In this retrospective descriptive cohort study of sTBI conducted at a single Level-I trauma center, those who received immunonutrition with n-3 PUFA-containing formulas for EN were slightly younger and more likely to be male, Hispanic/Latinx, have sustained non-CNS injury in addition to TBI, and have undergone emergent non-CNS surgery due to their injuries. The greater likelihood of being prescribed n-3 PUFA-containing immunonutrition in patients with polytrauma and in those requiring non-CNS surgery is not unexpected, given the high protein requirements believed to be necessary to support increased metabolic demand and promote wound healing after general traumatic injury and surgery8,14. Further, the results also suggest that in clinical practice, the decision to prescribe EN with immunonutrition is not based on the presence of trauma alone. Specifically, older patients, female gender, and those with isolated TBI may not be considered to require similar nutritional interventions compared to younger male patients with polytrauma. Particularly for isolated TBI, this might be an incorrect assumption, as sTBI patients may need up to 200% of their baseline metabolic requirements15, and with similarly high rates of malnourishment (68–76%) reported in both polytrauma and TBI populations8,16. In addition, given inflammation and infectious risk are universal regardless of trauma type, further clinical study is needed to better define the nutritional needs and clinical targets for TBI patients, especially in those at high-risk for malnutrition and poor functional outcomes.
Immunonutrition with n-3 PUFA supplementation compared to standard, polymeric EN formula was not associated with any statistically significant differences in clinical markers of inflammation and immunity. Specifically, peak 14-day WBC and CRP, 14-day fever burden and calendar days with fever were nearly identical between study groups. Although the incidence of culture-positive infection was lower in those receiving immunonutrition, this did not reach statistical significance (80.9% versus 86.0%, p = 0.42). Given the proposed mechanism of immunonutrition with n-3 PUFA supplementation is to attenuate systemic inflammation and promote immune function, these results might be interpreted as unexpected; however, several circumstances should be considered. These outcome measures were chosen because they are assessable from retrospective medical record review but are likely non-specific and crude measures of relevant anti-inflammatory activity that may be altered by several confounding factors. The study population was also severely injured (average GCS = 4.8), which may have blunted the effect of the nutritional intervention.
The culture-positive infection rate found in this study is higher than many hospital-acquired infection rates reported in moderate-severe TBI17,18 and hospitalized trauma patients19. Both trauma and TBI are thought to increase the risk of in-hospital infection20,21, while multiple surgical interventions further increase infection risk22. Given the features of our study population, subjects were at high infection risk. Lastly, rates of infection in TBI populations are based on early retrospective report, while true infection risk is likely underestimated. Our reported rate of lower respiratory infection (78.2%) is comparable to recent literature described in a prospective surveillance study (72.2%)23. Given nosocomial infection after TBI is associated with worse outcomes17,21, the results also highlight the clinical problem of infection risk following TBI and the need to identify novel approaches to prevent this common complication.
Despite no identified differences in clinical markers of inflammation and immunity, there were important differences in clinical outcomes observed between the two study groups. In multivariate linear regression analysis, EN with immunonutrition and n-3 PUFA supplementation resulted in 10.7 fewer hospital days compared to standard EN (-19.3, -2.0, p = 0.02). In pre-planned subgroup analysis, this appeared to be primarily driven by differences in those with polytrauma (-28.3, -4.9, p = 0.006). Similarly, ICU LOS was significantly shorter in the polytrauma subgroup. Although there was no statistically significant difference with in-hospital mortality between study groups in multivariate logistic regression analysis, there was a 0.7 rate ratio for mortality with treatment compared to standard nutrition (-0.6, 1.9, p = 0.53). From this study, it is unclear why immunonutrition with n-3 PUFA supplementation is associated with positive outcomes, but the results are sufficiently promising to warrant prospective study in clinical efficacy trials in this subgroup of the trauma population. It is possible that immunonutrition with n-3 PUFAs positively impacts energetics and optimizes nutritional status in ways that we were not able to capture in a retrospective study. Particularly for n-3 PUFA supplementation, the pre-clinical data is robust. In rodent models of TBI, eicosanoid n-3 PUFA therapy has been demonstrated to be neuroprotective against inflammation-mediated oxidative stress and pro-apoptotic signaling after TBI24–26, and can reduce pro-inflammatory signaling, maintain mitochondrial integrity, and reduce infarct size in stroke models27–29. Additional amino acid components of immunonutrition, specifically arginine and glutamine, have been shown to support the synthesis of several cellular proteins vital to oxidative metabolism, immune function, and cellular repair30–32.
