Friday, March 31, 2006

Can a beer belly save your life?

Last week on Car Talk, the Magliozzi brothers opened their weekly radio show with new research that relates a man's physique to his chances of surviving a car crash. According to a Medical College of Wisconsin study, "certain men are more likely to survive a car accident by virtue of bellies! It turns out that the extra layer of lard that many guys carry around in their midsections can actually help protect vital internal organs in the event of a serious accident." However, "there's a cut-off point...They say if you're too fat, it doesn't help, because then you've got other problems that hurt your chances of surviving." Listen to their hilarious banter here (clicking the link will open a RealPlayer file that's about 3 minutes long; in case it doesn't work I've provided a transcript* below).

If, like me, you don't have access to the full text of the original research article, you might also want to check out these slightly more serious summaries of the study: 'Spare Tire' Might Protect Men During Car Accident and Obese and Skinny Male Drivers Fare Worse in Car Crashes. Sure enough, the Car Talk guys basically got it right: male drivers with a body mass index (BMI) greater than 35 (i.e., obese) or lower than 22 (i.e., lean) were more likely to die after car collisions than men with intermediate BMIs. Men with BMIs around 28 (i.e., overweight) had the best odds of survival, perhaps due to a "cushioning effect."

So what does a beer belly look like in the anatomy lab? You'd probably expect obese individuals to have a relatively thick layer of fat just under the skin, and indeed they do. Of course, we all have fat just under our skin, i.e., subcutaneous fat. It's the fat you can measure with skinfold calipers.

But peel away the subcutaneous fat, and the beer belly, though smaller, is still there. That's because a second type of fat - visceral fat - is lurking around the intestines and other abdominal organs. Visceral fat accumulates in several anatomically distinct sites:
  1. Deep to the muscles of the abdomen. The inside of the abdominal body wall is lined with a very thin transparent membrane called the peritoneum. Sandwiched between the peritoneum and the muscles of the abdomen is a layer of fat - called extraperitoneal or retroperitoneal fat - that is too deep to grab with skinfold calipers. Some people have a little, some people have a lot. Everyone has at least some deposits of retroperitoneal fat around their kidneys, presumably for its cushioning effect.
  2. In sheets of tissue attached to the stomach and intestines. These sheets of tissue - called omenta and mesenteries - convey blood vessels, nerves, and lymphatics to the various organs of the abdomen. They can also contain huge quantities of fat.
  3. On the large intestine. The large intestine (or colon) has finger-like globs of fat - called epiploic appendages - dangling along its entire length. Epiploic appendages appear to serve as nothing more than a reservoir of surplus fat.
Although we tend to obsess about subcutaneous fat, it turns out that excess visceral fat is the real enemy. High levels of visceral fat are associated with metabolic syndrome, an increasingly common condition that can lead to heart disease and type 2 diabetes. So yes, a beer belly may save your life in a car crash, but in the long run your odds are probably better with a slimmer waist!

* Transcript of Car Talk, 25 March 2006, opening segment:
Ray: We're broadcasting this week from the Department of Evolutionary Biology here at Car Talk Plaza.

Tom: Yeah, this is very interesting, and it's good news for some people...This is a study from the Medical College of Wisconsin in Milwaukee. They found that certain men are more likely to survive a car accident by virtue of bellies! It turns out that the extra layer of lard that many guys carry around in their midsections can actually help protect vital internal organs in the event of a serious accident.

Ray: Sure, it's a built-in airbag.

Tom: That's the theory at least. There's a cut-off point...they say if you're too fat, it doesn't help, because then you've got other problems that hurt your chances of surviving. But who knew? I mean, this is brilliant stuff.

Ray: So you gotta have a beer belly, but it can't be huge. It's gotta be just the right size.

Tom: Here we thought that the beer belly was a useless appendage. In fact, we even refer to it as the "spare tire" - "spare" as in "unnecessary." Not so - it's an evolutionary advantage. What do you think of that?

Ray: Jeez, well, I'm encouraged, actually. Now, it does raise one question: what about women? Don't they have their own...

Tom: Airbags?

Ray: OK, this study doesn't address that I guess...

Tom: It doesn't address that but I was wondering the same thing, because the study does happen to note that men were twice as likely to die in the 22,000 accidents that they looked at. Why? We don't know. But I think someone should look into your theory of natural airbags. That might have something to do with it.

Ray: And what about the unnatural airbags? You would think those would provide an even, uh, larger amount of protection, wouldn't they? I mean, shouldn't someone study that, too? It could be us!

Tom: I'll get right on it!

Friday, March 24, 2006

Brain myths and facts

One of my favorite science writers, Carl Zimmer, investigates a surprising claim about brain power in his latest blog entry: You're a Dim Bulb (And I mean that in the best possible way). The claim is that a normally functioning brain only uses about 10 watts, which of course is much less power than your standard 75 watt light bulb consumes. It turns out that "10 watts" is a little low, but in the right ballpark. I decided to do my own calculations (details below*) and came up with 16 watts. Dim bulb indeed!

