Does Gut Size Predict Intelligence?

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The origins of our intelligence is arguably one of the biggest questions in science. Our enormous brain, or rather how it became that is the professional focus for many anthropologists, primatologists and biologists. Whereas psychologists pursue more contemporary aspects of cognitive function, brain size is an important issue for us too. How can we be curious about the brain but not wonder about the evolutioanry origins of its size? Yet, when I learned a theory of human intelligence revolved around our shrinking guts, I became curious. Could evolutionary theories of intelligence lead to new insight on the mind-body,  brain- gut relationship? Apparently yes…

Encephalization of humans is an interesting topic. This term, which I had to look up on Wikipedia, means to quantify the weight of the brain in relationship with body mass. We can encephalize a lot of different things- but humans, as we might smugly assume, are at the top of the EQ chart.

The first obvious question that comes to mind when we consider the phenomenal growth of the human brain in relation to our body is…How? How did our EQ get to be so big? Perhaps equally important is to determine what it costs to fuel the brain. It’s an organ that can only use a specific fuel source (glucose), and as you’ll see, requires a lot of it.

Investigating the matter further, we first learn that human brains are expensive. Human babies, the fattest mammalian species, attribute around 50% of nutrients consumed to fueling their growing cortex. The human brain’s mass specific metabolic rate (MSR) is approximately nine times higher than the average MSR of the body as a whole. To recap, the brain uses energy 9 times faster than the body, and all of its organs. One responsibility of having such large brains, entails an ability to provide it with fuel. Constantly.  This task is made more difficulty by virtue the fact that the brain cannot store energy reserves. There are no ways to grow extra brain tissue; just an ongoing pressure to keep fueling it.

In the name of keeping your brain fuelled, this might seem like a great moment to get something to eat.  But, ironically, our basal metabolic rate (BMR) isn’t that far out of step with other primates. If our adjusted BMR isn’t out of step with other ancestry, it must mean that there is compensation somewhere along the lines (or somewhere inside the lines, in essence, within other organs) that allow for this adjustment.

Here lies an interesting problem: in order to counterbalance for the racing agility of our brain, something else must have changed. Our brain uses masses of energy, but this is not reflected in our metabolic rate. So, where can the extra energy be going?

Brain Size, Gut Size

Let’s take a look at some of the key organs involved in staying alive, that might be able to self sacrifice in the name of keeping the brain happy:The Liver?   The size of the liver during the process of encephalization isn’t really up for debate; the brain needs a constant supply of glucose from the blood and a major role of the liver is replenish and maintain these levels by releasing glucose from the breakdown of glycogen  stores. These glycogen stores take up about 10% of the liver mass, and then it converts the rest of it from other alternative sites in the body. So changes to the liver aren’t happening.

The Heart?   The heart. The heart?! The heart isn’t changing size, and since its entire mass consists of a rhythmically contracting cardiac muscle, an attempt to reduce its metabolic rate would lead to some pretty weird consequences. And, by ‘weird’, I mean death: blood circulation wouldn’t happen at the appropriate rate, and since the brain requires high levels of glucose and oxygen, any reduction in metabolic rate would not be possible. Further, one of the most cherished arguments that pertain to human development, i.e. man the hunter, requires us to have a healthy heart. You can’t engage in aerobic activity (like chasing prey) if you don’t have a highly energetic heart.

So, those are two expensive types of tissue. What about other muscles?   Scientists argue against any change in skeletal muscle because the basal metabolic rate of muscles is already quite low in comparison with the expensive organs listed above.

This leaves an important organ: the gut. After studying a series of other primates, an important incongruency emerged: the gut seemed to function with the brain: as one got larger, the other became smaller. This symbiotic exchange implies the gut coevolved in step with the brain, and provides even more insight into our ancestry.

Expensive Tissue Theory versus Social Brain Hypothesis (or other ways to describe the origin of our intellect)

There are numerous theories to describe how we evolved those precious extra brain tissue folds, arguably the most expensive and precious bi products of millions and millions of years of evolution. Evolutionary biologists have hypothesized that man’s brain evolved due to a highly demanding social structure. Robin Dunbar has suggested that intelligence evolved as a means for surviving and reproducing in large and complex social groups. The behaviours needed to meet the goal of reproduction included altruism, deception, and coalition formation. These group dynamics give us an inkling of ‘who we really are,’ or Theory of Mind.

Aiello and Wheeler’s (1995) theory of coevolution, however, suggests something more basic is responsible for our increased brain size. Simply, that a higher quality diet provided the energy to fuel growth for more brain folds, which was afforded by a highly sophisticated and efficient gut that is able to break things down quickly. If we look at certain examples, the Expensive Tissue hypothesis may indeed hold some intriguing keys to our evolution. While the Social Brain Hypothesis does offer important insights into group behavior, it’s hard to determine if this didn’t happen as a consequence (not cause) of primates increasingly sophisticated cerebral structure.

The mind body, or gut brain axis will continue to dominate scientific studies for the foreseeable future. Already, there are so many nods- both scientific and anecdotal- to the reciprocity between the two organs: Going with our guts, biology’s new obsession with ‘The Second Brain’, without needing peer review we can all agree that intuition is more visceral than cerebral. Cognitive, conscious reasoning cannot explain everything.