Benefits
Shelf-stable immune modulation
Since the bacteria are already dead, nothing has to survive manufacturing, storage or stomach acid for the ingredient to stay the same as the day it was made. That is a genuine handling advantage over live probiotics. It is a manufacturing point rather than a health result, though: none of the cited studies compared this postbiotic against a live probiotic, or showed that stability translates into a better outcome for you.
Immune lab markers shifted in healthy adults
In the main cited trial, 60 healthy adults (30 men and 30 women) took 10 mg a day of heat-killed L. plantarum L-137 or a control for 12 weeks. Two laboratory measures of immune activity went up compared with control: lymphocyte proliferation in response to a test stimulus (P = 0.036) and the Th1 to Th2 ratio (P = 0.002). A second study reported more type I interferon production, but that paper combined human data with work in pigs. Two things to keep in mind: natural killer cell activity was measured in the main trial but was not among the markers that rose, and none of the studies tracked whether people actually caught fewer colds or infections. The Toll-like receptor explanation is a theory from laboratory work, not something these trials measured.
What has and has not been shown for the gut
This is laboratory theory rather than a finding in people. The idea that L. plantarum components tighten the junctions between gut lining cells and reduce 'leaky gut' comes from cell studies. None of the four studies cited on this page measured gut permeability, digestion or bowel habits. The only gut results come from an uncontrolled safety study of strain L-137: in 29 people who took 10 mg a day for 12 months, the Firmicutes to Bacteroidetes ratio in stool was lower at 12 months and stool short-chain fatty acids were higher at 6 months, compared with their own starting values. With no placebo group, this may reflect a change in gut bacteria but is not a proven gut benefit. The closest human data is a trial in overweight adults that reported changes in inflammation markers and blood fats, which is not the same thing as a gut barrier effect.
Mechanism of action
TLR2/TLR4 signaling and trained immunity
Heat-killed bacterial components are Pathogen-Associated Molecular Patterns (PAMPs) that bind Toll-like receptors on intestinal immune cells without triggering infection-level responses. In laboratory work this signaling can switch on cytokine production and help dendritic cells mature, and some researchers propose it also leaves innate immune cells in a longer lasting primed state sometimes called 'trained immunity'. That whole chain is a proposed explanation built from cell and animal research. The human studies cited here measured blood immune markers only; they did not test this mechanism and did not show faster or stronger recovery from any illness.
Clinical trials
Randomized controlled trial of heat-killed Lactobacillus plantarum L-137, 10 mg a day for 12 weeks, published in the Journal of Nutrition in 2006 (PMID 17116721). This trial tested the L-137 strain, not the SoWell brand, and the page does not state whether SoWell supplies L-137. Evidence from a different strain, such as L. paracasei MCC1849, does not apply here.
60 healthy adults, 30 men and 30 women.
Compared with control, the L-137 group had higher lymphocyte proliferation in response to a test stimulus (P = 0.036) and a higher Th1 to Th2 ratio (P = 0.002). These are laboratory immune measures; the trial did not show fewer infections or fewer sick days. The other three cited studies on this strain looked at type I interferon production (in humans and in pigs), inflammation and blood fats in overweight adults, and safety at high doses over a long period. Limitations worth knowing: this research comes largely from Japanese groups connected to the company that developed L-137, it has not been independently replicated, and it is all strain-specific.