At present, the association between chewing gum and POI is not fully understood. Increasing evidence that chewing gum reduces recovery time in abdominal surgical patients has been available for many years. The treatment effect varies somewhat according to the type of surgery and indication of surgery, but the effect is most accordance. It is well accepted that chewing gum as a type of sham feeding stimulates bowel motility.[10, 11] However, whether chewing gum reduces POI remains controversial. This study systematically estimates the effects of chewing gum for postoperative treatment of ileus following abdominal surgery and conjectures the potential benefits of its use, if possible, providing an inexpensive, well-tolerated, and widely available solution to ameliorate an old problem. Our meta-analysis of 17 RCTs provides evidence that chewing gum significantly reduces recovery time following abdominal surgery. Patients in the chewing gum treatment group, compared with the reference group, experienced a significant reduction of 0.31 days for time to first flatus, 0.51 days for time to first bowel movement, 0.72 days for LOS (Figs 3–5).

Mechanisms of Chewing Gum Reduce POI

The underlying mechanism involved in the association between chewing gum and POI is uncertain. One possible explanation is that chewing acts as sham feeding, stimulating the motility of human stomach,[48] duodenum,[11] and rectum.[10] Another explanation is chewing may trigger the release of gastrointestinal hormones[49] and increase the secretion of saliva and pancreatic juice, gastrin, and neurotensin.[48] Thus, it seems that the mechanisms are multimodal. However, for an intervention that is so cheap, effective, well tolerated, and free of side effects, it may be used clinically even before knowing the mechanism behind its success and important health and economic benefits.

Sources of Heterogeneity

Heterogeneity is a potential problem that may affect the interpretation of the results. The present meta-analysis showed that there was large heterogeneity between studies (Table 2). Subsequent subgroup analysis stratified by indication of surgery, type of surgery, and quality of study identified large heterogeneity mostly as well, indicating that indication of surgery, type of surgery, and quality of study contributed little to the existence of overall heterogeneity. We performed sensitivity analysis on these subgroup analyses. On the colectomy subgroup, the studies by Crainic et al.[31] had longer duration time in the treatment group than the control and had 17 missing values in the results. Dropping this study yielded opposite results in time to flatus, with a statistical significance of 0.49 days shorten (P = 0.012), but also with a large heterogeneity (I2 = 69.7%). Sensitivity analysis showed that the study by Asao et al.,[12] Hirayama et al.,[13] and Crainic et al.[31] had a large impact on our results. The study patients in their report were less than 30. Dropping these three studies from our analysis did increase the benefit seen from chewing gum with regard to time to first flatus and time to first bowel movement, but had no significance on LOS.

Twelve trials underwent open surgery, one underwent laparoscopic surgery,[12] and two underwent both.[26, 31] The results for the cohorts were analyzed separately on subgroup analysis. But on the laparoscopic surgery studies, only two of them[12, 26] reported time to first bowel movement and LOS as outcomes. Although a trend toward shorter duration of time to flatus, first bowel movement and LOS in patients who underwent laparoscopic surgery, none of the results were significant. Laparoscopic surgery is known to reduce the inflammatory response, and in so doing, promotes a faster recovery.[50] Many studies suggest a significant reduction in LOS on laparoscopic surgery compared with the open technique.[51, 52] The explanations of incongruent results in our subgroup analysis may be owing to: (i) the small number of cases and participants of the three trials increased the possibility that chance accounted for their results; (ii) the study by Crainic et al.[31] reported both open and laparoscopic procedure results, having incomplete data and even with longer duration time in treatment group longer than control; (iii) the study by Asao et al.[12] had relatively longer hospital stays than other studies (mean 13.5 days for gum-chewing group, 14.5 days for the control group). It is worth mentioning that no definitions of discharge criteria were given in any of the studies to help interpret LOS outcomes. These may explain the large heterogeneity between laparoscopic surgery studies in some degree. Therefore, considerably larger and more rigorous studies are needed to determine the effect of chewing gum on duration of postoperative hospital stay after laparoscopic surgery.

Study Strengths and Limitations

A major strength of our study is that all the included original studies used an RCT design, which has well comparability of the two groups and minimizes selection bias. Moreover, we included all abdominal surgery and then performed subgroup analysis based on different indication of surgery. There is sufficient evidence of 17 RCTs to conclude that chewing gum shortens time of outcomes in patients undergoing abdominal surgery. Although some studies had conducted a meta-analysis of relation between chewing gum and POI and demonstrated a significant effect, all were only about one surgery indication.[34–37] With the accumulating evidence and enlarged sample size, we have enhanced statistical power to provide more precise and reliable efficiency estimates. We have shown that despite variation in results from each study, overall, the published evidence supports the hypothesis that gum chewing reduces the duration of POI. In addition, no publication bias was detected in this meta-analysis, which indicated that the pooled results of our study should be reliable.

One potential limitation of the present meta-analysis was that only one study included a double placebo group to assess the treatment effect,[27] but because of their differing method of reporting results, not all of their data could be included in our analysis. Another study by Choi et al.[22] was excluded because of insufficient data that rendered the meta-analysis impossible. The study by Choi et al.[22] underwent open and laparoscopic cystectomy for bladder cancer surgery and found a significant time for the amelioration of ileus. A second limitation is the lack of blinding in most studies, leading to potential bias by the investigator recording the results. Double blinding should be difficult in this project, but blinding the observer is achievable and would reduce bias of the results. A third limitation is the substantial heterogeneity among studies. Despite assessing outcomes only in patients undergoing colectomy, it also identified large heterogeneity. Nevertheless, we were able to detect the major source of heterogeneity through the sensitivity analyses. In other hand, residual confounding is of concern. Uncontrolled or unmeasured confounding factors such as opioids, epidural analgesia, and early enteral feeding etc. potentially produce biases. Unfortunately, several studies did not state postoperative practice in this respect. Furthermore, because current data in efficacy of chewing gum after laparoscopic surgery are sparse, we were unable to assess consistent results for these outcomes. New trials with better design are necessary for patients who undergo laparoscopic surgery before gum chewing becomes a routine feature of the postoperative management.
In conclusion, the current evidence suggests that gum chewing following abdominal surgery offers significant benefits in reducing the time to resolution of POI and LOS. However, for the subgroup of colectomy and laparoscopic surgery, inconsistent results were presented. Well-designed, large-scale, blinded, randomized, controlled trials with a placebo arm studies were urgently needed to answer the question of whether gum chewing can significantly reduce POI in different abdominal surgeries.