For the endurance athletes, their performances are heavily dependent on having adequate supply of carbohydrate. Our body has a limited supply of carbohydrate compared to the amount of fat and protein. About 300-500 g of glycogen is stored in the muscles and another 75-100 g is stored in the liver (Bjorkman and Wahren, 1988). However, this amount is sufficient for us to run at moderate intensity for 20 miles (32 km)! An increase in exercise intensity will lead to an exponential increase in this utilization rate with the major contribution from muscle glycogen (Saltin and Karlsson, 1971).
It is not necessary for endurance athletes such as runners to engage in special dietary strategies such as carbohydrate loading during practices and races especially if the intensity is not high and the distance is relatively short.
Bjorkman, O., & Wahren, J. (1988). Glucose homeostasis during and after exercise. RL Terjung. New York: Macmillan, 100-115.
Saltin, B., & Karlsson, J. (1971). Muscle glycogen utilization during work of different intensities. In Muscle metabolism during exercise (pp. 289-299). Springer US.
It is common for endurance athletes to engage in glycogen loading or carbohydrate loading as it is more commonly called. They believe that this coupled with a modified training programme will maximize glycogen stores and lead to improved performance. Some researchers such as Hawley et al. (1997) as cited in Antonio (2008) suggested that glycogen loading of supercompensation can be beneficial for endurance bouts that last longer than 90 minutes. It is reported by Hawley et al. (1997) that glycogen loading can reduce fatigue during endurance training and increase duration of exercise bout by 20% while increasing workload or distance completed by 2-3%.
Accordingly to Antonio (2008), the classic carbohydrate loading method requires a 3-4 day glycogen-depleting regime which contained hard exercise coupled with a low carbohydrate diet followed by a 3-4 day repletion phase in which training volume and intensity were decreased whereas carbohydrate consumption was significantly increased. The drawback of this method is that workout quality suffered during the depletion phase and optimal performance was not achieved.
Sherman et al. (1981) as cited in Antonio (2008) proposed an alternative method. It required the athlete to consume a 50% carbohydrate diet for 3 days while slowly reducing training volume. The athlete will consume 70% carbohydrate diet from the fourth day while still reducing training volume. On the seventh day, the athlete will compete. The authors reported that this modified method is highly effective for loading glycogen with less risk of performance decrements than the classic method.
Antonio, J., Kalman, D., Stout, J. R., Greenwood, M., Willoughby, D. S., & Haff, G. G. (Eds.). (2008). Carbohydrates. In A. Haff, G.G (Eds.), Essentials of sports nutrition and supplements (pp. 301). Chapter Humana Press.
Hawley, J. A., Schabort, E. J., Noakes, T. D., & Dennis, S. C. (1997). Carbohydrate-loading and exercise performance. An update. Sports medicine (Auckland, NZ), 24(2), 73.
Sherman, W. M., Costill, D. L., Fink, W. J., & Miller, J. M. (1981). Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine, 2(2), 114.
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Consequences of endurance exercise
1. Muscle damage and soreness
Mild muscle damage actually stimulates the rebuilding process which results in new and stronger muscle protein. More severe damage can result in muscle stiffness and soreness, limiting recovery and inhibiting performance. There are three primary causes of muscle damage.
a) Contractile stress
- Muscle contraction (especially the eccentric phase) places great stress on the muscles -> small tearing of muscle fibre.
- An acute inflammatory response is triggered by an injury -> swelling at injured site -> further muscle damage. This response peaks after 24 hours which explains why soreness is often felt some time after the workout is completed.
b) Hormonal shifts
- Catabolic hormone, cortisol is released when blood glucose is low or during high
- Primary function of cortisol is to generate fuel for working muscles by activating gluconeogenesis, lipolysis and proteolysis. Proteolysis can cause muscle damage.
c) Free radical reactions
- Free radicals are generated during exercise which can damage muscle protein and membranes and may even affect proper functioning of the immune system.
- These radicals must be neutralized by antioxidants such as vitamins C and E.
2. Immune system suppression
Athletes who train intensely -> more likely to catch colds and infections. Moderately intense exercises stimulate the immune system but strenuous exercises coupled with work-life stress -> suppress immune function.
Several reasons exist for the immunosuppressive effects of strenuous exercise. These include an increase in blood cortisol and other stress hormones, and a reduction in blood glutamine and glucose. Cortisol is the main contributor which increases during strenuous exercise, low blood glucose and periods of mental stress. It lowers the concentration and activities of many important immune cells that fight infection. The immune system suppression can last up to 73 hours after exercise and significantly increase vulnerability to infection.
Antonio, J., Kalman, D., Stout, J. R., Greenwood, M., Willoughby, D. S., & Haff, G. G. (Eds.). (2008). Nutrition before, during, and after exercise. In A. Ivy, J.L. (Eds.), Essentials of sports nutrition and supplements (pp. 624-625). Chapter Humana Press.
All information presented on this site is meant for general purposes. It is not meant to replace health and medical advice from healthcare professionals.
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