Benefits
Diarrhea during antibiotics: research used different strains and is not cited here
That research was done with different strains. The Bio-K+ product contains CL1285 and LBC80R, not the Shirota strain covered by the studies cited on this page, and probiotic results do not carry across strains. It was also done in hospitalized adults on antibiotics as a medical prevention measure. C. difficile infection is a serious illness that needs medical care, not a self-care supplement decision. None of that work appears in this page's reference list, and larger trials since then have not consistently repeated the benefit.
Respiratory symptoms in older adults: no benefit in a large trial
In a placebo-controlled trial in 737 healthy nursing home residents aged 65 and over, two bottles a day of fermented milk with strain Shirota for 176 days did not reduce respiratory symptoms or improve the antibody response to a flu vaccine. The one positive respiratory trial cited here was in highly active adults, and results in one group do not carry over to another.
Stress and mood: no supporting reference on this page
No reference on this page supports a stress or mood benefit. Small studies in students around exam periods have been reported elsewhere, but they are not cited here, and the gut-brain explanation offered for them remains a hypothesis rather than a settled finding.
Constipation-related symptoms in women after childbirth
The constipation study cited on this page (Sakai 2015, Benef Microbes) was a randomized trial of fermented milk containing L. casei strain Shirota for constipation-related symptoms and haemorrhoids in women during the weeks after childbirth. It was not done in older adults. It also tested a fermented milk drink rather than a capsule, and a milk drink has its own effect on bowel habit, so not all of any benefit can be credited to the bacteria. Whether the same drink helps other groups has not been shown by the evidence cited here.
Fewer upper respiratory infections in one athlete trial
In a manufacturer-sponsored trial, 84 highly active adults drank fermented milk with strain Shirota (two bottles a day) or a placebo drink for 16 weeks; 58 finished. 66% on the probiotic vs 90% on placebo had a week or more of upper respiratory symptoms, with fewer episodes, but illness was not milder or shorter. Larger trials in 243 athletes and 737 older adults found no reduction, so this is mixed evidence.
Mechanism of action
Gastric acid and bile resistance
L. casei Shirota was selected in 1935 by Dr. Minoru Shirota for its acid and bile tolerance. Laboratory work reports that enough live bacteria survive stomach acid and bile to reach the intestine. Comparisons with other probiotics are not backed by anything cited here, and survival in the gut is not the same thing as a health benefit.
Possible effects on immune cells (proposed mechanism)
Some trials elsewhere have reported changes in natural killer cell activity, and laboratory work suggests bacterial cell wall fragments signal through immune receptors called TLR2 and NOD2. None of that work is cited on this page. A shift in a laboratory marker is not the same as getting sick less often, and this remains a proposed mechanism.
Lactic acid production and pathogen suppression
L. casei ferments the lactose in milk into lactic acid, which is what gives fermented milk its tart taste. In the gut, lactic acid lowers pH and inhibits gram-negative pathogens. The strain also produces small amounts of acetic and propionic acids.
Bacteriocin-like inhibitory substance production
In laboratory dishes, L. casei produces substances that slow the growth of bacteria such as C. difficile, Listeria monocytogenes and some E. coli. These are test tube results. They are not evidence that taking L. casei prevents infection in people, and no study cited on this page tested it against those bacteria.
Clinical trials
Same multicenter clinical trial as L. acidophilus entry: 255 hospitalized adults on antibiotics randomized to placebo, 50B CFU, or 100B CFU L. casei LBC80R + L. acidophilus CL1285 + L. rhamnosus CLR2 (Bio-K+). (Gao et al. 2010, Am J Gastroenterol)
255 hospitalized adults on antibiotics.
The percentages listed here previously were internally inconsistent (they showed the middle dose doing worse than placebo) and this trial is not in this page's reference list, so the figures could not be checked and have been removed. What can be said is that this was a hospital study of a three-strain combination product, and those strains are not the Shirota strain tested in the studies cited here. The large 2018 PLACIDE trial (n=2,981) and other big trials have not consistently repeated a probiotic benefit for diarrhea during antibiotics.
16-week, randomized, double-blind, placebo-controlled trial during winter training season. 84 highly active men and women in endurance-based activities drank fermented milk with L. casei Shirota (Yakult, two bottles a day, each with about 6.5 billion live bacteria) or a placebo drink. Outcomes: URI episodes, severity. (Int J Sport Nutr Exerc Metab)
Endurance athletes during winter training.
Among those who finished, 66% on the probiotic drink versus 90% on placebo had at least one week with upper respiratory symptoms, and episodes were fewer (1.2 versus 2.1), but severity and duration did not differ; saliva IgA was higher on the probiotic. Limits: only 58 of 84 finished, the trial was sponsored by the manufacturer (Yakult), and a larger trial in 243 athletes found no reduction.