Why does he have to poop?

Ah, kids. You have to love them.

We’ve all just gotten home, and my wife goes to take the dog out to – as they euphemistically say – ‘do his business’. My son goes over to the porch door, and asks what she’s doing. Bear in mind, of course, that he’s seen us take the dog out several times a day since he was born. But my wife, with her bounless patience, simply answers that he “has to go potty”.

My son looks at her. “Why does he have to poop?”

It’s actually a kind of fair question. Why do we? Wouldn’t it make more sense if we could metabolize everything? It sure would make more sense. Sadly, however, we don’t. And so, the things we don’t metabolize have to go somewhere. Or, at least, that’s how I assumed the process worked. And it was accurate, as far as it went. But it turns out there’s a little more to it than just that.

To start with, let’s recap the digestive process. The National Institute of Health has a pretty good one-page presentation titled Your Digestive system and How it Works. In brief, food goes into the mouth where your saliva begins breaking down starches. It then moves to the stomach, where your stomach acid begins breakind down proteins. From there the food and acid mix (called “chyme”) moves to the small intestine, where starches and proteins are further broken down along with carbohydrates and fats (thanks in part to pancreatic juices and bile), and where nutrients are absorbed into the blood. Whatever’s left is then dumped into the large intestine, which absorbs water and any remaining nutrients and then expels the remaining waste matter.  Voila! Poop! But… what is it? It’s all well and good to say it’s “waste matter”, but what does that mean?

I found two different sources, each of which agree in broad strokes on the composition of human stool. It is approximately:

  • 75% water
  • 7.5% bacteria
  • 2.5% – 5% “inorganic substance”
  • 2.5% – 5% fat
  • 0.5% – 0.75%
  • Undigested foods make up the remaining quantities

So why do dogs (or people, for that matter) poop? Because evolution gave us digestive systems that are good enough, not perfect. And because all chemical processes – and digestion is just another chemical process – leave waste behind. And because, eventually, we’d hit a point of diminishing returns. If nothing else, how much carbon does one body need?