Higher Body Mass Index Linked to Worse Intensive Care Outcomes After Severe Trauma

New registry-based research suggests that higher body mass index (BMI) is associated with increased organ failure, longer intensive care stays, and higher in-hospital mortality among severely injured patients, offering no evidence to support a protective “obesity paradox” in trauma-related critical illness.

The findings come from a retrospective analysis published in Frontiers in Anesthesiology, which examined intensive care outcomes among trauma patients included in the German TraumaRegister DGU® . Investigators analyzed data from 5,766 patients admitted to the intensive care unit (ICU) between 2005 and 2009 and conducted a matched cohort analysis to assess outcomes across BMI categories while controlling for injury severity, age, sex, injury pattern, and treatment approach.

Patients were grouped into three BMI categories: underweight (≤20 kg/m²), normal weight and overweight (20.1–29.9 kg/m²), and obese (≥30 kg/m²). Because very low and very high BMI categories were uncommon in the registry, the authors used broader groupings rather than standard World Health Organization classifications to preserve statistical stability. From the overall cohort, a matched sample of 768 patients was constructed using a 1:2:1 ratio to allow balanced comparisons across BMI strata.

Across the full cohort, higher BMI was consistently associated with greater morbidity during the ICU course. Organ failure occurred in just over 40 percent of patients overall, with a statistically significant linear increase across BMI categories. Obese patients experienced higher rates of cardiovascular and pulmonary failure, both of which rose in frequency with increasing BMI. Multiple organ failure followed a similar pattern, occurring more often among patients with obesity than among those with lower BMI.

Resource utilization also differed by BMI. In survivors, the duration of invasive mechanical ventilation increased with higher BMI, as did length of ICU stay and total hospital stay. Median ICU stays ranged from five days in underweight patients to eight days in obese patients, while total hospitalization increased from a median of 20 days to 27 days across the same groups. Among patients who did not survive hospitalization, obese individuals also had significantly longer durations of ventilation, ICU care, and hospital stay before death.

Mortality analyses showed a graded relationship with BMI. In the matched cohort, hospital mortality was approximately twice as high in obese patients compared with underweight patients, with a statistically significant linear trend across BMI groups. Early mortality within 24 hours of admission did not differ significantly, suggesting that the observed differences emerged later during the ICU course rather than during initial resuscitation.

A predefined subgroup analysis focused on patients with polytrauma—defined as serious injuries in at least two body regions—yielded similar patterns, although some differences did not reach statistical significance due to smaller sample size. In this subgroup, cardiovascular failure remained significantly more common among obese patients, while trends toward higher mortality and longer ICU stays were observed but were not statistically definitive.

The authors also examined baseline characteristics and mechanisms of injury. Obese patients were older on average and had a higher prevalence of pre-existing medical conditions. Traffic-related accidents were more common among patients with higher BMI, whereas underweight patients were more frequently injured as pedestrians, cyclists, or through falls. Injury severity scores, however, did not differ significantly across BMI groups, supporting the conclusion that outcome differences were not driven by more severe injuries in patients with obesity.

Laboratory findings at emergency department and ICU admission showed some statistically significant differences across BMI categories, but values largely remained within normal reference ranges and were not considered clinically meaningful. The study relied on standardized definitions of organ failure using Sepsis-related Organ Failure Assessment criteria, with failure defined as sustained dysfunction over at least two days.

Importantly, the investigators report no evidence of improved survival associated with higher BMI in this trauma population. While prior studies in other critical care settings have described an “obesity paradox,” in which overweight or obese patients appear to have better outcomes, this analysis did not replicate such findings in severely injured ICU patients. Instead, higher BMI was associated with longer and more complex ICU courses and higher overall mortality, though absolute differences in death rates were modest.

The authors note several limitations, including the retrospective design, reliance on BMI as a surrogate for body composition, and the historical nature of the data, which predate some modern advances in trauma and critical care. BMI does not distinguish between fat and lean mass or account for fat distribution, frailty, or metabolic health, factors that may influence outcomes independently. Residual confounding from comorbidities and survivorship bias cannot be excluded.

Despite these limitations, the study provides a large, carefully matched analysis of trauma ICU outcomes across BMI categories. The findings suggest that higher BMI is associated with less favorable intensive care trajectories following severe injury and that BMI alone should not be interpreted as conferring a survival advantage in this setting.