Discussion
Probiotics supplements are growing in popularity globallyIn searching for markers that distinguish response groups, we found that circulating levels of homovanillic acid (HVA) differentiated responders and non-responders with fairly high accuracy (71%). This difference in HVA between groups appears to be due to declining levels of HVA over the course of the study in the non-responder group. HVA is a product of human metabolism, a major metabolite of dopamine, and its circulating levels are indicative of dopaminergic activity.The microbiome is linked to host metabolism, and dietary interventions that target the microbiome can alter human immune status and metabolism.One predictive feature of probiotic response group was differences in dietary intake of total and added sugars, lactose, and sucrose throughout the study. Probiotic responders consumed more of these nutrients relative to non-responders, however it is important to note that since consumption of these nutrients did not differ between either group and the placebo group, differences were likely driven by both increased consumption by responders and decreased consumption by non-responders. This difference in sugar consumption between groups is counter-intuitive given the link between sugar consumption and poor metabolic health.The lack of an arm-wide response to the probiotic intervention could be a result of participant heterogeneity, insufficient sample size, or insufficient length of probiotic intervention. Longer probiotic interventions (> 3 months) have been shown to be beneficial in controlling fat mass and lipid profiles.