Although this was not a prospective study of early EN (< 48-72H from admission), prescription of enteral immunonutrition initiated with a mean of 50.8H after admission in our study population appeared safe. Although a comprehensive evaluation of ICU complications was out of the scope of this study, the use of early EN immunonutrition therapy was not associated with weight loss, increased NPO days, day 14 ileus, or increased infection risk compared to standard therapy. This was an important finding as results from the general ICU population have been conflicting regarding the safety of early EN, partly due to a concern for aspiration-related respiratory infections33. The capability to resolve inflammation without compromising immune function is one of the positive aspects of n-3 PUFA supplementation and immunonutrition, and because these components are naturally occurring with no known serious adverse effects, their supplementation is considered safe with a large therapeutic window12. Early EN may be a clinically important intervention in TBI. It is estimated that every 10 kcal/kg/day increase in energy intake is associated with a 30–40% reduction in mortality risk34.
Important limitations to this study should be considered. In our retrospective analysis, data quality was dependent on the accuracy of electronic documentation. Given this anticipated limitation, study outcome measures were selected that were objective and were commonly documented as part of standard practice. This study lacked more precise measures of nutritional status, which could not be accurately measured by chart review. Additionally, due to the retrospective nature of the study, we were unable to control all potential sources of bias and effect modification. The justification of selecting one nutrition intervention versus another was not standardized or documented. Nutrient components also differ between each EN formula, and we could not accurately control for % caloric, protein, fat and carbohydrate needs met in retrospective analysis. Also, those who received EN with immunonutrition including n-3 PUFA supplementation may have received differential management compared to those receiving standard therapy based on the patient characteristics discussed. As with any observational study, correlation does not prove causation. Given the stringent inclusion criteria, the study population was relatively small. A larger study sample would provide a greater power in detecting clinically meaningful differences, especially in subgroups. Future blinded, randomized trials are needed to elucidate the direct impact of immunonutrition formulas on the inflammatory response and short and long-term patient outcomes after moderate-severe TBI. Regarding generalizability, our US-based study population represented a very severe TBI cohort with a large proportion of Latinx and male subjects and may not be fully generalizable to other patient populations and regions of practice. The practicality of the intervention may also be influenced by the availability of specific enteral nutrition products in different sites; however, n-3 supplementation products are widely available in many countries and are relatively low-cost.
The emerging science in both pre-clinical and human populations of both n-3 PUFA supplementation and other amino acid immunonutrition warrants further study in prospective, pragmatic, randomized clinical efficacy trials. Additional correlations with biomarkers will be helpful in describing the molecular mechanisms as how immunonutrition improves clinical outcomes and to optimize proposed nutritional interventions. If mechanistic activity and positive impact on clinical outcomes can be demonstrated in clinical study, routine incorporation of n-3 PUFA supplementation and immunonutrition with EN would be a novel approach to optimize ICU nutritional status and promote recovery after moderate or severe TBI in thousands of patients annually. Products that provide enriched amounts of n-3 PUFAs and amino acids are already in clinical practice, are widely available, and are relatively low-cost, supporting their rapid translation and implementation into standards of ICU practice if benefit can be shown. Guideline recommendations do provide some support for these interventions in general ICU populations35,36, but further rigorous study is needed in patients with TBI37.