by Chad Kerksick PhD August 31, 2011 6 min read
From time to time, it’s really cool when you go back and find reinforcement for things that you are doing or that may be on your mind. It’s the reason why I think various messages inside religious books are interesting because it was so long ago, but yet may still apply a great deal to things you are dealing with today. The same can be said with aspects of research as the articles discussed today are from 1980 and 1991. That’s a good while ago for sure, but the findings are as relevant today as they were back then.
To begin our discussion, a few points will be reviewed. During exercise, our body uses carbohydrates, fats and proteins as ingredients for energy, which is needed by our body to power our muscles, complete chemical reactions and so forth. Carbohydrates are the overwhelming preferred source of fuel with fats and proteins coming in next. As exercise intensity increases in your workouts, the percentage of energy used by the body from carbohydrates increases. Why? The human body can burn carbohydrates much faster than it can fats or protein and this is important when exercising because the demand for energy goes up and energy needs to be delivered quickly.
One problem exists, however, and that is your body has limited carbohydrate stores: only around two hours or so worth if exercising at a moderate intensity of exercise and even less if the intensity is high. For example, a typical bout (35-40 minutes) of resistance exercise depletes stored carbohydrate by 30 – 35% (Robergs et al. 1991; Haff et al. 2000), so it stands to reason that much greater depletion occurs if the intensity increases and the duration continues for much longer.
When carbohydrate stores become depleted and exercise continues, the body responds in a couple of ways: 1) exercise intensity will drop off because of a lack of readily available energy stores and decreased intensity results in a decreased overload or training stimulus and 2) the body starts to burn more fat and more protein. Burning more fat sounds good, but burning protein isn’t good, especially if you are trying to put on a few pounds of muscle or follow a diet program and need to maintain your muscle mass and metabolism. Yes, as we have discussed in previous articles, more muscle translates into a higher resting metabolism (Stiegler and Cunliffe 2006).
This is where our old school research comes in, but make no mistake about it, the two research groups being discussed have been stalwarts in the research community of exercise science. In the first study, trained cyclists attempted to cycle for two hours at 70% of their maximum power in either a carbohydrate-depleted state or a carbohydrate loaded state (Wagenmakers et al. 1991). The athlete’s performance wasn’t of concern and instead the researchers measured a number of things in the blood that were indicative of protein and amino acid metabolism. Interestingly, when the cyclists attempted the ride in a carbohydrate depleted state they were not able to even complete the ride and the intensity had to be reduced to 50% of their maximum power. Keep in mind these were trained cyclists and not couch potatoes for research subjects, so their inability to maintain necessary power was surprising. Particularly when the same cyclists completed the same ride in a carbohydrate loaded state they were able to complete the two hour ride while maintaining 70 – 75% of their maximum power. Immediately, it becomes clear that adequate carbohydrate stores are necessary if you want to maintain maximum intensity in your workouts (Wagenmakers et al. 1991). I’ll go one step further and say if you can’t train at your maximum intensity, then you’re not going to get all you can out of your workouts. That’s right, just spinning your wheels…figuratively and literally!
Another important finding was that when the glycogen content in the muscles were compared to the activity of an important enzyme that breaks down amino acids in muscles, the lower glycogen content was related to higher enzyme activity and vice versa. Similar findings were also found between glycogen content in the muscle and other markers of protein and amino acid breakdown. Why did this happen? The researchers felt that the decreased level of glycogen in the muscle sent a signal to the brain to start “saving” carbohydrate and start breaking down other things for energy. As a result, protein breakdown increased. The take-home message from this finding is that exercising with a higher level of muscle glycogen helps to prevent increases in metabolic pathways that lead to the breakdown of amino acids and protein for energy (Wagenmakers et al. 1991).
The other study also had well-trained cyclists complete one hour of cycling at 60% of their maximum (Lemon and Mullin 1980). Before it’s discussed further, this exercise bout is what would be considered moderate exercise and something that most individuals could complete whether they were highly trained or not. This is important because it makes the research findings realistic for everyone and they just don’t apply to some crazy, animal of an athlete that ran, cycled or lifted for some ridiculous amount. Before, during and for four hours after they completed the exercise bout, measures of urea production, a marker of protein breakdown was measured. Like before, the cyclists exercised on two occasions: a carbohydrate depleted or a carbohydrate loaded state. When carbohydrate depleted, but not when carbohydrate loaded, the amount of protein breakdown (urea production) which occurred during the exercise bout increased every time it was measured and when exercise was completed, protein breakdown continued to rise even further. In fact, the authors estimated that breakdown of protein accounted for approximately 10% of the all energy required by the body. Another manner in which this study relates to you is the impact of completing a moderate exercise bout while being low on carbohydrate stores.
Certainly, I don’t expect that many of you regularly complete a regimen to purposefully deplete your carbohydrate stores as these cyclists did, but how many of you have worked out after not eating anything all day? Who has worked out after consuming suboptimal amounts of recovery nutrients like carbohydrates and protein before working out again? Finally, who considers their workouts to be longer than 60 minutes and greater than a moderate level of intensity? If you were saying yes to two or more of these questions, it’s likely your carbohydrate stores are taking a hit and that means your protein (muscle) stores are taking a hit as well.
In the end, there’s no perfect solution, as everyone is different, but making sure carbohydrates stores are topped off before starting a workout helps to ensure maximal intensity and performance and will also help your body to spare its protein stores. If you are trying to improve your body composition through diet and exercise and/or trying to add muscle, this means you.