Friday, October 02, 2009
While Travis and I often focus on the cardiometabolic and sometimes psychological complications of obesity, the consequences of carrying excess weight reach far beyond these two areas.
Take for example automobile collisions.
By and large, most cars on the road today are not built specifically to be driven by obese drivers. More importantly, the US Federal Motor Vehicle Safety regulations and New Car Assessment Program testing uses a crash test dummy with a BMI of ~25.5 kg/m2 to gauge how safe a given car, and what type of damage may be sustained by a passenger or driver of that car upon collision. Unfortunately, it is very likely that given the same type of vehicle and mode of collision, the mechanics of the trauma incurred by the driver and passenger will vary greatly when the BMI of these parties is 45 kg/m2 rather than 25.
Indeed, previous studies have found that increasing weight is associated with a greater mortality risk from a car crash, even when other factors such as seatbelt use, age, gender, seating position, and car weight are accounted for. Such a finding, however, does not allow us to deduce whether obese individuals are more injury prone given the same crash or simply if given the same injury sustained, obese people are more likely to die from complications.
For instance, some have previously hypothesized that excess fat among overweight and obese people may actually function as an “intrinsic airbag” or “protective cushion”, thereby potentially reducing the risk of injury during a car crash. These notions were purely hypothetical and not based on any scientific evidence.
In a recently published study in the journal Obesity, Kent and colleagues investigated the crash mechanics of obese versus normal weight subjects. Given that obese crash test dummies are not available, the researchers used 3 obese and 5 normal weight cadavers for their studies, in which the effects of a frontal collision travelling at 48km/h were investigated.
First, the authors found that during the crash the obese subjects generally experienced greater maximum forward movement from their seat towards the dash before their motion was arrested by the seatbelt – obviously, not so good.
Second, the obese cadavers tended to have a more reclined torso during the crash, with the hips sliding out of the seat more than among normal weight cadavers – a difference which may increase the risk of rib fractures and pulmonary trauma, as documented previously.
However, the authors observed that the increased hip excursion and decreased torso flexion forward in the obese cadavers may reduce the risk of the head striking some component of the vehicle interior in frontal or near-frontal impacts, thus explaining the previously documented reduced risk of head injury during a car crash among heavier drivers or passengers.
Thus, while the mechanics of the trauma seem to vary between normal weight and obese individuals, there seems to be not much evidence supporting the concept of obesity being protective against injury during a car crash. With the exception of a possibly lesser chance of head injury, obese subjects may actually be more injury prone during an automobile collision.
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Kent, R., Forman, J., & Bostrom, O. (2009). Is There Really a “Cushion Effect”?: A Biomechanical Investigation of Crash Injury Mechanisms in the Obese Obesity DOI: 10.1038/oby.2009.315
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