The Basics of Fat Burning

Energy in, energy out. The body normally burns a mix of carbohydrate, as glucose, and fat for fuel. How much of either depends on your physical activity and if, or what you have eaten recently. When you use more energy than you take in from food and drink, the body burns stored fat and carbohydrates, and then even protein, to fuel your everyday activities even if you are not exercising That’s what happens when people starve of course; the body starts to eat itself. Depending on your family history -- your genetics -- and the way you eat and exercise to create this energy deficit, your body may decide to get conservative and drop your metabolic rate to try to hold onto body weight. Some of us seem to have inherited this tendency more than others, the origins of which may be in the early periods of human evolution where 'feast or famine' was more or less the norm. Glucose, fat and protein. Even so, starvation always works eventually and the body starts to break down its own tissue for fuel. Stored carbohydrate called glycogen is quickly used up, then goes the fat stored under the skin and around the internal organs. Protein in muscle is then broken down to create glucose to keep the brain working and you conscious. Fat and glucose are the body’s two main energy sources. Fat you know well, glucose comes mainly from carbohydrate foods like rice and bread and potatoes and protein is supplied mainly by meat and beans and dairy products. The amino acid building blocks of protein foods can be converted to glucose in emergencies. Your body always burns a mix of fat and glucose except at very high intensities, and the ratio of the fat and glucose in 'the burn' varies with intensity and time of exercise. Fat burning zone. You may have noticed that some bikes and treadmills at the gym have a setting that says “fat burning zone”, which implies a setting for intensity or speed. The reason for this is that the body burns a greater percentage of fat at a slow pace (or after about 90 minutes of exercise). The fat burning zone, a low intensity speed zone is mainly a gimmick, and here is the reason. Even though you burn more fat going slowly, you still burn some fat at much faster speeds or intensity. It all boils down to how much energy you expend in totality. For example, if you compare exercising at a slow rate that burns 60 percent fat and 40 percent glucose and a higher intensity or duration that burns only 30 percent fat and 70 percent glucose, you may still burn more fat at the higher intensity. A typical example. Exercise (1) is the slower 60/40 mix and exercise (2) is the faster, 30/70 mix of fat and glucose fuel. Walking on a treadmill for 30 minutes -- 180 calories used -- 108 calories of fat burned Running on a treadmill for 30 minutes -- 400 calories used -- 120 calories of fat burned You can see from this example that the bottom line really is how much energy you expend -- and that is the ultimate fat burning measure. The theoretical fat burning zone is mostly a convenient myth. Weight Training Does it Better -- Or Does It? Muscle burns more fat. Weight training is increasingly recommended as a fat-busting tool because some experts say extra muscle burns more energy than body fat at rest, so if you develop more muscle and have a higher muscle to fat ratio than before, you must burn extra energy and more stored fat as a result. This is true and has been shown in metabolic studies. However, the differences are not that dramatic; perhaps less than a few tens of calories per day for each pound of muscle increased, for most people. Does that mean you shouldn’t worry about weight training? Certainly not, because weight training has many other benefits for health and performance, not the least of which is extra muscle. It’s just that this advantage has been somewhat overstated and we need to get this fat burning thing right in order to develop the best weight loss and performance programs. Getting the afterburn. Okay, so extra muscle does not provide that much advantage, but what about the afterburn? The 'afterburn', or the amount of energy you use after you stop exercising, has been promoted as an important slimming idea. If you can get afterburn, which is really another way of saying your metabolism increases for several hours or longer after a particular exercise, then that’s a bonus because you burn fat during the exercise and after you cease as well. Will the fun ever stop! However, this idea has recently been reconsidered as well. An article in the Journal of Sports Science reported that despite some promising early studies of this effect, the idea has not proven to be as useful as first thought. Exercise scientists call this afterburn effect EPOC, which stands for Excess Post-exercise Oxygen Consumption. The authors of that study say that the high intensities required -- greater than about 75 percent of maximum heart rate -- are probably beyond what most people wanting to lose weight can cope with in sustained exercise. So the afterburn advantage from lifting weights or running fast is there, but you need to be able to sustain that intensity, which means a lot of hard work. No secrets there, I'm sure. We also need to consider how fuel is used preferentially according to how your body stores are maintained. After you do a vigorous or long workout, your blood and muscle glucose will be much lower than before you started. Low glucose stores signal the body to burn fat preferentially. So after hard exercise that uses a lot of glucose, the body switches to burning fat. That's why all energy expenditure is important, not just fat burning during exercise.