Join Date: Dec 2005
Re: Cardio: Burning Fat or Carbs??
I've been doing a lot of research on this topic recently, so I thought I'd share a couple points. I did this post primarily to assemble the information for myself, so excuse the verbosity. This is probably more information than the OP (or anyone else) would want, but here it is anyway. I welcome any additions or corrections.
First, it is true that at lower heart rates (60-70% max HR) you burn proportionately more fat than carbs. This is why a lot of exercise machines and exercise charts label the 60-70% HR zone the "fat loss" or "weight loss" zones. It can be less taxing on the body (knees, back, shins, etc.) than higher intensity workouts. Its probably also safer for someone who is very out of shape, because it allows them to build their cardiac and muscle endurance in gradual steps. In sum, low intesity cardio is a safe, traditional, time-proven method for weight loss.
However, starsofglass and pipermac are both correct that at higher intensities (70-85% max HR), although you will burn proportionately less fat, you may burn more net fat just because your energy needs are higher. Indeed, there are a lot of people today trying to dispel the "myth" (their word, not mine) that lower HR exercise is better for weight loss. In some respects I think this thinking is a big factor behind the increased popularity of high-intensity interval training (HIIT) in recent years. And, there is quite a bit of science and practical experience supporting the theory that HIIT can be superior to traditional low HR cardio.
Just to confuse you further, lets take it another step. Your body has two fuel tanks available: carbs and fat. Carbs (glycogen) convert easily into available energy, so you can think of the carbs as the "high octane" fuel. On the other hand, burning fat for energy (lipolysis) is very inefficient, but the fat fuel tank is much larger than the carb fuel tank. Because of this, your body tries to use the fat fuel tank as much as possible in order to save the high-octane fuel (carbs) in case of need. So when your muscles are only moderately stressed (low HR), you body has the luxury of using more from the fat fuel tank. However, as your exercise intensity increases (high HR), energy can't be delivered from the fat fuel tank at a sufficient rate to keep up. So your body switches more and more to the carb fuel tank.
However, prolonged exercise at high HR has two "bad" effects (putting aside the question of your cardiac health). First, at some point you may be working so hard that you switch from aerobic to anaerobic exercise. The rule of thumb is 90% max HR, but this varies widely. Anaerobic means "without oxygen" and it simply indicates that you are using energy so quickly that your body doesn't have time to add oxygen to the fuel burning process. This isn't inherently dangerous in itself -- sprinters, weight lifters and other athletes that depend on short, quick bursts of power -- are generally working in the anaerobic zone. But, when glycogen (carbs) are burned without oxygen, it produces lactic acid, which builds up in the muscles and causes discomfort and fatigue. Thus, you can't sustain prolonged anaerobic activity.
Secondly, prolonged intense cardio will stimulate the release of cortisol into your body. Cortisol is a hormone that gives access to a third fuel tank - protein. Unfortunately, this fuel tank is actually your muscles themselves, and cortisol actually causes your own muscle tissue to be burned for energy (catabolization). Cortisol in the system (which is also triggered by stress) additionally promotes the formation of fat, especially around the midsection.
One last point: you can give your engine an overhaul. By training exclusively (or almost exclusively) at low heart rates, some researchers and trainers believe that you can increase the efficiency of the fat fuel system. Your carb fuel tank (glycogen) has limited supplies, maybe 70-100 minutes depending on the person and when they last ate and/or exercised. Once its used up, you've only got the fat fuel tank to rely on, so people who exercise/compete for long periods need to get better gas mileage out of their system (so to speak). Long-distance runners and bikers train their bodies to be more efficient in the fat burning process so that even at higher heart rates, they are still relying on their fat fuel tank for a larger portion of their energy needs. In order to accomplish this tune-up, some people advocate exercising exclusively at low heart rates (60-75%). A runner, for example, may find that this forces them to slow way, way, way down at first. But in the long run, the theory goes, that runner will train their body to burn fat more efficiently and thus use more of the fat fuel tank even at higher HRs. This results in better, faster performance -- and more fat burning. A full discussion of low HR training is beyond the scope of this post, but a Google search for Maffetone and/or Hadd (two of the leading advocates) will put you on the trail.
Of course, what this implies is that low HR cardio training can actually be the better way to burn more fat in the long run. Suffice it to say, the advocates of high intensity cardio hold fast to their contrary view.
Personally, I have tried both HIIT and low HR training and I don't know which is better. I've lost body fat with both. I do feel more "fit" having done six weeks of low HR training, but I also think that low HR training is much more time demanding (longer cardio sessions) than HIIT for equivalent gains. Since I am training for a marathon next year, I will probably stick with low HR training; increased endurance is now as important a goal for me as fat loss. Obviously, other people have different goals, etc. In any event, hopefully this illustrates that both high and low intensity cardio have benefits, and I don't know that one is better or worse than the other. In the end, I believe it depends on what you want to accomplish and what will motivate you to keep at it.
Last edited by Berner; 12-22-2005 at 09:44 AM.