Another Risk of Obesity: Car Accidents

I always thought that every situation, no matter how bad, had some sort of upside. Could there be an advantage to being obese?

Yes, I know that this is Precision Nutrition, a site dedicated to helping people lose weight and eat healthier, but I always thought that every situation had some sort of advantage.

For example, you might think that carrying excess body fat might be an advantage in a car accident. Maybe additional fat might help pad the impact a bit. It makes sense — after all, one of the purposes of our body fat is to pad our organs and some of the pointy bits of our skeleton (such as our pelvic bones).

Thus, would having a higher body mass index decrease your change of injury in a car accident?

Calculating obesity: BMI

Good old body mass index — or BMI for short. It’s the easiest way to figure out if someone is overweight or obese.

Now, many fitness enthusiasts and bodybuilders are going to say “Well, I have a high BMI and I’m considered obese.” For those of you who don’t know, BMI is calculated like this:

(Or just google BMI calculator.)

As you can tell from the equation, BMI doesn’t actually look at how much fat you actually have — it just assumes that any extra weight you have is from fat and not muscle.

Since the vast majority of people don’t have enough muscle to dramatically change their BMI, it’s not a big deal. However, people with a lot of muscle — like bodybuilders and powerlifters — often have BMIs in the “obese” range even if they have single digit body fat percentages.

Nevertheless, since (unfortunately) highly muscular and fit people are a very, very small percentage of all people, BMI remains a good measure of obesity.

For all its flaws, BMI is a pretty good measure of obesity. It’s easy to measure, and it shouldn’t be too hard to look at somebody to figure out if they have a high BMI because of too much fat or too much muscle. (If in doubt, ask them to pick up something heavy or “Which way to the gun show?”)

Risk of obesity

While obesity and being overweight is associated with a lot of chronic diseases including some cancers, diabetes (type II), cardiovascular disease, hypertension, and stroke, what about acute risks? In other words, are there immediate, short-term risks of being obese as well as long-term ones?

This week’s review looks at obesity and risk of injury during a car accident. Is it an advantage or disadvantage to carry extra weight in the form of fat if you’re in a car accident?

Zhu S, Kim JE, Ma X, Shih A, Laud PW, Pintar F, Shen W, Heymsfield SB, Allison DB. BMI and risk of serious upper body injury following motor vehicle crashes: concordance of real-world and computer-simulated observations. PLoS Med. 2010 Mar 30;7(3): e1000250

Methods

The methods for this study were straightforward: First, the researchers analyzed data on car accidents and BMI, and then they used crash simulations to figure out the risk of injury depending on BMI.

Car accident data

For the first part of the study, researchers collected data from the Crashworthiness Data System (CDS) from the US database of car accidents (National Automotive Sampling System). To keep the analysis simple, researchers examined only frontal-collision and passengers over 18 years old, which left over 10,000 passengers for analysis.

The researchers looked at BMI (body mass index) of passengers in car accidents and the severity of their injuries.
Injuries were grouped by body regions: head, face, thorax, abdomen, spine, upper extremities (arms and hands) and lower extremities (legs and feet).

Crash simulation

In the second part of the study, researchers simulated crashes with computerized crash test dummies using obese, average and light dummies.

Male and female obese dummies had a BMI of either 30 or 35, with the female dummies being shorter and lighter. There were also standard dummies (average) with a BMI of 25 and very light dummies with a BMI of 22 (5^th percentile) .

The computer model for the crash was based on the accident data; researchers used MRI data on body fat distribution for the dummy models [1].

Figure 1 – Standard and obese dummies used for computer simulations (Figure 3 from Zhu et al 2010).

Results

Figure 2 below shows some of the results. Read BMI from left-to-right along the bottom.

Key findings:

• In men the risk of injury goes up dramatically as BMI goes above 30. The light blue line shows male injuries in general; the dark blue line shows severe injuries in males. Notice how both these lines go up as BMI increases, and the severe injuries skyrocket past a BMI of 30, especially severe spinal injuries.
• On the other hand, for women, BMI doesn’t seem to have much of an overall effect on risk of head, face, chest (thorax) or spine injuries, although there’s a slight increase in severe facial injuries.

Figure 2 – Risk of head, face, thorax or spine injury in a car accident and obesity (Figure 1 from Zhu et al. 2010).

There’s one major exception to women’s lower risk, which isn’t shown: abdominal injury. Both extremes, low BMI (<22) and high BMI (>39), had huge increases in the risk of severe abdominal injury.  (See Figure 3 below.)

Researchers explain that underweight women had less padding (one function of fat) than normal women, so that increases their chance of injury, but severely obese women have so much extra weight that the increase in momentum increases risk of injury (and thus the potential bonus of fat padding is canceled out).

Men’s risk increased too, but not as dramatically. Men tend to put on weight around their middles, so even underweight men may be more protected than underweight women.

However, there are still a few factors unexplained — but the key message is that obesity does increase risk of certain injuries.

Figure 3 – Risk of abdominal injury in a car accident and obesity (Figure 2 from Zhu et al. 2010).

Summary

Using both real accidents and a computer model based on over 10,000 accidents, researchers found that men with higher BMIs (>30) were more likely to have severe car accident injuries of the face, head, chest, and spine than men a BMI less than 30.

Meanwhile, BMI didn’t seem to matter in women, since there was minimal relationship between BMI and injuries, except abdominal injuries where there was a U-shaped relationship with both obese women (BMI>39) and underweight women (BMI<22), who had increased risk of injury.

Why the difference between men and women?

The researchers speculate that body fat distribution, body shape, and centre of gravity may explain why obese men and women have different risk of injuries.

Obviously, women have a lower centre of gravity compared to men; a higher centre of gravity in a car crash means the upper body would have more force at the same speed because of the weight. More force means bigger boo-boo.

Another reason for the difference between men and women is that the car cabin is designed for a 1.78 m tall, 77.1 kg man. Obese men are further away from the optimal size than obese women (since women would be relatively shorter and lighter). Being bigger than the optimal size might be an important factor.

You’re probably thinking that since this is a simulation that we shouldn’t put too much stock into it, and you’d be right if there weren’t any real world studies that confirm the simulation. However, just a few days ago, a study came out that supported this simulation using obese cadavers [2].

Bottom line

This study surprised me, since I figured the opposite would be true. I assumed more weight would be an advantage in a car accident, but not so — at least in men. (Thinking about it, I guess that whole force = mass x acceleration thing is probably relevant.)

Gee, obesity is starting to look more and more like another addiction, cigarette smoking. While smoking cigarettes doesn’t increase your risk of injury in a car crash, it does tend to increase your chance of being in one. And of course, we all know how substance use + driving ends. (Ending up as a star on Cops is the least of your problems.)

When we’re thinking about the health effects of obesity, we should consider all the effects — not just the obvious ones.

References

1. Shen W, Chen J. Application of imaging and other noninvasive techniques in determining adipose tissue mass. Methods Mol Biol 456 (2008): 39–54.
2. Kent RW, Forman JL, Bostrom O. Is there really a “cushion effect”?: a biomechanical investigation of crash injury mechanisms in the obese. Obesity (Silver Spring). 2010 Apr;18(4):749-53. Epub 2009 Oct 1.