Probiotics - pricey supplements designed to support the trillions of bacteria blossoming in our guts - have become a big business, with a market that is projected to exceed US$57 billion in the next five years.

"Probiotics are probably the single most important new food category to emerge in the last 20 years," Scott Bass, the head of the Global Life Sciences team at law firm Sidley Austin LLP and an adviser for the FDA on its first dietary supplement website, told Business Insider.

The idea behind the pills is simple: foster the growth of beneficial bacteria in the gut and curb the growth of the bad bacteria to improve digestion, boost the immune system, and even lower rates of certain diseases.

Putting that idea into practice, however, has proven a bit more complicated than some scientists initially envisioned.

So far, the effects of existing probiotic supplements have been all over the map - sometimes they help, but most of the time, they don't.

Nevertheless, supplement-makers continue to advertise their pills as beneficial for everything from weight loss to treating lactose intolerance.

The problem is that while most probiotic formulas contain tens of millions of beneficial bacteria, like Lactobacillus acidophilus, fewer than a hundred or so of those bacteria actually make it into your gut.

"Thirty billion Lactobacillus sounds good, but after going through the stomach acid, only about 43 of them survive," Ian Orme, a distinguished professor of microbiology and pathology at Colorado State University, told Business Insider.

These "good" bacteria are supposed to replace the "bad" bacteria (like Bifidobacteria) and help you feel better.

"In other words these 43 or so bacteria politely ask the million or so anaerobic Bifidobacteria to please leave," said Orme. "Yeah, sure."

There are some specific incidences where the research suggests that the pills could actually help.

A rigorous 2014 review of probiotics research concluded that the supplements could be especially helpful for newborns with intensive needs.

Adding "good" bacteria to the guts of infants at risk of developing the life-threatening gut disease necrotising enterocolitis, for example, significantly reduced the chances that they'd come down with the disease.

More recently, researchers have been experimenting with supplements called synbiotics, which combine a probiotic bacterial strain with what's called a prebiotic - essentially a type of sugar designed to feed the beneficial bacteria and help it thrive in the gut.

The idea is that the pre- and the pro-biotic would work together to provide a combined benefit - while the probiotic settles in and pushes out the "bad" bacteria, the prebiotic hangs around and acts as its food supply, ensuring that the supplement sticks around and does its job.

Just this month, as part of the first large-scale clinical trial of its kind, researchers working in rural India found that newborns who were given a synbiotic were at a substantially lower risk of developing sepsis, a potentially fatal condition characterised by severe infection.

Some small studies have suggested that synbiotics could provide benefits to a range of other conditions influenced by the gut microbiome as well, including obesity, diabetes, and non-alcoholic fatty liver disease, but larger-scale clinical trials focusing on each of those conditions are needed.

So if you see a probiotic - or a synbiotic - for sale at your local health-foods store, know that the existing research backing up its claims is very limited.

This article was originally published by Business Insider.