Abstract: The searches identified 9900 titles and, after excluding duplicates, 6433 titles were initially screened. After the initial title screen, 6266 were excluded. Abstracts were screened for 167 studies and 33 articles were identified as meeting the inclusion criteria, of which 13 were included in the review after an assessment of the complete manuscripts. Seven trials evaluating IE nutrition were included in the review, of which 6 were combined in a meta-analysis. These studies showed a low to moderate level of heterogeneity and significantly reduced total post-operative complications (risk ratio (RR) 0.67 CI 0.53 to 0.84). Three trials evaluating PN were included in a meta-analysis and a significant reduction in post-operative complications was demonstrated (RR 0.64 95% CI 0.46 to 0.87) with low heterogeneity, in predominantly malnourished participants. Two trials evaluating enteral nutrition (RR 0.79, 95% CI 0.56 to 1.10) and 3 trials evaluating standard oral supplements (RR 1.01 95% CI 0.56 to 1.10) were included, neither of which showed any difference in the primary outcomes. There have been significant benefits demonstrated with pre-operative administration of IE nutrition in some high quality trials. However, bias was identified which may limit the generalizability of these results to all GI surgical candidates and the data needs to be placed in context with other recent innovations in surgical management (eg-ERAS). Some unwanted effects have also been reported with components of IE nutrition in critical care patients and it is unknown whether there would be detrimental effects by administering IE nutrition to patients who could require critical care support after their surgery. The studies evaluating PN demonstrated that the provision of PN to predominantly malnourished surgical candidates reduced post-operative complications; however, these data may not be applicable to current clinical practice, not least because they have involved a high degree of 'hyperalimentation'. Trials evaluating enteral or oral nutrition were inconclusive and further studies are required to select GI surgical patients for these nutritional interventions.

Plain-language summary: Results showed that studies evaluating oral drinks with added nutrients to assist fighting infections ('immune enhancing') given before an operation could reduce total complications from 42% in the control group to 27% in those who received the drinks, while infections were reduced from 27% in the control group to 14% in the group given the drinks. Parenteral nutrition reduced total complications from 45% in the control group to 28% in the group receiving parenteral nutrition. There were no benefits demonstrated for either enteral or standard supplement drinks. Thus, some benefits have been demonstrated from giving nutritional support to patients before an operation with immune enhancing drinks and with parenteral nutrition. However, studies on parenteral nutrition were over 20 years old and during that time there have been many changes to surgical practice. Quality assessment of studies on PN was generally low. Immune enhancing drinks have only been tested with selected surgical patients and some unwanted effects have also been reported using these drinks in critical care patients; it is therefore unknown whether there would be detrimental effects by giving these drinks to patients who could potentially require critical care support after their surgery. No benefit of standard oral or enteral feeding was demonstrated and more research is required in these areas with malnourished patients.