The main outcomes of our pilot study were providing TBI patients with early enteral feeding, short-term (seven days) caloric restriction associated with fewer episodes of gastrointestinal intolerance, improved glycemic control, shorter duration of mechanical ventilation, and a trend towards reduced length of hospital stay compared to the early standard enteral feeding.
Previous studies investigating the association between the level of caloric intake and clinical outcomes in critically ill patients rendered conflicting results [6, 9, 13–14, 16–17, 23–25, 40–42], possibly due to heterogeneity in their design, patient population, feeding strategies and formulas, as well as diversity in the route of delivery and feeding rate and timing.
Although some similarities were present between the design of our study and that of the previous ones [13, 25, 41], to the best of our knowledge, this study was the first attempt to compare the effect of short-term EEPU versus EESF in young non-malnourished adults with TBI admitted to ICU; besides, enteral feeding of patients in the EEPU group was initiated at a rate of 30% of calculated energy requirements, and gradually advanced until the patients received 100% of the calculated energy requirements by the end of week one after initiating the intervention.
Our findings did not demonstrate a significant difference in 28-day and ICU mortality rates between the two groups, which could be due to various reasons. Caloric intakes of both study groups were below what was planned, particularly during the first week in the EESF group (achieved: 77.2% vs. planned: 100%). Difficulty to achieve the nutritional targets in ICU, specifically in the intervention studies planning to reach target calorie goals was well documented by former studies [9, 13, 25, 38, 41].
In our study, extended episodes and frequencies of gastrointestinal intolerance including slow gastric emptying, elevated gastric residual volumes, diarrhea, distention, and cramps resulted in lower energy intake, particularly in the EESF group compared to the EEPU group. Accordingly, a trend towards a higher number of NPO days was recorded in the EESF group compared with the EEPU group (1.1 ± 1 vs. 0.6 ± 1).
Results of some studies suggested higher risk of Clostridium diffcile-associated diarrhea with use of specific pharmacological interventions such as proton-pump inhibitors and certain classes of antibiotics [43–45]. However, we believe that a significant difference in the use of drugs was not present between our study groups, as the percentage of patients receiving antibiotics (22% vs. 23%) and acid suppressants (25% vs. 21%) in the EEPU and EESF groups were alike.
Similarly, Rice et al. [41] and Ibrahim et al. [13] proposed that aggressive early enteral nutrition was associated with increased gastrointestinal complications in critically ill patients with acute respiratory failure. On the other hand, the gradual increase in caloric intake in the EEPU group possibly diminished our chance to provide patients with sufficient caloric restriction to observe a significant difference in the mortality rate, which is in accordance with several previous observational studies [14, 17].
Furthermore, the small sample size possibly undermined the likelihood of observing significant treatment effects, as it was based on the anticipated treatment effects resulting from observational studies [46]. Contrary to the commonly used outcome measure of 28-day all-cause mortality in randomized controlled trials, long-term outcome measures such as 90-day and 180-day mortality rates might detect the effect of calorie restriction on the survival and mortality rate of critically ill patients more efficiently. In this regard, results of larger RCTs with long-term outcome measures and follow-ups in different clinical settings such as PermiT Trial [10] are awaited.
Our data showed that the patients in the EEPU group had fewer days on mechanical ventilation and less days of hospitalization. The results of clinical trials and observational studies still lack conclusive evidence on this issue. In the pioneering cohort study of 48 critically ill surgical patients, Villet et al. [24] demonstrated that cumulative energy deficit is associated with prolonged duration of mechanical ventilation and ICU stay.
Discordantly, Dvir et al. [23] propounded that maximum negative energy balance does not have a significant effect on mechanical ventilation duration as well as length of ICU and hospital stay. Conflictingly, results of a retrospective analysis on 40 trauma and surgical obese patients by Dickerson et al.16 demonstrated that the patients in hypocaloric group had fewer days on mechanical ventilation compared to the patients in full-caloric group (16.6 ± 11.7 vs. 27.4 ± 17.3 days). Similar to our findings, Ibrahim et al. [13], pointed out that mechanically ventilated medical patients in hypocaloric group had a reduced incidence of ventilator-associated pneumonia (30.7% vs. 49.3%, P = 0.020) and fewer days of hospital stay (16.7 ± 12.5 vs. 22.9 ± 19.7 days).
Most recently, the results of EDEN study, a multicenter trial conducted by the National Heart, Lung and Blood Institute on acute respiratory distress syndrome, showed that patients with acute lung injury who received full enteral feeding suffered from gastrointestinal intolerance more than trophic feeding group [47].
Our findings showed that progressive advancement of caloric intake in TBI patients resulted in more efficient glycemic control and a concomitant decrease in the incidence of hyperglycemia. As with similar previous studies, Mc Cowen et al. [48] reported no significant differences in the incidence of hyperglycemia in critically ill patients who received either the isocaloric total parenteral nutrition (TPN) regimen (25 kcal/kg, 1.5 g/kg protein) or the hypocaloric TPN regimen (1000 kcal/day, 70 g of protein); however, the study was limited due to fixed energy intake in the latter group regardless of the patients’ weight and the difference in TPN formulation between the two groups. The results of EDEN randomized trial [47] delineated that the mean plasma glucose value was lower in the trophic-feeding group during the first six days of enteral nutrition compared with full-feeding group.
The present study should be highlighted through its strengths and limitations. Its strengths include blinded design (RCT), random assignment of the enrolled patients using concealed envelopes, and well-balanced distribution of patients in the two study groups. All our patients were healthy young adults who had a minimum history of chronic diseases or malnutrition before admission to ICU. Follow-up examinations were completed for all the patients.
Our study had several limitations; it was a single-center study, mainly conducted in the trauma ICU and therefore, we cannot be sure that our findings are generalizable to other patient populations. Unfortunately, we did not achieve the predicted energy requirements, particularly in the EESF group. According to the consensus guideline recommendations [29, 49–50] and the practical routine in our ICU, energy requirements were calculated using the American College of Chest Physicians equations rather than the calorimetric methods; hence, we cannot be confident that our estimated energy requirements are based on the actual metabolic demands of all our patients.
Given these limitations, further study is warranted to optimize nutritional strategies and routes of delivery and to ascertain that all patients receive energy requirements based on the actual metabolic demands as well as their entire calculated protein requirements. By undertaking a large, randomized, controlled, multicenter trial, the quality of these data could be promoted.
In conclusion, the strategy of early enteral permissive underfeeding for up to seven days may be associated with favorable effects on certain clinical outcomes such as fewer gastrointestinal complications, improved glycemic control, and reduced length of mechanical ventilation when compared with standard early enteral feeding.