Cars as a metaphor for understanding obesity

If we want to understand the accumulation of excess body fat, it's tempting to focus our attention on the location that defines the condition: adipose tissue.  Ultimately, the key question we want to answer is the following: why does fat enter adipose tissue faster than it exits?

It follows that if we want to understand why obesity occurs, we should seek to understand the dynamics of fat trafficking in adipose tissue, and the factors that influence it.  Right?

I don't think this is right, and here's a metaphor that explains why.

The speed of a car depends primarily on the force that its wheels exert on the asphalt below them.  If we want to understand why cars move quickly sometimes, and slowly at other times, we should seek to understand the dynamics of how force is transferred from the wheels to the asphalt, and the factors that influence it, right?

As you may have already surmised, that wouldn't be a very effective way of understanding car speed. To understand car speed, we have to move up the causal chain until we get to the system that actually regulates speed-- the person in the driver's seat.  Looking at the problem from the perspective of the wheels is not an effective way of understanding the person in the driver's seat.  Once we understand the driver, then we also understand why the wheels move how they do.

Similarly, in obesity, we have to move up the causal chain until we find the system that actually regulates body fatness.  The only known system in the human body that regulates body fatness is the brain.  Once we understand how the brain regulates body fatness, we'll understand why fat enters adipose tissue faster than it exits sometimes, eventually leading to obesity.

We already know a lot about how this process works, and that's why I focus my attention on the brain and behavior rather than the biochemical mechanics of adipose tissue.

Do Blood Glucose Levels Affect Hunger and Satiety?

You've heard the story before: when you eat carbohydrate-rich foods that digest quickly, it sends your blood sugar and insulin levels soaring, then your blood sugar level comes crashing back down and you feel hungry and cranky.  You reach for more carbohydrate, perpetuating the cycle of crashes, overeating, and fat gain.

It sounds pretty reasonable-- in fact, so reasonable that it's commonly stated as fact in popular media and in casual conversation.  This idea is so deeply ingrained in the popular psyche that people often say "I have low blood sugar" instead of "I'm hungry" or "I'm tired".  But this hypothesis has a big problem: despite extensive research, it hasn't been clearly supported.  I've written about this issue before (1).

A new study offers a straightforward test of the hypothesis, and once again finds it lacking.

The study