Soil-based probiotics have been touted for their ability to improve digestion, stimulate the immune system, and help maintain a healthy gut microbiota. On the other hand, some people believe that SBOs should be avoided at all costs, due to their spore-forming nature and ability to compete with resident gut microbes. So, should you be taking soil-based probiotics? If so, which formula or brand is best? Read on to find out.
Until recently, I recommended Prescript Assist as the best soil-based probiotic. While some of the microbes in the old probiotic formula were not well-studied, Prescript Assist had been used in several clinical trials with excellent results and no reported adverse events.1,2 It was also well tolerated, even by those with IBS, severe gut dysbiosis, or other gut pathologies.
In late 2017, the manufacturer made major changes to the ingredients of Prescript Assist, so it is no longer using the formulation that was studied in clinical trials. While the original formula contained 29 different strains, the new formula contains 28 strains, only 9 of which are from the original formula. There’s very limited evidence for most of the strains included in the new formula.
I’ve been asked by many people what my new recommended alternative to Prescript Assist is, so I decided to do some digging. This guide is a result of 50+ hours of independent research on soil-based organisms and the products commercially available today.
First, though, I’ll review what soil-based organisms are, the controversy, and the evidence for the most commonly used strains.
What are soil-based organisms?
The term soil-based organism (SBO) encompasses over 100 different species of microbes naturally found in soil. Some of these have been isolated and adapted for use as probiotics. Most SBO used in soil-based probiotics are spore-forming. This means that when they replicate, they form a small spore that is protected by a hard coating, making them resistant to heat, acid, and most antibiotics. This also means that they are more likely to survive the upper gastrointestinal tract and reach the large intestine compared to other commonly used probiotic strains.
The controversy and the evidence
There is currently a lot of debate within health circles about the safety of SBOs. Advocates claim that they are probiotics that normalize bowel function, aid in digestion, beneficially stimulate the immune system, and help re-seed the gut microbiota. They also praise SBOs for their ability to resist stomach acid and the lack of need for refrigeration.
On the other hand, opponents of SBOs argue that because of their spore-forming nature, they proliferate rapidly, and may compete with our resident gut microbes, rather than helping to bring the gut microbiota back to normalcy. In some individuals, opportunistic SBOs could overgrow, with the potential to become pathogenic.
My take: Several individual SBO strains have been shown to be beneficial, with no adverse effects, in dozens of randomized, placebo-controlled, human clinical trials. However, other strains have very limited or no clinical evidence and may be able to cause infection in people with a compromised immune system.
Rather than labeling all SBOs as good or bad, I think we need a more nuanced discussion – one that considers the evidence for each particular strain, microbe, and formula. That’s what I’ll try to provide in the next few sections. If you just want to see my recommendations, feel free to skip down to the “Conclusions” section at the end.
Common soil-based species used in probiotics:
Before I do an analysis of the soil-based probiotic formulations on the market, it’s important to understand which species are most commonly used.
I searched PubMed and Google Scholar for every available human clinical trial on the seven most commonly used soil-based microbes:
Before I share my findings below, it’s important to note that microbes are denoted by their genus, species, and strain. The list above provides the genus and species but does not tell you anything about the strain. Strain is incredibly important since two strains of the same species can have very different characteristics.3 Unfortunately, most probiotic formulations do not provide information about which strains they contain.
To see my full analysis of the available clinical trial literature for each strain, click here to download my Excel spreadsheet. For just the key findings, read on: