I have always thought that reasoning from anatomy to the human diet was ridiculous. Humans are unlike any other animal in one very important and relevant respect: we have harnessed fire to make food easier to digest. Cooking is a form of predigestion. Indeed, cooking can be thought of as a form of external digestion. As humans, we have outsourced our digestion: from gastrointestinal tract to fireplace.

This outsourcing provides the human species with a competitive advantage. Because our guts no longer need to do the work of digestion, there is less evolutionary pressure to maintain gut function and length. All of those resources that the body dedicates to maintaining gut length now can be used to maintain other organs, such as the brain. All of that time to chewing food, humans can now use for other activities, like hunting, gathering, and socializing in increasingly complex and sophisticated ways.

Humans, through fire, were liberated, body and mind. Fire was the first revolution of mankind. This is why the Greeks dedicated to this milestone one of the most interesting and terrible of all myths: that of Prometheus.

Fire changed us forever. By giving evolution permission to shrink our guts, fire gave license to the expansion of our brains. By freeing us from the yoke of perpetual chewing, it subjected these enlarged brains to the novel and powerful selective pressures of social life, which further enhanced our cognitive capabilities. And by unlocking the endless bounty of nature–which had hitherto been indigestible–fire made humans resilient to famine and paved the way for our spread across the globe.

That is the story told by Richard Wrangham, anyway, in his influential book Catching Fire. And believing it, I also believed that the relationship between gastrointestinal tract structure and digestive function, as evident in the other animals in the world, was broken in humans. While the herbivores of yore might have very long digestive tracts, humans needn’t. We simply do not follow the same rules as the other animals. So if humans did have gastrointestinal anatomy much like a carnivore, then they might be either a carnivore… or not.

Thus, my judgment was closed: anyone arguing from digestive anatomy to function was, at best, a lost cause. And engaging with such arguments, a waste of time. Besides, every diet tribe claims human gastrointestinal anatomy as their own anyway. Therefore, we simply should not try to derive the optimal human diet from anatomy, but from modern nutritional science. Easy peasy.

That was until Game Changers, anyway. A great monument to pseudoscience, I have been motivated to check everything: every crevice of every fact. Besides, despite all my logical certainty, my curiosity was the greater force. I have always wanted to know: exactly what was human digestive anatomy like?

So when Julianne Taylor tweeted about this–that humans have a digestive tract length similar to that of lions, contradicting the claims from Game Changers–I had to check this out.

And what followed was this:

I really appreciate Julianne. We have followed and interacted with each other for a long time, and I really appreciate the work that she does. So I dove into her references. Because I wanted to really understand.

What follows me is my own fact-checking of Julianne’s fact-checks. I wrote this thread after I discovered that her reference was incorrect: humans do not have the same gut-to-body-length ratio as lions or other carnivores. Without further ado, posted with some minor edits…

I don’t think the reference Julianne uses is credible. The human GI tract is about 8.5 meters. The length from neck to anus is around 0.7-0.8m.

9/0.7 = 12
9/0.8 = 11

Human ratio is therefore 11-12x. Game Changers exaggerates but in this case is close-ish.

Let’s look at human GI tract. Here is a paper with 200 subjects: 100 males and 100 females. It shows average is about 8 meters, not including the upper GI tract. Including the upper GI tract takes us to about 8.5 meters.

Now the length from neck to anus in humans is about 0.7-0.8 meters. I’m tall and I measured about 0.8 meters.

Why measure from neck to anus? Because that’s how you would measure a quadruped animal’s body length: neck (or head) to base of tail. That’s how lions are measured. (More on this in a moment.) Measuring to the legs does not work because human legs are elongated compared to what they would be if we were quadrupeds. (Try walking on all fours with legs extended.) Likewise, the human leg joint is pointed downward in the hip, giving additional length. So to make a fair comparison, the legs must be excluded. Otherwise humans for reason of their long legs will have an artificially depressed ratio. In fact, the paper that Julianne cites includes the legs of the humans but excludes the legs of the animals, very erroneously depressing the ratio!

Now, the length of the lion GI tract. This paper gives the lion’s GI tract length at about 8.7 meters.


Yet another gives a very similar value (Panthera leo):


Yet another does the same (Felis leo):


According to the San Diego Zoo, the body length of a lion, measured from head to anus, is about 1.9 meters. Let’s cut down to 1.75 metes to get rid of the head. (We are estimating here, but we want to get things basically right.)

Therefore 8.7 meters / 1.75 meters = 5

Which is strikingly similar to the ratio given in Game Changers.

So according to these calculations, humans have more than double the length of GI tract compared to the African lion.

11-12 vs. 5

This is why is it so important to check and double check references. It is not good enough to just find a paper on the Internet. Many papers are plainly wrong.

For instance, the paper that @juliannejtaylor derived her figures from was entitled “Human adaptations to meat eating”.

The title is a red flag. Maybe not bright red, but definitely red. Why are we talking about human adaptations to meat eating? It sounds like a position paper, i.e. biased. But it gets worse, because the paper is not well-written and the refs are nearly untraceable.

Don’t believe documentaries and don’t believe googled papers on the Internet. Most human beings are not objective or scientific, and the incentive structures that would encourage objective information communication are not in place. Quite the contrary.

And in fact, at face value, I even believed Julianne’s thread without looking deeper.

Until I was alerted by @AdamRMG to look deeper. And the above is what I found. Thanks Adam.

I still think @nutrition_facts sucks for endorsing this terrible film, but the human GI tract ratio:body length is very different from a lion’s, and it is close to the figures provided by Game Changers.

This implies that, at least with respect to total intestinal length, humans are most similar to omnivorous or herbivorous and not carnivorous animals. I won’t claim to understand the subject at greater depth than this. I don’t. But we progress small pieces at a time. Perhaps someday I will be able to discuss this at greater length.

In the meantime, I will communicate what the nutrition science says, and evolutionary arguments will continue to take a back seat.

If you like my stuff, donate below. Any contribution, no matter how small, makes a big difference.

I will also be doing an AMA on December 7-8th.

As mentioned, you will need to be a donor to participate. So do so now if that is something you would like to do.


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