Of all macronutrients, our bodies have the greatest capacity to store fat. Our muscles and liver store only about 500g of glycogen. In contrast, our skin and viscera can store many kilos of fat in various parts of the body. It’s easy to assume that the body will burn fat whenever we exercise, but that’s just wishful thinking. The body has a favourite fuel and that is glycogen. Then, there’s protein. Yes, the body can metabolise lean body weight to yield energy – which is bad news. What do we do to ensure that it’s mostly fat that gets burned?
Glycogen is clean fuel that “burns” quickly under stressful conditions. If exercise intensity goes beyond 70% VO2max (maximal aerobic capacity), fat cannot be broken down fast enough to meet the energy requirements of the body. Here, VO2max is used as a yardstick because intensity of exercise is relative to the individual. For most people, sprinting, heavy weight-lifting and high intensity sports like rugby are high intensity activities that selectively draw on muscle glycogen as the main source of fuel and is rapidly depleted. On the other hand, exercises at moderate intensity of 50-70% VO2max derive about half of required energy from glycogen and another half from fat. Exercising at relatively low intensity of 50% VO2max is fuelled mainly by fat.
Having said that, don’t be too happy, thinking that you can burn fat just by walking to the refrigerator and back to the couch. Low intensity activities burn very little fat. There are other parameters to consider.
Duration of Exercise
In a long distance race, a well-trained, well-fed and well-paced athlete’s muscle glycogen is depleted after about 180mins. Running at a higher speed, resulting in anaerobic respiration, glycogen depletion can occur in as little as 30-45mins. That’s why it’s so important to keep a suitable pace in long distance races. If you go too fast, you’ll “burn out” in no time even though the lactic acid accumulated is not wasted if you repay your oxygen debt. Fat can only be used to power a reasonably intense activity under aerobic conditions. Under anaerobic conditions, meaning exercise intensity remains high, protein will be broken down.
These findings point to an ideal strategy for fat loss incorporating a high intensity workout to deplete glycogen followed by a low intensity interval to burn fat.
Individual VO2max is used as yardstick because physically fit people have more fat oxidising enzymes like hormone sensitive lipase. Thus, the fit individual’s body is more efficient in breaking down storage fat into free fatty acids. The guy who walks to the refrigerator and back to his couch has a body that is a lot less efficient in burning fat; never mind if the activity is sufficiently low intensity.
Besides that, trained athletes have better blood supply to their muscles and a larger number of mitochondria in their muscles cells. A high level of aerobic fitness allows faster burning of fat under aerobic conditions, sparing glycogen for more intense activities (relative to that body).
Some health gurus have suggested going on a low carbohydrate diet to encourage the burning of fat. If you’ve understood what you have been reading so far, you’ll see that that’s a bad idea. A low carbohydrate diet will cause muscles to lose glycogen. The body’s capacity for intense activity will decrease because as mentioned earlier, you need oxygen to fuel fat metabolism. Any level of activity that careens into anaerobic territory will force the body to break down protein. Fat may continue to burn under low intensity aerobic exercise, but in practice, muscle is almost inevitably broken down if such diets are adopted long term.