In previous Part 1 and Part 2 of this serie, we mentionned the importance of carbohydrates (CHO) in the recovery process which starts during the training session and continues post-workout. We also realized the impact of proper CHO feeding during exercise to delay peripheal fatigue within the working muscles. There have been some very interresting theories about CHO ingestion during exercise and the onset of central fatigue. The evidence on the subject is very thin. Some experts say CHO feeding and glucose level regulation could have an impact on central fatigue, some say it doesn't. Nevertheless, the whole mecanism surrounding this phenomenon is fascinating and worth's some attention, even though nobody knows for sure what's happening.
CHO and Central Fatigue
The central nervous system (CNS) is the master commander within the body. Any physiological and biomechanical action originates from the CNS. It relies on glucose as it's sole source of energy to work properly. It can't use any other macronutrients to fuel itself, therefore glucose needs to be available for your CNS, be it by liver glycogen degradation, neoglucogenesis or circulating blood glucose level.
Think about it for a second. What stops you when exercise becomes too difficult? Sure there is massive leg pain, heavy breathing and general discomfort but all those signs of ''pain'' are actual signals sent to your brain. It then analyses those signals and they result in a perceived effort. A general physical en psychological sensation that originates from your brain. Your perception of the effort you are actually putting in is a major factor influencing your capacity to keep going or not. I can tell you I had tons of incoming signals to my brain yesterday during my 3 minutes efforts! Your brain controls your sensations during exercise, your motivation and mood. Remember what it uses as fuel: CHO. Maintaining blood glucose at optimal level as been shown to lead to high CHO oxidation rate, higher blood glucose, reduced perceived exertion in subjects and lower cortisol and growth hormone concentration. Adequate CHO availability to the brain can enhance these physical and psychological parameter and help you achieve your training session or race with higher performance and/or less fatigue.
Now the most interesting part (and debatable one) is the relation between blood glucose level and circulating blood free fatty acids (FFA). If blood glucose level is not maintained, insulin decreases and concentration of hormones epinephrine, cortisol and growth hormone increases, which means blood FFA also increases. An increase in blood FFA as been shown to lead to an increase in blood free-Tryptophan (f-TRP), an amino acid that can be converted in the brain into serotonin. Tryptophan (TRP) generally circulates in blood attached to albumin, a protein found in blood which plays a transporter role. When blood FFA increases, f-TRP also increases because FFA fights with TRP's binding site on albumin. Free-Tryptophan can make it's way to the brain and be converted into serotonin, a major CNS neuro-transmitter associated with arousal, mood, sleepiness and lethargy. Higher concentration of serotonin are generally associated with higher concentration of it's major byproduct, a specific acidic metabolite.
Some studies have shown a relation between fatigue and high concentration of serotonin and it's metabolite. They also investigated dopamine level, another neuro-transmitter involved in muscular control, motivation and arousal, along with it's major metabolite which has stimulating effects on the CNS, as opposed to serotonin, which has suppressive effects. They found dopamine level and it's metabolite were lower when fatigue occured whilst serotinin and it's metabolite level were higher. An inverted relationship which potentially explain fatigue originating from the CNS. All of this very complexe phenomenon originates from blood FFA higher concentration caused by lower blood glucose levels.
Take this with a grain of salt, as some studies have found no relation between brain serotonin levels and the onset of fatigue. As with any study, one says something, the next one says something else. But I thought the whole complexe relationship between CHO and the subsequent chain reaction was a very interresting one to write about and it would certainly require more investigation.