Heat-killed Lactobacillus plantarum L-137 (HK L-137) promotes immune function in animals. In healthy people, T-cell proliferation was shown to be enhanced by taking 10 mg HK L-137 daily for 12 weeks. However, the safety and efficacy of higher doses or longer treatments have not yet been investigated in humans. To investigate the high-dose and long-term use effects of HK L-137 on immune-related safety and on host intestinal bacterial flora, 15 healthy volunteers took a daily HK L-137 (50 mg) preparation for 4 weeks. An additional 29 participants who regularly visited a clinic for health care took HK L-137 (10 mg) daily for 12 months. Measures for anthropometrics, hematology, biochemistry, and urinalysis were taken at scheduled timepoints for all participants. Stool and blood samples were also collected and evaluated for microbes and short-chain fatty acids (SCFA); isolated T-cells were assessed for levels of proliferation induced by phytohemagglutinin in the long-term study. Adverse events or shifts in clinical measures from normal ranges due to the dietary intervention were not observed in the high-dose or long-term studies. Long-term intake also did not result in immune exhaustion due to any chronic immunostimulation; ex vivo T-cell proliferation was significantly greater at 12 months than at baseline (p < 0.01). In addition, the Firmicutes/Bacteroidetes ratio in stool samples was significantly lower at 12 months than at baseline (p < 0.05) due to the long-term intake of the HK L-137. Lastly, fecal SCFA concentrations were significantly greater (p < 0.05) at 6 months than at baseline. From these data, it can be concluded that the efficacy of HK L-137 is maintained with no overt adverse effects as a result of high-dose and/or long-term consumption.
Keywords: Bacteroidetes; Firmicutes; Heat-killed bacteria; Lactobacillus plantarum L-137; T-cell proliferation; adverse event; microbiota; safety; short-chain fatty acids.