BackgroundMalnutrition is a common problem in critically ill patients in UK NHS critical care units. Early nutritional support is therefore recommended to address deficiencies in nutritional state and related disorders in metabolism. However, evidence is conflicting regarding the optimum route (parenteral or enteral) of delivery.ObjectivesTo estimate the effect of early nutritional support via the parenteral route compared with the enteral route on mortality at 30 days and on incremental cost-effectiveness at 1 year. Secondary objectives were to compare the route of early nutritional support on duration of organ support; infectious and non-infectious complications; critical care unit and acute hospital length of stay; all-cause mortality at critical care unit and acute hospital discharge, at 90 days and 1 year; survival to 90 days and 1 year; nutritional and health-related quality of life, resource use and costs at 90 days and 1 year; and estimated lifetime incremental cost-effectiveness.DesignA pragmatic, open, multicentre, parallel-group randomised controlled trial with an integrated economic evaluation.SettingAdult general critical care units in 33 NHS hospitals in England.Participants2400 eligible patients.InterventionsFive days of early nutritional support delivered via the parenteral (n = 1200) and enteral (n = 1200) route.Main outcome measuresAll-cause mortality at 30 days after randomisation and incremental net benefit (INB) (at £20,000 per quality-adjusted life-year) at 1 year.ResultsBy 30 days, 393 of 1188 (33.1%) patients assigned to receive early nutritional support via the parenteral route and 409 of 1195 (34.2%) assigned to the enteral route had died [p = 0.57; absolute risk reduction 1.15%, 95% confidence interval (CI) −2.65 to 4.94; relative risk 0.97 (0.86 to 1.08)]. At 1 year, INB for the parenteral route compared with the enteral route was negative at −£1320 (95% CI −£3709 to £1069). The probability that early nutritional support via the parenteral route is more cost-effective – given the data – is < 20%. The proportion of patients in the parenteral group who experienced episodes of hypoglycaemia (p = 0.006) and of vomiting (p < 0.001) was significantly lower than in the enteral group. There were no significant differences in the 15 other secondary outcomes and no significant interactions with pre-specified subgroups.LimitationsBlinding of nutritional support was deemed to be impractical and, although the primary outcome was objective, some secondary outcomes, although defined and objectively assessed, may have been more vulnerable to observer bias.ConclusionsThere was no significant difference in all-cause mortality at 30 days for early nutritional support via the parenteral route compared with the enteral route among adults admitted to critical care units in England. On average, costs were higher for the parenteral route, which, combined with similar survival and quality of life, resulted in negative INBs at 1 year.Future workNutritional support is a complex combination of timing, dose, duration, delivery and type, all of which may affect outcomes and costs. Conflicting evidence remains regarding optimum provision to critically ill patients. There is a need to utilise rigorous consensus methods to establish future priorities for basic and clinical research in this area.Trial registrationCurrent Controlled Trials ISRCTN17386141.FundingThis project was funded by the NIHR Health Technology Assessment programme and will be published in full inHealth Technology Assessment; Vol. 20, No. 28. See the NIHR Journals Library website for further project information.