Abstract: We included 19 RCTs (1099 participants). The mean age of participants ranged from 47 to 66 years. Most studies (12/19) had fewer than 25 patients in each treatment group. Most studies (16/19) used immunonutrition formulas containing arginine, but there was variation in the actual products and amounts used, and in the length of intervention postoperatively. Follow-up time for outcome measurement varied considerably across studies, ranging from five days to greater than or equal to 16 months. Primary outcomes We found no evidence of a difference in the length of hospital stay (mean difference -2.5 days, 95% confidence interval (CI) -5.11 to 0.12; 10 studies, 757 participants; low-quality evidence). Similarly, we found no evidence of an effect of immunonutrition on wound infection (risk ratio (RR) 0.94, 95% CI 0.70 to 1.26; 12 studies, 812 participants; very low-quality evidence). Fistula formation may be reduced with immunonutrition; the absolute risks were 11.3% and 5.4% in the standard care and immunonutrition groups, with a RR of 0.48 (95% CI 0.27 to 0.85; 10 studies, 747 participants; low-quality evidence). We found no evidence of a difference in terms of tolerance of feeds ('adverse events') between treatments (RR 1.33, 95% CI 0.86 to 2.06; 9 studies, 719 participants; very low-quality evidence). Secondary outcomes We found no evidence of a difference between treatments in all-cause mortality (RR 1.33, 95% CI 0.48 to 3.66; 14 studies, 776 participants; low-quality evidence). Other postoperative complications such as pneumonia and urinary tract infections were not commonly reported. The risk of postoperative fistula formation may be reduced with immunonutrition, but we found no evidence of an effect of immunonutrition on any of the other outcomes that we assessed. The studies included in this review were generally small or at high risk of bias (or both). We judged the overall quality of the evidence to be low for the outcomes length of hospital stay and all-cause mortality, and very low for wound infection and adverse events. Further research should include larger, better quality studies.

Plain-language summary: We included 19 studies that recruited 1099 adults in total (studies ranged in size from 8 to 209 participants, but most (12 out of 19) had fewer than 25 participants per treatment group). The studies focused on people who were given immunonutrition or a standard feed before and after or only after their surgery. The studies varied in the length of time over which people were given the feeds, but this was usually at least five days. The evidence is current to February 2018. We did not find evidence of a difference in the length of hospital stay but there was wide variation between the individual studies in what they showed. We found some evidence that people who had immunonutrition may be about half as likely to have breakdown of their surgical wound called a fistula (a channel between the inside of the throat and the surface skin). We found no evidence that immunonutrition had any effect on wound infection (but not all studies were clear in how they measured this) or death. Study feeds were generally well tolerated and there was no evidence of a difference in adverse events such as diarrhoea between treatment groups. Other clinical complications such as pneumonia and urinary tract infections were not commonly reported, but there was little evidence of a reduction with immunonutrition. Most studies included in this review were small and poorly reported, which means that their results may be less reliable. More studies are needed that are larger, of better quality and conducted within current healthcare systems.