After a few complaints of my last post being too long, I’m going to split this one up into small, easily digestible parts. Too long? I mean, the last post was like 2000 words—how did you people make it through college?

For the first part I’ll go over how our bodies burn energy, as well as the two types of energy we use for fuel.

Fat:

Fat is your body’s primary fuel source. We are constantly burning fat while sitting, sleeping, standing, walking—any light to moderate activity we engage in will burn fat. Obviously, the more intense the activity, the more fat we’ll burn. However, there is a point where we receive diminishing returns from fat as an energy source. The reason is individual fat cells are, well, fat. Only so many fat cells can fit through our capillaries at a time. As our activity level increases in intensity, our cells demand more energy, and past a certain point fat cells are literally too big to get through our capillaries in time to meet that demand. Think of fat as your body’s diesel fuel—low intensity, high mileage, endurance fuel. For faster, harder, and more intense, your body has to turn to glucose.

Glucose:

Glucose is your body’s high octane gasoline. It’s not a very efficient fuel, and you don’t get anywhere near the mileage as fat, but sometimes you just gotta go fast. Well, maybe not you, but your hunter-gatherer ancestors needed glucose to chase their dinner, and to avoid becoming dinner.

Your body prefers to burn fat rather than glucose. How do we know this? Because we store more fat than glucose. Our capacity for fat storage is nearly unlimited—the more we store, the fatter we get. Glucose, on the other hand, is stored primarily in the muscles, and there is a definite limit to how much glucose our muscles can hold.

A few of our body parts require glucose for energy, like the brain and red blood cells, and the glucose stored in our muscles can fuel our bodies for about twelve hours at rest. If we add intense exercise, a person in good shape will have about two hours worth of glucose to burn, depending on the type of activity. So what the hell does all this mean? Do we need to keep replenishing our glucose stores before they run out?

Good God no! What it means is despite all of our civilization, biologically we are still hunter-gatherers. We are designed to burn fat most of the day while we forage for nuts and berries on the way to the hunting grounds, burn even more fat as we jog around the wildebeest herd comparing notes with each other on which poor little gnu is the likeliest catch, and then burn even more fat mixed with some high octane glucose as we chase that delicious self-propelled barbeque down, stab the hell out of it like Caesar in the Senate House, and high-five each other while we drag it back to our cave.

That little hunting scenario is a good image to use to get a rough idea whether you’re burning glucose or not.  If you’re able to talk then you’re probably burning mostly fat.  If you don’t have enough air in your lungs to waste on idle gossip with your treadmill neighbor then chances are you’re tapping into your glucose reserves.

Typical Paleolithic Era outfit with an ergonomic design that allowed freedom of movement for hunting and gathering (post-Ice Age).

A big misconception I find most people have is that fat/glucose burning is an either/or situation, and to burn more body fat one should engage in low intensity aerobic activity. This is absolutely not true. Fat burning does not switch off just because your muscles start burning glucose—glucose is burned on top of and in addition to fat, and if your muscles are burning glucose it’s because you can’t possibly burn any more fat and keep the same intensity level.

Constant fat-burning is how our bodies are designed to operate, but it is certainly possible to throw a monkey in the matrix and burn only glucose, forcing our bodies to store more and more fat until we blame the dryer for shrinking our pants. I’ll go over nutritional sabotage in the next post, but for now just picture filling your car up with rocket fuel. It doesn’t make your car go faster—it just burns up your engine.