Probiotics are living microorganisms, typically bacteria or yeasts, which confer a health benefit on the host when consumed in adequate amounts. These beneficial microbes are similar to the ones naturally residing in the human digestive tract and are often found in fermented foods. The concept of introducing these helpful organisms to the diet has been explored for over a century to support the balance of the gut microbiota. Among the many strains available, Lactobacillus casei strain Shirota (LcS) stands out as a highly researched example with a long history of safe consumption and documented effects.
Origin and Discovery of the Strain
The L. casei Shirota strain was discovered in the 1930s by Japanese microbiologist Dr. Minoru Shirota. His goal was to isolate a unique lactic acid bacterium that could survive the journey through the harsh environment of the human digestive tract. This was based on the understanding that for a probiotic to be effective, it must reach the intestines alive in sufficient numbers. Dr. Shirota specifically selected and cultivated this strain for its exceptional resilience, leading to its initial isolation and widespread commercial use as a functional food ingredient.
How the Strain Survives and Works
The ability of L. casei Shirota to survive passage through the gastrointestinal tract is a defining characteristic attributed to its natural properties. The strain demonstrates a high resistance to the low pH levels of the stomach acid, allowing a significant number of the live bacteria to pass through this acidic barrier. This tolerance is paired with an ability to withstand the bile salts secreted in the small intestine, which are generally toxic to many other microorganisms.
Once it reaches the large intestine, the strain exerts its effects by transiently colonizing the intestinal wall and interacting with the existing gut microbiota. Consistent consumption of the strain has been shown to increase the population of beneficial bacteria, such as total lactobacilli and bifidobacteria, in the gut. This modification of the gut environment helps suppress the growth of potentially harmful microorganisms, contributing to a more balanced microbial ecosystem.
Documented Effects on Digestive and Immune Health
Research has demonstrated that regular consumption of L. casei Shirota can support digestive regularity and comfort. Studies on individuals with chronic constipation have shown that the strain can significantly improve both the frequency of defecation and stool consistency. This improvement is associated with an increase in the number of ideal, sausage-shaped stools according to the Bristol Stool Chart.
The strain plays a role in modulating the body’s immune responses. In human trials, daily intake of L. casei Shirota has been shown to increase the activity of natural killer (NK) cells, which are a component of the body’s innate immune system. This immune-modulating effect may contribute to the body’s overall defense mechanisms.
The strain has also been studied for its potential to help manage stress-associated symptoms, which often include digestive issues. Trials involving medical students under examination stress showed that consumption significantly suppressed the increase in salivary cortisol, a stress hormone. It also reduced the incidence of stress-related gastrointestinal symptoms, suggesting an influence on the gut-brain axis, potentially through vagal nerve signaling.
Practical Consumption and Safety Profile
The L. casei Shirota strain is most commonly consumed via a fermented milk drink, known commercially as Yakult. The typical recommended daily intake involves consuming a dose that delivers a high number of live bacteria, generally in the range of 6.5 billion to 10 billion colony-forming units (CFU) per serving. Continuous daily consumption is necessary because the strain is transient, meaning its presence in the gut decreases and returns to baseline levels shortly after intake is stopped.
The strain has a long history of safe use and is generally recognized as safe (GRAS) by regulatory bodies in the United States. For the majority of healthy adults, consumption is well-tolerated with no serious side effects reported in studies. However, as with any microbial product, caution is advised for individuals who are severely immunocompromised or critically ill, and they should consult a healthcare professional before use.