Everyday a lot of people are exercising very hard trying to lose weight, and hoping they only lose fat but not muscles. Do they choose the right exercise to maximize fat loss? Dr. Sten Ekberg is a holistic doctor and a former Olympic decathlete. He gives some very good explanation on what exercise to do for fat loss based on personal conditions.
There are two ways our body produces energy. The preferred way is oxidative phosphorylation, which means we make energy in the presence of oxygen. When there is enough oxygen, the fuel, either fat or glucose, is sent to mitochondria to be oxidized and be converted into ATP, the energy of the cell. A glucose molecule can make 38 ATPs, and a fat molecule is larger in size and can make more than 38 ATPs. This process is very efficient and thus it is the preferred method to make ATP. During rest and low intensity activities, our body makes ATP 85% from fat and 15% from glucose.
The alternative way is called glycolysis. When oxygen is not sufficient, the ATP generation is made by breaking down glucose 100%. It happens inside the cell, not in the mitochondria, but in the cytoplasm. This process is not efficient at all. One glucose molecule can only produce 2 ATPs. This is an emergency way of making energy. Our body is so smart that it wants to preserve glucose as much as possible.
Now comes to exercise. Walking is considered aerobic, which means the heart rate is below 120 bpm for most people. You will breathe normally without huffing and puffing. Running is considered anaerobic, which means the heart rate is about 140 -160 bpm for most people. And you will be huffing and puffing through the exercise. During aerobic exercise, because there is enough oxygen, most energy comes from fat burning. In addition, the cortisol level stays low. When switching from aerobic to anaerobic exercise, more and more energy comes from glucose, and less and less from fat. Also cortisol level rises and it causes the liver to break down glycogen to glucose for fuel. As glucose level rises, insulin also rises which inhibits fat burning if you are not fat adapted. When you are close to exhaust the glycogen in the body, the body will start to break down amino acids and protein for fuel, which is not good. Also if you are not fat adapted, you will become more hungry and have more cravings for carbs in order to restore the lost glucose. Then if you overeat the carbs, you will likely generate more new fat, which will offset the fat loss you made from the hard work.
So, who should do running and who should do walking? My conclusion is: for people who are lean and fit, and who are used to run, they should do running for sure. For people who are not fit, obviously walking is the best choice. They should try to walk as fast as they can to be close to their aerobic threshold to maximize fat burning. They can try running in short intervals from time to time to gradually increase their aerobic threshold to improve fat burning efficiency.