This is a claim you hear often, especially among bodybuilders, but also among some personal trainers. The most extreme version is aerobics makes you fat, the reasoning being that it leads to loss of muscle, which lowers your metabolism, causing you to gain fat because you're now eating too much for your slow metabolism. These types of claims are based on some truth, as we'll see, but are highly exaggerated. Your muscles are not going to waste away to nothing because you run half an hour per day.
There are some mechanisms by which aerobic exercise can interfere with muscle growth or cause actual muscle loss. The first is that concurrent aerobic exercise and strength training lead to competing adaptations in muscles. For example, steady state aerobics leads to endurance adaptations such as increased mitochondria (aerobic energy factories) and aerobic enzymes in the muscle cells, while strength training can lead to hypertrophy, or growth in muscle fibers. The bottom line is that doing both of these activities has been shown to cut muscle growth about in half compared to just doing strength training [Docherty, 2001; Gordon, 1967].
For those of us that are doing strength training for fitness this is not a big deal, it just means it will take longer to build up muscle mass. But for bodybuilders it's interfering with proficiency in their specialty. So many bodybuilders will minimize aerobics or take measures to reduce the interference. I think this is where the seed of this "muscle wasting" idea was first planted. But note that aerobics in these studies has been shown to reduce the rate of muscle growth, which is a far cry from causing muscle loss. Interestingly, the opposite interference does not seem to occur: adding strength training does not interfere with cardio improvements. Many of us that are into aerobic training will supplement it with upper body strength work, and there's no problem with concurrent training in that case.
There is another way that cardio can interfere with strength training, and that can be by just taking up too much time. I remember a time when I was trying to lose weight so I did about 90 minutes a day of cardio. I tried to do a token amount of resistance training but had little time and was worn out anyway. I had much more success when I cut back to a more reasonable hour per day and left more time to lift. I talk about the appropriate balance of different types of training in another article.
There are a couple mechanisms by which excessive aerobics can lead to actual muscle loss, however, due to overtraining and/or poor nutrition. Too much aerobics can lead to increased production of catabolic hormones like cortisol (often referred to as a "stress hormone"), which can subsequently cause breakdown of muscle tissue. But aerobics in moderate amounts is a relaxing activity, which leads to a net decrease in cortisol. Only excessive amounts of aerobic activity leads to elevated cortisol levels in the bloodstream after the activity is complete. A study which specifically examined how much aerobic exercise is needed found that cortisol elevations did not occur when running for 40 or 80 minutes, but only occurred in runs of two hours [Tremblay, 2005]. Ironically, high volume resistance training can cause the same effect [Stone, 1998], but I've never heard anyone being warned not to lift because it causes your muscles to waste away!
The other mechanism is that if your body does not have enough blood glucose, it can manufacture it by breaking down protein. If not enough protein is available from food, it will get it from muscle tissue [Berning, 1998]. Again this is only likely if you're training excessively, or undernourished. The most obvious example of this is "hitting the wall" in the marathon or cyclists "bonking" on long rides. You can get irritable and have impaired judgment as the brain, which can only run on glucose, is not getting enough fuel. I've experienced both of these and they're no fun. I don't know if my body broke down muscle for fuel but afterwards it sure felt like my muscles had been broken down, or at least beat up. But both times this occurred to me after about 3 hours of exercise without taking in any fuel. On the other hand, many people that are into aerobics, thinking of carbs as fuel, will bump up their consumption of bad carbs like white flour products or sugary drinks or "power bars" which are basically glorified candy bars. This can lead to an
unhealthy lifestyle of poor nutrition justified by overtraining.
There is a way people who do a lot of cardio can end up protein deficient: endurance exercise increases the demand for protein. It's actually provides a small but not negligible amount of fuel (you've probably heard that cardio is fueled by a mixture of carbs and fat, depending on the intensity level, but there's a bit of protein in the mix, too), and protein is needed to repair any tissue damage caused by the exercise [Noakes, 2004]. Strength trainers are well aware that they need more protein, but people who do cardio often are not. In addition, since many who do cardio are trying to lose weight, they're probably cutting back on calories at the same time, which if you do it by just reducing portion size can decrease protein intake. The typical recommendation for protein is 0.25-0.45 grams per pound of body weight, but endurance athletes can require more like 0.55 to 0.65 grams per pound [Sharkey, 2001].
So don't overtrain and don't underreat, and don't eat junk. Follow common sense procedures like easy day/hard day, don't do hours per day of cardio, and don't try to lose more than about a pound of weight per week. Make sure you're doing a balance of cardio and resistance training, and your muscles will be just fine.
References:
· Berning, J, "Energy Intake, Diet, and Muscle Wasting", in in Overtraining in Sport, Kreider, R, Fry, A, and O'Toole, M, eds, Human Kinetics, 1998.
· Noakes, T, Lore of Running, Human Kinetics, 2002.
· Sharkey, B, Fitness and Health, Human Kinetics, 2001.
· Stone, M, and Fry, A, "Increased Training Volume in Strength/Power Athletes", in Overtraining in Sport, Kreider, R, Fry, A, and O'Toole, M, eds, Human Kinetics, 1998.
· Tremblay, M, Copeland J, and Van Helder, W, "Influence Of Exercise Duration On Post-exercise Steroid Hormone Responses In Trained Males", Eur J Appl Physiol, 94(5-6):505-13, 2005.
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