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Obesity and car crashes - does fat provide a protective cushion against injury?

Friday, October 02, 2009 Posted by Peter Janiszewski, PhD

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.

Peter

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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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2 Response to "Obesity and car crashes - does fat provide a protective cushion against injury?"

  1. Lauri Said,

    My dh works in the research side of transportation. This is news to him. Studies are done for a BMI of 35 or less and they show it's more a matter of how the vehicles collide than the size of the occupants -- not including children. Anyone with a BMI of 45 or greater is already straining the heart.

    (you say)"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." Actually, the April edition of the American Journal of Public Health has a study showing this is not hypothetical. The study was extensive, involving 22,107 accidents in the United States over a 5 year period.

    Posted on October 5, 2009 at 10:24 AM

     
  2. Peter Janiszewski, PhD (Cand.), MSc Said,

    Hi Lauri,

    What exactly is news to your 'dh'?

    Transportation research is obviously not my area of expertise. The authors state that the dummies used in the research are of a 25.5 BMI - not a BMI of 35 or less - this would be rather difficult to pull off logistically - do they use multiple crash test dummies all of varying weight? A bit impractical if you ask me, but again I'm not an expert on this.

    Also, this study only investigated one mode of impact - head on collision at 48km/h. Thus, while your suggestion that mode of impact may be more relevant than weight of passengers could be true - this was not the point of the study.

    I am aware that prior studies have been done on weight and car crash mortality. The problem here is that the findings are often confounded - if there is a higher mortality for obese people is it due to: A) obese are more prone to death due to complications of the injury sustained, or B) are obese more likely to be seriously injured given the exact same type of impact in comparison to lean individuals. (B) was the issue investigated in this study.

    I checked the April edition of the American Journal of Public Health and could find no such study - could you provide the link to the article - would be much appreciated.

    The authors of the study I discussed had submitted their work for publication in March of this year, so it is possible the study you are referring to was not yet published - thus was not included in their discussion.

    Thanks for the comments, Lauri - and again, please forward a link to the study - I am interested in reading it.

    Posted on October 5, 2009 at 11:00 AM

     

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We are PhD students in the School of Kinesiology and Health Studies at Queen's University in Kingston, Ontario. Our research focuses on the relationships between obesity, physical activity, and health risk. This blog is our attempt to consider the many "cures" for obesity that we read about on a daily basis. Enjoy.

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