Billions of years ago, life on earth divided into two great kingdoms: animals, which move, and plants, which do not. Our ancestors chose movement, and that basic biology hasn’t changed since. When you get in shape, when you exercise, when you dance, you are sharing the ancient chemistry of movement with every other animal on the planet. We can move because we have muscles that contract. Our muscles are sophisticated machines that use oxygen to burn fat or glucose (blood sugar) in millions of tiny engines called mitochondria, which then produce the energy for contraction. It’s straightforward internal combustion, just like your car but without a flame. The mitochondria are the key to muscle contraction and to the evolution of movement on earth. Bacteria developed mitochondria two billion years ago to burn oxygen. Not to produce energy, but to get rid of the oxygen that was just then creeping into the atmosphere and that turned out to be highly toxic stuff, both then and now. It’s toxic because it’s explosive on a molecular level. That’s why fires burn when you add oxygen and why they go out when you remove it. The ability to burn oxygen inside cells is what gives animals the power to move, but free oxygen is dangerous; it burns holes in our DNA, leading to cell death and ultimately to things like heart disease and cancer. Since storing and handling oxygen is such risky business, we have elaborate oxygen detoxification systems that work around the clock to protect us. The antioxidants in the fruits and vegetables we eat soak up the remaining free oxygen (so eat a lot of them) and with all these systems working hard, we get by pretty well. Bacteria didn’t have any of this. Instead they used the oxygen to burn sugar in their mitochondria, producing harmless water and carbon dioxide as exhaust. Five hundred million years ago, bacterial mitochondria somehow moved inside the cells of our primitive ancestors, who harnessed them to their muscles and gave birth to aerobic metabolism. It was access to the unlimited supply of cheap, oxygen-based energy that fueled the explosion of higher life-forms from then on. Bacterial mitochondria make all higher animal life possible, and they live in every muscle cell of every animal on the planet today, including yours. All animal motion is fueled by the mitochondria inherited from bacteria—the energy you use to walk in the park, run a marathon, scratch your nose or swim a lap. The DNA in your mitochondria is still bacterial, not human. You inherited it like some ancient, permanent trust fund. Incidentally, plants inherited photosynthesis from algae the same way we stole mitochondria from bacteria, so all life energy on earth today comes from machinery developed by either algae or bacteria. Pathways to Higher Energy With that brief look at the last few billion years to put things in perspective, let’s talk about getting in shape. Aerobic fitness is all about making more energy in the muscles. That means building more mitochondria and bringing them more fuel and oxygen. Mitochondria can burn either fat or glucose. It’s like having a car that can run on either diesel (fat) or gasoline (glucose), depending on your needs: diesel for long-haul road trips, high-octane gasoline for speed and acceleration. Your muscles prefer to burn fat most of the time, because it’s a more efficient fuel, but for hard exercise—for speed and power—you burn glucose. At rest, and with light exercise, you burn 95 percent fat and 5 percent glucose. Most fat isn’t stored in your muscles; it’s stored around your belly and hips and in a few other prime locations. Your body has to bring it to your muscles through your circulation. That’s harder than it seems, because your blood is largely water and fat doesn’t dissolve in water. Fat has to be carried in special proteins called triglycerides, which your doctor probably mentioned during your last checkup. The trouble with this, from your muscles’ perspective, is that your capillaries can handle only a few triglyceride molecules at a time. So each capillary can deliver only a trickle of fat to your mitochondria. With consistent aerobic training, your body builds vast new networks of capillaries to bring more fat to your muscles. Eventually, however, you are delivering as much fat as you possibly can, and if you want to go faster, or harder, you need to start bringing glucose to the mitochondria to use as a second fuel. With harder exercise you keep burning fat in the background, but all the extra energy comes from burning glucose. Most of the glucose is stored in your muscles ahead of time, but your circulation gets a double workout, first bringing in more glucose and the oxygen necessary to burn it, then carrying away the exhaust, especially the carbon dioxide. Any way you look at it, circulation is the basic infrastructure of exercise. Steady aerobic exercise, over months and years, produces dramatic improvements in your circulatory system, which is one of the ways exercise saves your life. Exercise stresses your muscles, and they release enough C-6 to trigger C-10. The C-10 released by the adaptive micro-trauma of exercise drives the creation of new mitochondria, the storage of more glucose in the muscle cells and the growth of new capillaries to feed them. Your muscles get hard as you get in shape because they’re stuffed full of all the new mitochondria, capillaries and extra glucose. It’s a fun image—that newly hardened muscle full of all the stuff you grew by exercising. ©MMVI, CBS Broadcasting Inc. 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