In the story, I mention that a complex interplay of hormones keeps Tarzan taking in the same amount of energy he is using. There are many hormones involved in that, but I’ll just mention a couple of them here.

In a metabolically healthy individual, glucose is the primary fuel used after a meal that contains starch or glucose. As glucose drops, fat is released from the fat cells and becomes the primary fuel between meals. Here is how that happens:

• Insulin is released in response to glucose; it tells the fat cells to hold onto their fat. As glucose levels drop back to baseline levels insulin levels do the same.
• Leptin is a hormone that is released by our fat cells. It sends a signal to the hypothalamus (a structure in the brain) so that the hypothalamus knows that we have fat reserves.
• When we eat a meal and fat is stored, our fat cells increase their leptin signal. But when insulin is elevated, the leptin signal is not as strongly received.

So here is how that plays out:

• We eat a meal that contains starch or sugar.
• Glucose levels rise, and insulin rises in response, so we are primarily burning glucose for fuel.
• As glucose and insulin levels drop, fat starts flowing out of the fat cells and becomes the primary fuel source.
• At the same time, the hypothalamus is now getting the signal from the leptin that we have plenty of energy in our fat stores and causes hormones to be released to tell our body that we have enough energy stored, so we don’t need to keep eating, and we don’t need to conserve energy.

Here’s how that went wrong for Tarzan:

• When he started eating meals with a lot of sugar and refined carbohydrates and became resistant to insulin, his insulin stayed elevated longer after meals.
• That means it took longer for Tarzan’s hypothalamus to get the signal that he had enough fat reserves. (“Tarzan’s Hypothalamus” would be a dreadful name for a rock band.)
• So even though Tarzan had just eaten a meal, his brain still thought he was starving.
• Hormones were released that caused Tarzan to crave food, and to conserve energy.
• Tarzan’s hormones were telling him to sit on the couch and eat potato chips.
• When he was metabolically healthy his hormones were telling him between meals to stop eating and to get moving.

Tarzan wasn’t metabolically unhealthy because he was eating more calories than he was burning; he was eating more than he was burning because he was metabolically unhealthy.