Does aerobic exercise cause muscle wasting?
This is a statement that you hear often, especially among bodybuilders, but also among some personal trainers. The more extreme version is that aerobics makes you fat, the reasoning being that it leads to muscle loss, which slows your metabolism and makes you gain fat because you’re now eating too much for your slow metabolism. These types of claims are based on some truth, as we shall see, but are greatly exaggerated. Your muscles are not going to wear out because you run 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 strength training lead to competitive adaptations in the muscles. For example, steady-state aerobic exercise leads to endurance adaptations, such as increased mitochondria (aerobic energy factories) and aerobic enzymes in muscle cells, while strength training can lead to muscle fiber hypertrophy or growth. The bottom line is that doing both activities has been shown to cut muscle growth in half compared to strength training alone. [Docherty, 2001; Gordon, 1967].
For those of us who are doing strength training to get fit, this isn’t a big deal, it just means it will take longer to build muscle mass. But for bodybuilders it is interfering with competition in their specialty. Many bodybuilders will minimize aerobic exercises or take steps to reduce interference. I think this is where the seed of this “muscle wasting” idea was first planted. But keep in mind that aerobics in these studies have been shown to reduce the rate of muscle growth, which is a far cry from causing muscle loss. Interestingly, the opposite interference does not appear to occur: adding strength training does not interfere with cardiovascular improvements. Many of us who are into aerobic training will supplement it with upper body strength work, and there’s no problem with concurrent training in that case.
There’s another way that cardio can interfere with strength training, and that may simply be by taking too long. I remember a time when I was trying to lose weight, so I was doing about 90 minutes of cardio a day. I tried to do a token amount of resistance training, but was short on time and exhausted anyway. I had much more success when I cut back to a more reasonable time per day and allowed more time to lift weights. I talk about the proper balance of different types of training in another article.
However, there are a couple of 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 like cortisol (often referred to as the “stress hormone”), which can subsequently cause the breakdown of muscle tissue. But aerobics in moderate amounts is a relaxing activity, leading to a net decrease in cortisol. Only excessive amounts of aerobic activity lead to elevated levels of cortisol in the bloodstream after the activity is completed. A study looking specifically at the amount of aerobic exercise needed found that cortisol elevations did not occur when running for 40 or 80 minutes, but only occurred in two-hour runs. [Tremblay, 2005]. Ironically, high-volume resistance training can cause the same effect. [Stone, 1998]but I’ve never heard anyone warn you not to lift weights because it causes muscle wasting!
The other mechanism is that if your body doesn’t have enough glucose in the blood, it can make it by breaking down protein. If there isn’t enough protein available from food, you’ll get it from muscle tissue. [Berning, 1998]. Again, this is only likely if you are overtraining or undernourished. The most obvious example of this is “hitting the wall” in the marathon or cyclists “hitting” on long rides. You may become irritated and have impaired judgment as your brain, which can only run on glucose, isn’t getting enough fuel. I have experienced both and they are not fun. I don’t know if my body broke down the muscles for fuel, but afterwards I felt like my muscles had been broken down, or at least hit. But both times this happened to me after about 3 hours of running without fuel. On the other hand, many aerobic exercisers, thinking of carbs as fuel, will increase their intake of bad carbs like white flour products or sugary drinks or “energy bars,” which are basically glorified chocolate bars. This can lead to a
unhealthy lifestyle of poor nutrition justified by overtraining.
There is a way that people who do a lot of cardio can end up protein deficient: resistance exercise increases the demand for protein. It actually provides a small but not insignificant amount of fuel (you’ve probably heard that cardio is fueled by a mixture of carbs and fats, depending on the level of intensity, but there’s also a bit of protein in the mix), and protein is needed to repair any tissue damage caused by exercise [Noakes, 2004]. Strength trainers are well aware that they need more protein, but people who do cardio often aren’t. Also, since many cardio exercisers are trying to lose weight, they’re probably cutting calories at the same time, so cutting back on portion sizes can lower your protein intake. The typical recommendation for protein is 0.25 to 0.45 grams per pound of body weight, but endurance athletes may require more than 0.55 to 0.65 grams per pound [Sharkey, 2001].
So don’t overtrain and don’t undertrain, and don’t eat junk. Follow common sense procedures like easy day/hard day, don’t do hours a day of cardio, and don’t try to lose more than a pound of weight per week. Make sure you do a balance of cardio and resistance training, and your muscles will be just fine.
References:
· Berning, J, “Energy Intake, Diet, and Muscle Wasting,” 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, “Increasing 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 Men,” Eur J Appl Physiol, 94(5-6):505-13, 2005 .
