Yes, it's true. I stuffed my face with pizza. And you know what else? It was just what I needed at that moment. Let me explain.
I recently went on an 11-day cycle touring trip with my wife, during which we rode and camped our way through broad swaths of Washington and Northwest Oregon. We carried all our gear on our bikes, and the riding was often hard due to long days, steep hills, and loose gravel trails.
If you've ever been on an extended bike touring or backpacking trip, you know how much you need to eat to maintain your performance. On this particular trip, I had the misfortune of catching a gastrointestinal bug that cut my appetite for several days. As we continued on our trip and my body's calorie stores declined, my energy levels began to flag. My legs became increasingly tired, and I had to start walking up hills that I would normally be able to handle.
I knew I had to replenish my body's calorie stores as quickly as possible, and to do that, I'd need to deploy the world's most powerful tool for putting calories into bodies: junk food. And that's what I did. My appetite started to return when we reached Cascade Locks, Washington, and we stopped in to a pizzeria for lunch. We ordered a 15" pizza, which arrived with an absurdly thick layer of toppings and cheese. Then, I proceeded to devour about two-thirds of it. I estimate I ate about 2,200 Calories.
The most interesting thing, for our purposes, is that I was able to eat that much pizza in the first place! Normally, my appetite would stop me after about 3-4 slices, and eating more than that would be uncomfortable. But I ate twice that amount without any discomfort, hopped on my bike, and rode off feeling energized. How is that possible? The answer illustrates some key principles about how the brain regulates appetite and body fatness.
How it works
Since energy is such a critical part of survival and reproduction, the brain regulation of body energy is highly evolved and very complex, but we can roughly divide it into two systems (1):
- A system that regulates food intake on a meal-to-meal basis. This is the satiety system, and it's centered primarily in the brainstem (particularly the nucleus tractus solitarius).
- A system that directly regulates body fat levels. This is the energy homeostasis system, and it's centered primarily in the hypothalamus (multiple nuclei).
The energy homeostasis system measures the size of body fat stores, primarily using the fat-secreted hormone leptin. If your body fat stores begin to decline, the system kicks in to try to restore the lost fat (this is a key reason why weight loss is so difficult and often fleeting). It does this by decreasing the amount of energy the body expends to some degree, but primarily by increasing appetite and the overall drive to eat food. It particularly enhances the drive to eat calorie-dense foods like pizza that are highly effective at increasing body fat levels.
The satiety system receives a variety of signals from your stomach and small intestine that inform it of what you've just eaten. At that point, your brain knows the volume of what you just ate, as well as its calorie density, and its content of fat, protein, and carbohydrate. It integrates those signals together, and gradually reduces your motivation to eat as you take each additional bite. This is accompanied by a conscious feeling of fullness.
These two systems interact with one another extensively. Probably the most important way in which they interact is that the energy homeostasis system sets the gain on the satiety system. In other words, if your fat stores are depleted, your energy homeostasis system tells your satiety system to become less sensitive to the signals it's receiving from the digestive tract. This means it takes more food than usual to feel full-- sometimes, much more.
And that's exactly what happened to me. My fat stores were depleted, and my energy homeostasis system knew it. It sent a signal to my satiety system, instructing it to delay the satiety signal substantially so I could stuff my face and quickly make up the calorie shortfall. It made me particularly attracted to calorie-dense junk food, the most effective way to put calories into bodies.
You might feel like your stomach is full after you eat a big meal, but it usually isn't. The human stomach has a remarkable capacity to stretch and accommodate absurd amounts of food. That sensation of impending stomach rupture doesn't actually come from your stomach-- it comes from your brain. It's a highly processed signal that your brain produces by interpreting signals from your stomach in the context of your body's current energy stores.
The human brain is naturally attracted to foods that deliver large, concentrated loads of easily digested calories. That's because eating those types of foods whenever possible kept our ancestors alive and making babies for millions of years under extremely rugged circumstances. In certain atypical contexts, such as the one I was in during my bike trip, those foods are literally good for you. But that same ability to deliver calories so effectively becomes a liability when it's exaggerated to an unnatural degree by advanced food technology, readily available all the time, and placed in the context of a population that lives a pampered lifestyle. Our food technology and lifestyles have evolved, but the hard-wired brain circuitry that sets our appetite and our food affinities hasn't.
And that's why I only let myself eat pizza a few times per year.
Stay tuned for a much deeper exploration of these subjects in my upcoming book, The Hungry Brain. It should be on shelves in late 2016.