Effects of transcranial direct current stimulation on the gut microbiome: A case report

The imbalance of the gut microbiome or dysbiosis has been implicated as a cause of obesity [1] and brain disorders [2]. Eat well is one of the most effective tools to relief metabolic or brain disorders. However, this practice is difficult to be followed by health agents and patients. Transcranial direct current stimulation (tDCS) is a non-invasive neuromodulation method that has been widely investigated in recent years [3]. The method consists of applying a low intensity continuous electric current over the scalp. When applied for 20e30 minutes it is able to alter significantly the cortical excitability. For this reason, this method has been used in the treatment of food craving and overweight [4]. On the other hand, none of the studies evaluated the impact of tDCS on the gut microbiome. Considering the bidirectional mechanisms of the brain-gut axis, it is conceivable to think that tDCS could also induce a significant change in the composition of the gut microbiome and, therefore, induce therapeutic effects. A 38-year-old woman diagnosed with overweight and cravings for sugary foods ten years ago, according to the WHO specification, presented herself in October 2019 at the Neuromodulation Clinic in Porto Alegre-RS. According to her unsuccessful weight loss attempts history, a 10-week treatment of transcranial direct current stimulation (tDCS) was provided. Therefore, the patient filled out the free and informed consent form, agreeing with the use of the tDCS and microbiome analysis using the 16S molecular technique. The tDCS involved the application of weak direct electric current (2 mA) through two electrodes (5 5 cm) embedded in saline solution and placed on the scalp, with the anode on the right dorsolateral prefrontal cortex and the cathode on the contralateral supraorbital region, for 20 minutes a day, twice a week [5]. This montage was based on previous studies for the treatment of cravings by drugs and food [4]. Three stool samples were also collected by swab for intestinal microbiome analysis in the pre-intervention moments, after a session of tDCS (acute effect) and after 20 sessions of tDCS (chronic effect), through the sequencing of the human fecal 16S ribosomal RNA gene. No dietary intervention was performed during treatment. It was observed a decrease in the bacteroidetes/firmicutes ratio from 8.5 to 5.2 after a single session of tDCS and from 5.7 after 20 sessions, represent an improvement of 38.8% and 33% for acute and chronic effects respectively. The patient showed a quick and significant decrease in the firmicutes proportions which were most prevalent, with an increase in Roseburia intestinalis and Faecalibacterium prausnitzii, a gradual and sustained effect until the 10th week. There was also an increasing number in the bacteroidetes