Does Aerobic Exercise Cause Muscle Wasting?

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 its 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 theres 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 Ive 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 youre 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. Ive experienced both of these and theyre no fun. I dont 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. Its actually provides a small but not negligible amount of fuel (youve probably heard that cardio is fueled by a mixture of carbs and fat, depending on the intensity level, but theres 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, theyre 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 dont overtrain and dont underreat, and dont eat junk. Follow common sense procedures like easy day/hard day, dont do hours per day of cardio, and dont try to lose more than about a pound of weight per week. Make sure youre 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 OToole, 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 OToole, 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.

Im 54 and a mechanical engineer with a Ph.D. from Stanford. Biking and fitness have been my main hobby for many years. With my engineering background I am well versed in biomechanics. Im also self-taught about exercise physiology through many years of reading and research, and certified as a Personal Trainer by ACE. For more fitness information please visit my website http://www.bikeandfit.com

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Metabolic Profiling - The Key to a Fitter, Healthier and Slimmer you

Many people spend hours grinding away at the gym, or follow one unsuccessful diet with another so why do so few of them achieve their goals?

Firstly being on an endless diet will not help you lose weight, no doubt in the first few weeks gains will be made but in most cases it becomes unsustainable particularly if you are on an extremely low calorie diet, 1000 calories a day is not enough to sustain basic life function, so the body goes into starvation mode and slows down the metabolism and holds on to stored body fat. For many dieters I would suggest that to lose weight you need to eat more!

Your resting metabolic rate; the number of calories used to maintain basic life function such as brain activity, heart & lung function, tissue growth and repair, internal organ function. To this you need to add your daily activity calories, then any additional exercise calories, this will give you your daily calorific requirement. To achieve sustainable weight loss you need to get a small deficit from this number, most dietitians and nutritionists recommend a deficit of no more than 500 calories a day.

But the big question, what is my RMR number? How many calories do I burn as I go about my daily business? And how many calories do I use when exercising?

For this a metabolic profiling assessment is the answer.

This two part assessment, RMR and exercise will give you daily requirement plus the calories you burn during exercise as well as being able to determine your optimal exercise heart rates for fitness improvements and weight loss.

You are hooked up with a Hannibal Lecter-style breathing mask to an analyzer and software program that uses the same science used by sports performance laboratories around the world. Each part of the assessment takes about 10 minutes the first part which is very easy you are just relaxing in a chair this is followed by a progressive exercise test performed on a treadmill, bike, or cross trainer.

Once the information is gathered the software produces a 12 week exercise training program and those all important calorie numbers.

As well as not knowing the amount of calories you need to fuel up to achieve your goals few people know at what heart rate intensities they need to work at to be successful in improving fat burning capabilities, aerobic and fitness performance. The great thing after taking the test you will know for each heart rate exactly how many calories you burn and perhaps more importantly where those calories come from sugar or fat. You will be given the highest work rate where fat is the dominant fuel, after this point a greater amount will be coming from sugar, you will also find out at what heart rate you stop to burn fat and all the fuel comes from sugar! You will also learn your anaerobic threshold an important point with regards to improving our fitness.

Most people are aware that to adopt a healthy lifestyle will help in obtaining weight loss goals, as well as improving fitness, health and wellbeing, and that to do this we must be mindful of how we fuel the body and must take regular exercise, so many regimes start with all the best intentions only to fall by the wayside because of lack of results. You can diet but not eat enough and send your body into starvation mode, and you can pound out mile after mile on the treadmill and just be burning sugar, in other words you can guess how many calories you need and how hard you need to workout in the gym or the alternative is to find out exactly what you need to do by taking a metabolic assessment, I believe this is the key to a fitter, healthier, and slimmer you.

Metabolic assessment centre's are becoming more readily available across the country to find out more or where your nearest centre is visit http://www.humanlabvo2.com or contact Richard Baker on (020) 8543-5288 or email richard@humanlabvo2.com

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