Nutrients 2023 , 15 , 3466
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suggested that the lower tryptophan levels in the probiotic group could have been caused by enhanced serotonin biosynthesis. In the present study, the biomarker brain-derived neurotrophic factor (BDNF) was measured. BDNF is important for the survivability and growth of neurons and thus affects learning and memory [33]. It has been shown that intake of two Bifidobacterium strains for 12 weeks by older adults ( ≥ 65 years) increased the BDNF level significantly compared to placebo [34]. However, no change over time in BDNF after intake of LPHEAL9 was found in the moderately stressed subjects, aged 21 to 52 years, included in the present study. Previous studies including relatively young subjects support the present results of no differences in BDNF [35–37]. Elevated stress has been connected to increased pro-inflammatory properties; therefore, three pro-inflammatory markers were used in this study. Galectin-3 has been shown to be involved in different diseases associated with chronic inflammation, cancer, and type 2 diabetes [38]. To our knowledge, the effect of probiotics on galectin-3 in moderately stressed subjects has not been studied previously. No clear results for this biomarker were found in the present study, probably due to the subjects included being healthy with normal levels of galectin-3 at baseline. The same theory can be applied to CRP, where no changes were observed over time. Fractalkine is another pro-inflammatory marker that is prominent in the intestinal epithelium [39]. It is produced in response to different inflammatory stimuli and has been shown to be involved in conditions such as atherosclerosis, cancer, rheumatoid arthritis, and IBD [40]. In an earlier study on LPHEAL9, the level of fractalkine was significantly lower in the LPHEAL9 group compared to the placebo group after an acute stress test (TSST) [4]. In the present study, there was a trend for a similar result, with a higher fractalkine level found in the placebo group ( p = 0.085), while the fractalkine level remained unchanged in the LPHEAL9 group ( p = 0.883) after 12 weeks of intervention compared to the baseline ( p = 0.133 between groups). In the present study, the results may have been influenced by the stressful situation of facing a cognition test, since the blood sample for fractalkine analysis was taken just before the initiation of these tests. The anti-inflammatory marker TGF- β was also measured, and comparable to the results of the pro-inflammatory marker fractalkine, a change in TGF- β was seen in the placebo group ( p = 0.027), while no change over time was observed in the LPHEAL9 group. The more stable inflammatory status in the probiotic group compared to the placebo group may be one underlying mechanism of the observed positive effect of LPHEAL9 on cognition. Different gastrointestinal metabolites, such as short-chain fatty acids, branched-chain amino acids (BCAA), and peptidoglycans, can also affect the gut–brain axis [41]. It is not known how LPHEAL9 influences the different metabolites. However, a recent publication showed that intake of LPHEAL9 together with L. paracasei 8700:2 for six months by children with celiac disease autoimmunity affected the fecal metabolome, significantly decreasing the BCAA threonine level and increasing the 4-hydroxyphenylacetate level compared to the placebo group [42]. This study has some limitations. As this was an exploratory trial without any pre- existing data as a basis for power calculations, the study was underpowered for some variables, which restricted the interpretation of the data. Furthermore, a fecal microbiota analysis could have added data for a suggestive mechanism of the actions behind the observed effects of LPHEAL9. A final limitation of this study was that placebo effects were observed for several variables. However, this may be difficult to overcome when evaluating subjective measurements in healthy and fairly young individuals and when the baseline scores for some of the endpoints were near normal, making it difficult to demonstrate an improvement. In future studies, measurements less influenced by a placebo response should preferably be selected, for example, by including additional performance challenges. 5. Conclusions The present study was designed to evaluate the effect of the probiotic bacteria Lacti- plantibacillus plantarum HEAL9 on moderately stressed but otherwise healthy individuals.
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