Last week I wrote about one of the hypotheses for overtraining syndrome; muscle glycogen depletion. Our bodies are very complex, however, and this is only one small piece of a larger puzzle. This week let’s examine the central fatigue hypothesis.
Central Fatigue Hypothesis
I lost sleep for nearly a month after I hit the wall of overtraining. I stayed in bed, tired, for hours just thinking and wishing for sleep. I have tried several techniques for sleeping including avoiding food and exercise before bed; I was still lucky to get 2 hours of sleep per night. In addition to (and partially due to) the loss of sleep I became depressed and eccentric. I am normally a happy person but I found no pleasure in most of the things that I loved. Why?
Tryptophan and Serotonin
There are a few key players involved in altering our central nervous system as a result of too much exercise. The first of which is a neurotransmitter that you should be familiar with, serotonin. Serotonin can also be referred to as 5-hydroxytryptamine (but let’s just call it 5-HT). In order to make 5-HT, the body needs tryptophan. Studies have linked an abundance of 5-HT to fatigue. Have you ever wondered why people talk about the tryptophan in your Thanksgiving dinner? Maybe you’ve eaten so much turkey that you dozed off for a little while?
Branched Chain Amino Acids
Branched chain amino acids (BCAAs) are my favorite workout drink. The following day I am not sore (or not as sore as I COULD be). Amino acids are a basic building block used by our bodies to construct protein. Every single protein that exists consists of a sequence of 20 amino acids. Of the 20 amino acids that exist, there are 9 that we must ingest through our diet. These include threonine, valine, leucine, isoleucine, methionine, phenylalanine, tryptophan, histidine, and lysine. Among these nine are BCAAs. Valine, leucine, and isoleucine are the only three amino acids that the liver cannot break down. That means they actually enter into our bloodstream and circulate.
BCAA’s and unbound tryptophan both compete with each other for access into the brain through the blood brain barrier. When we exercise the level of free unbound tryptophan in our blood is elevated. Exercise also depletes the BCAA s from our blood by oxidizing them in the skeletal muscle. The altered amino acid concentrations favor tryptophan for entry into the brain allowing an increase in 5-HT synthesis.
Unfortunately this theory hasn’t been thoroughly examined and fatigue is very subjective. This theory does not explain the other symptoms of overtraining, and as I stated before, our bodies are amazing and incredibly complex. The brief summary I’ve provided here is very watered down but I hope you were able to get something out of it. Please see the references below for more information, they are great reads and very informative.
What are some things you’re wondering about with regards to your body and fitness or nutrition?
Shimomura, Yoshiharu, Taro Murakami, Naoya Nakai, and Robert Harris. “Exercise Promotes BCAA Catabolism: Effects of BCAA Supplementation on Skeletal Muscle during Exercise.” The American Society for Nutritional Sciences 134.6 (2004): 15835-5875. Web.
Nicastro, Humberto, Claudia Ribeiro Da Luz, Daniela Fojo Seixas Chaves, Luiz Roberto Grassman Bechara, Vanessa Azevedo Voltarelli, Marcelo Macedo Rogero, and Antonio Herbert Lancha, Jr. “Does Branched-Chain Amino Acids Supplementation Modulate Skeletal Muscle Remodeling through Inflammation Modulation? Possible Mechanisms of Action.” Journal of Nutrition and Metabolism (2012): n. pag. Web.
Bailey, S. P., J. M. Davis, and E. N. Ahlborn. “Neuroendocrine and Substrate Responses to Altered Brain 5-HT Activity during Prolonged Exercise to Fatigue.” Journal of Applied Physiology 74.6 (1993): 3006-012. Web.
Meeusen, Romain, Philip Watson, Hiroshi Hasegawa, Bart Roelands, and Maria F. Piacentini. “Brain Neurotransmitters in Fatigue and Overtraining.” Applied Physiology, Nutrition, and Metabolism 32.5 (2007): 857-64. Web.
Lee, Sang-Won, Yeon-Soo Kim, Tae-Won Jun, Jin-Hee Seo, Kijeong Kim, Mal-Soon Shin, and Chang-Ju Kim. “The Impact of Duration of One Bout Treadmill Exercise on Cell Proliferation and Central Fatigue in Rats.” Journal of Exercise Rehabilitation 9.5 (2013): n. pag. Web.