TABLE 2.

Mechanism research of included studies.

Reference	Detecting items	Research methods and techniques	Main results
Brain-gut peptides (neurotransmitters and GI hormones)
Hu et al., 2021	Serum NE and PP levels	Corresponding commercial kits	No significant alteration in NE and PP levels with TEA nor with sham TEA
Li, 2015	Serum VIP and 5-HT levels	ELISA	Decreased serum VIP and 5-HT levels in MA and greater than CONT (P < 0.05)
Zhenzhong et al., 2015	Colon tissue SP and VIP levels	Immunohistochemical staining	Decreased SP and VIP expression in both EA and Mox
Zhao et al., 2015	Sigmoid tissue 5-HT, 5-HT3R and 5-HT4R levels	Immunohistochemical staining	Decreased 5-HT3R and 5-HT4R expression in both EA and Mox; decreased 5-HT expression in Mox and greater than EA (P < 0.05)
Zhan et al., 2014	Serum 5-HT levels	ELISA	Decreased serum 5-HT levels in both MA and CONT
Cerebral activities
Zhao et al., 2015	Brain activation	fMRI	Decreased voxel values of PFC in EA
Chu et al., 2012	Brain activation	fMRI	Significantly higher activation at right insula, pulvinar and medial nucleus of the thalamus was observed in EA compared to SA
Neuroendocrine functions
Hu et al., 2021	Serum NE and PP levels	Corresponding commercial kits	No significant alteration in NE and PP levels with TEA nor with sham TEA
Schneider et al., 2007	Salivary cortisol levels Heart rate and blood pressure	Radioimmune assay ECG	Decreased cortisol concentrations in MA not in SA Parasympathetically decreased heart rate response in MA not in SA during orthostasis
Mental status
Sun et al., 2021	Depression	HAMD	Decreased HAMD scores in MA + EA and greater than EA (P < 0.05)
Zhao et al., 2015	Depression and anxiety	HAMD and HAMA	Decreased HAMD and HAMA scores in both EA and Mox (P < 0.05 or P < 0.01)
Inflammation and hypersensitivity of the bowels
Hu et al., 2021	Serum IL-10 and IL-6 levels	Corresponding commercial kits	No significant alteration in IL-10 and IL-6 levels with TEA nor with sham TEA
Wu et al., 2013	Serum IFN-γ, IL-2, IL-4, IL-10 and IFN-γ/IL-4	Not mentioned	Improved serum IL-4 and IL-10 in MA and greater than CONT (P < 0.05); decreased serum IFN-γ/IL-4 in MA and greater than CONT (P < 0.01)
Zhao et al., 2015	Rectal sensory thresholds	VAS	Significant increases in the urgent defecation perception thresholds and maximum pain perception thresholds were observed in both EA and Mox groups after treatment (P < 0.05 or P < 0.01)
Chu et al., 2012	Rectal sensation	Likert scale	A significant correlation (P < 0.005) between subjective rectal pain rating and brain activation was observed in hypothalamus, bilateral thalami and bilateral insula
Genetic polymorphism
Guo et al., 2021	5-HTTLPR	Genetic polymorphism	SS genotypes take over a higher proportion in IBS-D patients; curative effect of acupuncture was better in LS and SS genotypes than LL and the same genotype in CONT

CONT, control group; EA, electroacupuncture; ECG, electrocardiogram; ELISA, enzyme-linked immunosorbent assay; fMRI, functional magnetic resonance imaging; HAMA, Hamilton Anxiety Rating Scale; HAMD, Hamilton Depression Rating Scale; IFN-γ, interferon γ; IL, interleukin; MA, manual acupuncture; LS, SS, and LL, three genotypes in 5-HTTLPR; Mox, moxibustion; NE, norepinephrine; PFC, prefrontal cortex (the senior center of feeling pain); PP, pancreatic polypeptide; SA, sham acupuncture/TEA; SP, substance P; TEA, transcutaneous electrical acustimulation; VAS, visual analog scale; VIP, vasoactive intestinal peptide; 5-HT, 5-hydroxytryptamine; 5-HT3R, serotonin receptor 3; 5-HT4R, serotonin receptor 4; 5-HTTLPR, serotonin transporter.