Friday, April 12, 2013

The Digestive System, Part 1: Teeth and Spit

If you’re like most people, everything you know about your own digestive system can be summed up by the following diagram, which I drew on my phone:
Human digestive system: Mouth, butt, tubes and shit
But is that all there is to the digestive system? Is the journey from food to poop really just a bland, 3-step trip through a poorly sketched flipper baby?

Actually, no!

As a matter of fact, it’s pretty fucking awesome. You see, our digestive systems have evolved with us over millions of years, shaping us into the efficient, fatass omnivores we are today. So come with me—won’t you?—on a magical journey through your own innards. It’s a journey through acid and bile, into the depths of a world that's shrouded in mystery, yet closer than your own ass. It's a world where an alchemical brew of ions and enzymes can transmute the basest foods into fuel for your body. It's a world where there are a thousand times more bacteria than you have brain cells.

We'll enter this world the same way most everything does: through the mouth.


Unless you like to yell at your food until it loses heart, digestion begins in the mouth.

Now, tear off your jaw and take a good look at your teeth. (Or use a mirror, I guess.)

Did you notice that not all of your teeth are the same shape? Then congratulations! You’re a mammal.
Mammals are the only surviving clade of animals with truly differentiated teeth, a trait as useful as it is ancient. Our differentiated teeth date back 300 million years, to before we could properly be called mammals. Back then, it was our early protomammalian ancestors, the synapsids, who ruled the world as the dominant land animals.

Yeah. Before the dinosaurs even got around to evolving, your ancestors were strutting around and lording their fancy differentiated teeth over everyone. It was a nonstop party until the stupid archosaurs rose to dominance and our noble ancestors (who were by now true mammals) found themselves pushed into the background. So, the next time you see a bird, punch it in the face and say, “That’s for what you did to the synapsids!” Then eat it with your differentiated teeth.
Differentiated teeth

Now, put your bloody, severed jaw back into its socket and try eating something. You’ll notice that your very front teeth, the incisors, are used primarily to sheer the food into manageable bites. The next teeth over, the pointy canines, anchor your bite so you can better tear off the sturdy bits. Now, pay careful attention to what you’re doing with your premolars, the double-pointed teeth that bridge the space between your canines and molars. Most people never even notice it but, as you chew, the premolars are slowly rocking your food backward, deeper into your mouth.

The premolars are pointed on the end but wide at the base, and they fit into each other in a nesting shape. While your incisors work like a pair of scissors, making bite-sized slices, your premolars work like one of those interlocking meat shredders that they use to make pulled pork. The motion of the premolars slices the food into itty bitty pieces and moves those pieces, conveyor-belt-style, to the back-most teeth. These are the molars, and they're where you get to the down and dirty. In humans, as in most mammals, the molars are there to grind your food to paste. By the time food gets to the molars, it’s already been reduced to little tiny pieces, but the molars just go fucking apeshit on it, reducing the food to a homogenous goo.

The ability to process food so dramatically before it even goes down your throat is an incredible aid to digestive efficiency, especially when eating hard-to-process plant material.

Salivary glands
Salivary glands and associated apparatus in green. Yes, they're seriously that big.

But wait, that's not all!

Your teeth get a big assist from saliva. You produce about a liter of saliva per day, which is very useful from a digestive standpoint, but super gross from almost any other. The most obvious function of saliva is to moisten your food, making it easier for your molars to turn it into a pasty homogenate. What you may not realize is that the extra moisture sticks the paste together into little balls of bolus. Bolus balls make it easy for your tongue to manipulate the food in your mouth, and for your esophagus to move it down your throat.

Less obviously, saliva begins the process of breaking down large nutrients into smaller forms that can be efficiently absorbed lower in the digestive tract.  Salivary amylase goes to work on starches, breaking them into their component molecules, the sugars. It has to work fast, however, as its activity drops to zero in the acid environment of the stomach. Contrary to some sources, however, salivary amylase is not entirely destroyed in the stomach. Some of it does survive the trip, and continues its work later in the digestive system.1

I can’t find a peer reviewed source that indicates just how much starch is converted to sugars by salivary amylase, but you can do a qualitative experiment of your own to see just how quickly amylase starts to act. If you chew something starchy (like a baked potato or Al Gore) for more than a few seconds, you’ll begin to notice the appearance of a faint, sweet taste. That’s sugar being liberated from the starches. Pretty cool, huh?

As the human diet evolved to accommodate more starchy grains, salivary amylase genes were duplicated, resulting in increased production by the salivary glands. Consequently, we have more copies of the gene than other apes, who adopted a largely frugivorous and/or folivorous diet. Even within human populations, gene copy numbers for salivary amylase vary slightly but significantly, and they correlate well with historic diets. Humans from agricultural populations have around seven copies, whereas humans from hunter-gatherer or pastoral populations have around five.2

Though amylase hogs the spotlight, it’s not the only digestive enzyme in the salivary mix. Foremost among the also-rans is lingual lipase, an enzyme secreted from glands on the tongue, which does for fats what amylase does for starches. Lingual lipase grabs hold of a triglyceride (shown at right,) and starts yanking fatty acid chains off like it was a vengeful cheerleader pulling bloody clumps of hair off the skank she caught flirting with her boyfriend. The liberated fatty acids are absorbed later in the digestive process.

But lingual lipase has a twist to it. Unlike amylase, which loses its activity in the stomach, lingual lipase thrives in that acidic environment. So it keeps working while your food is getting bored with waiting around in the stomach.

And you thought your tongue was just for tasting things and oral sex.

*  *  *

If you liked this article, why not check out the others in the series?

Digestive System, Part 1: Teeth and Spit (this article)
Digestive System, Part 2: Swallowing
Digestive System, Part 3: Down the Tubes
Digestive System, Part 4: B-12 as Temptress
Digestive System, Part 5: The Duodenum
Digestive System, Part 6: The Jejunum
Digestive System, Part 7: The Ileum
Digestive System, Part 8: Liver and Cecum
Digestive System, Part 9: The Colon
Digestive System, Part 10: The Bitter End 

Citations and References:
  1. Fried, et al. Passage of salivary amylase through the stomach in humans. Digestive Diseases and Sciences. October 1987, Volume 32, Issue 10, pp 1097-1103.
  2. Coyne and Hoekstra. Evolution of Protein Expression: New Genes for a New Diet. Current Biology. Volume 17, Issue 23, 4 December 2007, Pages R1014–R1016.

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