Then again, maybe the comparison isn't fair. Incandescent light bulbs are notoriously inefficient -- at least 90% of the energy they consume is wasted as heat. According to this site, a 23 watt compact fluorescent bulb produces as much light as a 75 watt incandescent bulb, and lasts over 10 times as long. But I digress.

Carl also refers to the best known brain myth of all, the hopeful notion that we use only 10% of our brain, suggesting that we all have huge reservoirs of untapped mental potential. I'm sure we all have untapped potential, but as Carl notes, the 10% myth is just plain wrong. We use essentially all of our brain (although, at any given moment, perhaps only 1% of its neurons are active). Check out these fine web pages if you're not convinced:
To balance out the brain myths, here are some fun brain facts:
  • The average adult human brain weights about 1400 grams (3 lbs.), or about 2% of total body weight. These are just averages - there can be considerable variation in both brain mass and body mass.
  • Although it represents only 2% of the body's mass, the brain consumes about 20% of the energy used by the entire body at rest. That's over twice as much energy as the heart uses.*
  • The number of neurons in the human neocortex is around 20 billion. That number is larger than the age of the universe in years (13.7 billion), but smaller than the number of stars in our galaxy (200-400 billion).
  • The total number of synapses (connections between neurons) in the neocortex is estimated to be more than 160 trillion. That works out to an average of about 8000 synapses per neuron. Obviously the cartoons of a "typical neuron" in biology textbooks are a little oversimplified!

* Here are some more specific details for the curious. According to Elia (1992), the metabolic rate of the brain is 240 kcal/kg/day, and the metabolic rate of the heart is 440 kcal/kg/day. Although you can see that heart tissue is more metabolically active than brain tissue, the heart as a whole is smaller than the brain as a whole, so the heart ends up consuming less energy than the brain.

For example, a 1400-gram brain burns about 336 kilocalories per day (16 watts), while the heart, weighing in at 330 grams, burns 145 kcal/day (7 watts). Note that kilocalories are equivalent to the "calories" that weight watchers keep track of. Also note that 240 kcal/kg/day is a bit higher than the brain metabolic rate assumed by Bill Leonard in Carl Zimmer's blog. I'm not sure what to make of the discrepancy.


Elia, M. (1992) "Organ and tissue contribution to metabolic rate." In: Energy Metabolism: Tissue Determinants and Cellular Corollaries. Edited by Kinney and Tucker. Raven Press, Ltd. New York. pp.61-77.

Friday, March 17, 2006

The "backward" chest X-ray in Scrubs

An astute future doc on the SDN forums noticed recently that the opening sequence of Scrubs (a sitcom which I confess I haven't seen yet) features an incorrectly oriented chest X-ray. In accordance with the universal standard for viewing X-ray images (or "plain films" as radiologists seem to prefer these days), you're supposed to imagine viewing a patient that is facing you, so the left side of the film is the patient's right side. The Scrubs title shot (see below) clearly shows a heart pointing the wrong way (i.e., to the right instead of left). Also, the diaphragm bulges higher on the wrong side (left instead of right). Flip the image and everything looks normal.

It turns out that the gaffe was intentional:
The chest X-ray in the title sequence was hung backwards during the first season, then corrected briefly for season 2, but then returned to being backwards. Bill Lawrence states that having the X-ray backwards was intentional as it signified that the new interns were inexperienced. This error was parodied in "My Cabbage" (original airdate: February 28, 2006), with Cabbage (an Intern), attempting to read a chest X-ray backwards.

(from the extensive entry on Scrubs in Wikipedia)
Sounds reasonable. But there is another, much less likely but much more anatomically interesting possibility. Perhaps the patient in the chest film has situs inversus. In this rare (1 in 10,000) congenital condition, an individual's internal organs appear to be the mirror image of the normal arrangement. The liver is on the left instead of the right, the spleen is on the right instead of the left, the left lung has 3 lobes instead of 2, etc. There is also some evidence that brain anatomy in situs inversus is inverted too.

For me, the most curious thing about situs inversus is that people with it usually have a normal life expectancy. Apparently in most people it's just an uncommon but normal variation, kind of like red hair or left-handedness. Indeed, the only real risk of situs inversus is confusing a clinician! For example, appendicitis normally causes pain in the right lower quadrant of the abdomen. In patients with situs guessed it, left lower quadrant. There's always something to keep doctors on their toes.

P.S. For more information, check out this blog entry on situs inversus. It has a link to a nice New York Times article, and many comments by readers with situs inversus.

Tuesday, March 14, 2006

Design flaw in the duodenum?

I love it when the word of the day from is related to anatomy. Here is today's:

duodenum (doo-uh-DEE-nuhm, doo-OD-n-uhm, dyoo-) noun

The first portion of the small intestine (so called because
its length is approximately twelve-finger breadth).

[From Medieval Latin, short for intestinum duodenum digitorum (intestine of twelve fingers), from Latin duodeni (twelve each), from duodecim (twelve).]

