Does aerobic exercise cause muscle loss?

Does aerobic exercise cause muscle loss?

This is a claim you hear a lot, especially among bodybuilders, but also some personal trainers. The most extreme version is that aerobics makes you fat. The reason for this is that it causes muscle loss, which lowers your metabolism and causes you to gain fat because you are now eating too much for your slow metabolism. These types of claims, as we will see, are based on some truth but are greatly exaggerated. Your muscles won't waste away to nothing because you walk half an hour a day.

There are a few mechanisms by which aerobic exercise can interfere with muscle growth or cause actual muscle loss. The first is that simultaneous aerobic exercise and resistance training result in competing adaptations in the muscles. For example, steady-state aerobics results in endurance adaptations such as increased mitochondria (aerobic energy factories) and aerobic enzymes in muscle cells, while resistance training can result in hypertrophy or growth of muscle fibers. The bottom line is that both activities have been shown to approximately halve muscle growth compared to strength training alone [Docherty, 2001; Gordon, 1967].

For those of us who do strength training for fitness, this isn't a big deal, it just means it takes longer to build muscle mass. But for bodybuilders, this compromises competence in their specialty. So many bodybuilders will minimize aerobic exercise or take measures to reduce the disruption. I think this is where the seeds of this idea of ​​“muscle loss” were planted. However, keep in mind that aerobic exercise has been shown in these studies to reduce the rate of muscle growth, which is far from causing muscle loss. Interestingly, the opposite interference doesn't seem to occur: adding strength training doesn't affect cardio improvements. Many of us who are interested in aerobics will complement it with upper body strength exercises, and in this case there is no problem with simultaneous training.

There's another way cardio can interfere with strength training, and that's by simply taking up too much time. I remember a time when I was trying to lose weight, so I did cardio for about 90 minutes a day. I tried to do a lot of strength training but was short on time and still exhausted. I had much more success when I went back to a more reasonable hour per day and allowed more time for lifting. I talk about the appropriate balance of different types of training in another article.

There are a few mechanisms by which excessive aerobic exercise can lead to actual muscle loss due to overtraining and/or poor nutrition. Too much aerobic exercise can lead to increased production of catabolic hormones such as cortisol (often referred to as the “stress hormone”), which can subsequently lead to the breakdown of muscle tissue. However, aerobic exercise in moderate amounts is a relaxing activity that results in a net reduction in cortisol. Only excessive amounts of aerobic activity result in increased levels of cortisol in the bloodstream after the activity has stopped. A study that specifically looked at how much aerobic exercise is required found that cortisol increases did not occur with 40 or 80 minutes of running, but only with two hours [Tremblay, 2005]. Ironically, high-volume strength training can cause the same effect [Stone, 1998], but I've never heard anyone warned not to lift because it will lose your muscle!

The other mechanism is that when your body doesn't have enough blood sugar, it can make it by breaking down protein. When there is not enough protein available from food, it is obtained from muscle tissue [Berning, 1998]. Again, this is only likely if you exercise excessively or are malnourished. The most obvious example of this is “hitting the wall” during a marathon or “bonking” cyclists on long rides. They can become irritable and impair judgment because the brain, which can only run on glucose, does not receive enough fuel. I've experienced both and they're no fun. I don't know if my body was breaking down muscle for fuel, but afterward it sure felt like my muscles were broken down or at least beaten up. But both times this occurred to me after about 3 hours of training without taking in any fuel. On the other hand, many people who are into aerobics and view carbs as fuel will increase 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 one

unhealthy lifestyle with poor nutrition justified by overtraining.

There's one way people who do a lot of cardio can become protein deficient: endurance training increases protein needs. It actually contains a small but not negligible amount of fuel (you've probably heard that cardio is fueled by a mix of carbohydrates and fat depending on the intensity, but the mix also contains a bit of protein) and protein is needed to repair tissue damage caused by exercise [Noakes, 2004]. Strength trainers are aware that they need more protein, but people who do cardio often don't. Since many who do cardio are trying to lose weight, they are likely reducing calories at the same time, which if you do so by just reducing portion size can reduce protein intake. The typical recommendation for protein is 0.25 to 0.45 grams per pound of body weight, but endurance athletes may need closer to 0.55 to 0.65 grams per pound [Sharkey, 2001].

So don't overwork, don't undertreat and don't eat junk. Follow common sense procedures like easy day/hard day, don't do hours of cardio per day, and don't try to lose more than a pound of weight per week. Make sure you are doing a balance of cardio and strength training and that your muscles are in good shape.

References:

· Berning, J., “Energy Intake, Diet and Muscle Wasting,” in Overtraining in Sports, Kreider, R., Fry, A. and O’Toole, M., eds., Human Kinetics, 1998.

· Noakes, T, History of Running, Human Kinetics, 2002.

· Sharkey, B, Fitness and Health, Human Kinetics, 2001.

· Stone, M and Fry, A, “Increased training volume in strength/strength athletes,” in overtraining in sports, 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 postexercise steroid hormone responses in trained men,” Eur J Appl Physiol, 94 (5-6): 505-13, 2005.

Inspired by Richard King