The problem? There’s a gap between research on the efficacy of postbiotics and over-the-top promises made by companies selling them as supplements. Can postbiotics really help “melt inches off my waist overnight?” Of course not. Here’s a look at what postbiotics are, what they are meant for, and why you don’t need to buy any unproven supplements just yet.

Probiotics, or “good” bacteria, have many well-known health benefits. When the correct probiotic strain is prescribed for a specific ailment, there’s strong science to support their efficacy. Now researchers are studying whether the dead microbes, or postbiotics, are also beneficial.

When small living microbes such as bacteria and fungi die, they break down into smaller parts such as amino acids, cell wall fragments and lysates, which may offer health benefits. These are postbiotic, or quite literally, “after life.”

To make supplements, scientists purposely inactivate or “kill” live microorganisms through spray-drying, radiation or pasteurization to create postbiotics.

Put simply, prebiotics are the “food” that feeds probiotics (which are “live,” so they need to eat). When probiotics die, some of the waste products can have health benefits and are known as postbiotics.

Are postbiotics effective?

Because postbiotics are already “dead,” they are likely to have a longer shelf-life than live probiotics and, thus, a wider application for use in supplements and foods. But it’s too soon to know whether postbiotics can be just as beneficial and effective as probiotics. Scientists are currently investigating.

“Since postbiotic refers to cell wall material and metabolites from the ‘good’ bacteria, it makes sense that consuming their metabolites directly would provide benefits,” said Andrea M. Liceaga, an associate professor of food science at Purdue University in West Lafayette, Ind. But many of the effects have been reported based only on in vitro studies without living subjects, Liceaga said. “More clinical trials are needed to fully understand their effectiveness in human health.”

The key to making postbiotic supplements is learning which microbes confer a health benefit and at what dose. To date, in vitro and animal studies have documented specific postbiotic components that show antioxidant, antimicrobial and anti-inflammatory effects; inhibit the growth of cancer cells; and protect immune health. But these studies need to be replicated in humans before specific supplement recommendations can be made.

A few postbiotic supplements that have been tested in humans are available in Europe. For example, Pylopass, made of spray-dried dead cell fragments of L. reuteri, is used to help control ulcers caused by H. pylori. And Lacteol Fort, made from nonviable cells from L. acidophilus, is used to treat irritable bowel syndrome.

These brands aren’t readily available in the United States. Instead, American stores are more likely to sell postbiotics that contain butyrate, or a novel three-in-one prebiotic, probiotic and postbiotic combination.

The evidence for that use is mixed. “Even if you take all of the butyrate in the world and eat fast food everyday, you won’t lose weight,” said Gregor Reid, distinguished professor emeritus at Western University in London, Ontario. “You can’t look for this magic bullet unless you change your lifestyle and diet.”

Some people don’t naturally make enough butyrate in the gut, and this may be linked to weight gain. If your gut is not making enough butyrate, it may signal a problem with your digestive system that can’t simply be fixed by taking butyrate supplements. “If you start taking probiotics, eating fermented foods and taking prebiotics, you will start to develop a better gut microbiota and therefore will produce more butyrate and get healthier,” Reid said. “If you just take butyrate supplements, it won’t solve your problem. Companies making claims like that should be facing FDA scrutiny.”

And what about those three-in-one combo pills? “You have to be cautious,” warned Mary Ellen Sanders, the executive science officer of ISAPP, based in Centennial, Colo. “If you use the terms ‘prebiotic,’ ‘probiotic’ or ‘postbiotic,’ each of the substances has to be independently tested and shown to be beneficial in humans,” Sanders said. “You’d have to contact the company to see if they have such research, and it may be unlikely that they do.”

In an ideal world, specific types of postbiotics would be sold only once clinical studies have established a health reason and a correct dosage, but some supplement companies have skipped that step (and, yep, it’s legal). Reid explained that a supplement “fails the definition of probiotic or postbiotic if it’s never been tested.” ISAPP defined these terms hoping that globally accepted definitions will bring consensus among researchers and manufacturers, and help guide consumers when purchasing products.

Are postbiotic supplements worth buying?

Years ago, when probiotics were a new supplement on the market, it was difficult to know which one to purchase for a specific health benefit. That problem was solved when the term “probiotic” was defined, and the U.S. Probiotics Guide was later established. This database lists probiotic-containing foods and supplements by brand name, probiotic strain, medical use and dosage, and is relied upon by physicians, dietitians and consumers to ensure that the right probiotic is used for the right reason.

At this point, there’s no such database for postbiotics. Hopefully one will be developed over time as more research is conducted.

“For postbiotics, there may be advantages, but we need more information on effect and dosage,” said Reid, who believes that if a company is making money by calling something a postbiotic, it should conduct studies to prove efficacy.

“These companies know how to suck people in to buy their products,” Reid said. “If a product says it is ‘postbiotics,’ ask them questions: which postbiotics, what’s the effect, what is the dose, and where’s your evidence?’”

I did ask these questions to two U.S.-based postbiotic manufacturers, each of whom sent me dozens of studies on postbiotics in general. But neither company provided any research on the specific claims they were making on the products they sell, since they have not done any clinical research. That means the effects and dose of their products are truly unknown. While there may be no harm from taking these products, there may be no benefit, either. They certainly won’t melt inches off your waistline overnight.

Sanders doesn’t think postbiotics are going to have the same range of benefits as live microorganisms. “We can look forward to more high-quality clinical trials about what effects they have and who can benefit,” Sanders said. “But should everyone be taking postbiotics? Current evidence does not support this.”

Postbiotics are a promising idea with lots of research on the way, and hopefully scientists will learn more about the right dose of which postbiotic to take for different health issues.

Until then, ask postbiotic companies to provide you with proof that their product is effective. If it hasn’t been tested and has no proof of efficacy, why would you take that supplement?

Dietitian Cara Rosenbloom is president of Words to Eat By and specializes in writing, nutrition education and recipe development. She is the co-author of “Food to Grow On.”

By Percy