An illustration of a duodenum:
And a view from the inside:

So, exactly how long is twelve finger breadths? Anatomy textbooks say the length of the duodenum is about 25 cm (just under 10 inches). The width of my four fingers, side by side, is about 7 cm, which means that I would need just over fourteen fingers to reach 25 cm, not twelve. Maybe if early anatomists all had bony fingers like me, they would have dubbed the first part of the small intestine the quattuordenum.

The duodenum's major anatomical claim to fame is its major duodenal papilla. That's the little bump (papilla means "little pimple" in Latin) where two important tubes - the common bile duct and the main pancreatic duct - converge and dump their contents (see the figure below). The common bile duct carries bile, a greenish biodegradable detergent that is manufactured in the liver, concentrated and stored in the gall bladder, and released into the duodenum to digest fats. The main pancreatic duct carries pancreatic juice, which contains bicarbonate (the active ingredient in baking soda) for neutralizing acid from the stomach and at least 19 different enzymes for breaking down proteins, fats, sugars, and nucleic acids.

The flow of bile and pancreatic juice is regulated by a tiny circular muscle called the sphincter of Oddi. At mealtime the sphincter of Oddi relaxes and the juices flow. Incidentally, for years anatomists have been pushing to get rid of eponyms in favor of more descriptive terms, so the sphincter of Oddi is more properly known as the - take a deep breath - sphincter of the hepatopancreatic ampulla. Uh huh.... I think this is one case where the eponym will never die.

Because of my enduring fascination with unintelligent design, I can't help but wonder if there is any good functional reason for both bile and pancreatic juice to enter the duodenum via a single opening. Normally this arrangement isn't a problem: both secretions come in handy whenever the stomach squeezes another glob of partially digested goo into the duodenum. However, things can get ugly if you have gallstones. Although it isn't common, a stone can form that is small enough to travel down the bile duct, but too big to squeeze through the sphincter of Oddi. A stone lodged near the papilla obstructs the flow of bile and pancreatic secretions. Blocking bile flow is bad enough, causing pain and jaundice, but blocking the flow of pancreatic juice can lead to acute pancreatitis, a potentially life-threatening condition in which the pancreas literally starts to digest itself. Acute pancreatis has many causes, but the most common is a gallstone clogging the drain.

"But wait," chimes in the anatomically informed reader, "isn't there a second pancreatic duct - the accessory pancreatic duct - that is connected to main duct but drains into the duodenum at a different papilla? Couldn't that accessory duct serve as an alternate route if the main duct is obstructed?" The answer is yes, but only in about 60% of the population. The remaining 40% would be out of luck, because their accessory duct drains only into the main duct, not into the duodenum directly.

So is there any good functional reason for bile and pancreatic juice to drain at the same point? I can't think of any, and neither can a colleague here at UVM whose does research on gall bladder function for a living. It would make more sense for everyone to have an alternate drainage route for pancreatic juice, not just 60% of us. But, hey, no body is perfect. :-P

Wednesday, March 01, 2006

Nasal irrigation

Staving off the common cold for years at a time may be a pipe dream, but I'm willing to keep trying. So last night, inspired in part by this NPR report, I finally tried nasal irrigation. The concept seems simple enough: squirt salt water up your nose in order to rinse out your nasal cavity and sinuses. As discussed in a recent, surprisingly readable review article called Nasal Irrigations: Good or Bad? (click here for the complete article), nasal irrigation is apparently safe and effective for treating many conditions affecting the nasal cavity and sinuses (especially sinusitis and rhinitis). Some people claim that regular irrigation actually prevents colds. But exactly how it works remains a mystery. Does nasal irrigation just rinse away excess mucus, which traps viruses and other infectious agents? Or does it somehow enhance the function of cilia, the hair-like microscopic whips found on cells throughout the respiratory tract that normally brush mucus away?

There is also no consensus about the best way to perform a nasal irrigation, or about the ingredients one should use. I decided to go with one of the recipes in the review article:
After mixing it up, I leaned over the kitchen sink and administered the saline using an awkward combination of "positive pressure" (squirting with a bottle), "negative pressure" (inhaling), and gravity (pouring it in with my head tipped back). I haven't decided yet which method I like best, but they all did the trick. It wasn't as unpleasant as I imagined; in fact, I kind of liked it, in much the same way that I enjoy cleaning my ears with Q-tips after a shower. It's nice to give a little attention to neglected body parts like the nasal cavity and the ear canal.

One thing I'd recommend is letting your nose drain as much as possible on its own before blowing your nose in the standard way (i.e., blowing it forcefully while letting air escape through only one nostril at a time). By blowing my nose too soon I ended up forcing saline into my left middle ear (the space immediately behind the eardrum) via the eustachian tube (a structure I describe in more detail in a previous post). Oddly enough, it felt exactly like getting water in my ear canal during swimming. Fortunately, the fix was quick and easy - I just tipped my head to the right and the wayward saline came pouring out! Good thing I paid attention in anatomy class. 8-)