. rhamnosus and B. lactis.

Irritable Bowel Syndrome. Several recent studies have found that multispecies probiotics can reduce irritable bowel syndrome (IBS) symptoms. In France, researchers showed that study subjects’ IBS symptoms and inflammation-promoting bacterium decreased after taking B. animalis lactis, Streptococcus thermophilus, L. bulgaricus and L. lactis.

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