Chronic fatigue is caused by a depletion in glycogen stores. The remedy is eating a diet high in carbohydrates.
Most athletes have experienced burn-out, also known as chronic fatigue, at some point in their athletic careers. Chronic fatigue is when an athlete becomes persistently exhausted during consecutive days or months of strenuous training to the point that their form and performance markedly suffer. Chronic fatigue usually occurs after a dramatic increase in training or late in a season; hence an additional name, overtraining. Chronic fatigue is a phenomenon that is found across many sports. In fact, a study was recently released looking at measures athletic trainers of professional soccer teams took to combat chronic fatigue (Recovery in soccer, 2013. Nedelec M, et al.).
It has long been known that the physiological root of chronic fatigue is a depletion of muscle glycogen stores (Effects of repeated days of intensified training on muscle glycogen and swimming performance, 1988. David L Costill, et al.). The simplest way to combat chronic fatigue is by eating a diet high in carbohydrates during periods of physically taxing training. The aforementioned 1988 study had twelve elite swimmers more than double their daily mileage for ten consecutive days. They found that the swimmers who were unable to complete the training had significantly lower levels of muscle glycogen. Interestingly, the group of swimmers that were unable to complete the training had a low carbohydrate diet to begin with and did not increase their caloric intake despite the increase in training. The differences in glycogen stores of those able to tolerate the training increase (group "B") and unable to tolerate the training increase (group "A") can be seen in figure 1 below.
What is it about glycogen reserves that are so important? Glycogen is the body's primary source of energy for exercise lasting longer than ten seconds up to 20 minutes or longer depending on the exercise intensity. As glcogen stores, found in both the liver and muscle itself, are used up, the body begins shifting over to fats. The problem is that fats require more oxygen to catabolize to an equivalent amount of energy (measured in ATP). When glycogen is used up, exercise capacity is severely limited. To make matters worse, the brain can only utilize blood glucose for energy (under non-fasting conditions). Whatever glucose is left is needed for the brain. The muscles must switch to almost 100% fat. In marathon runners this switch is often referred to as "hitting the wall." It usually occurs in a trained marathon runner around miles 17-20.
Obviously having large glycogen stores is important for marathon runners, but what about athletes whose sports require a much shorter duration of continuous exercise? They are affected as well because even as glycogen begins being used up the body is switching over to more costly fat metabolism; this occurs even with relatively short durations of exercise. For example, a 90 minute soccer match leaves players' glycogen reserves severely depleted (The copenhagen soccer test: physiological response and fatigue development, 2012. Bendisken M, et al.). The way that glycogen depletion creates fatigue may be through restriction of Ca2+release, which causes muscle fiber contraction (Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle, 2011. Ortenblad, et al.). As exercise intensity (%VO2max) increases, the body obtains a larger percentage of its energy from carbohydrates (see figure 2). As a side note for those looking to burn weight, while intense exercise burns carbohydrates, moderate exercise is what burns unwanted fat.
Recovery is an energy intensive process. Carbohydrate ingestion aids recovery (The use of recovery methods post-exercise, 2005. Reilly and Ekblom). Most likely the way that depleted glycogen reserves hinder recovery is via reduced energy availability for recovery. There are many companies in the business of selling recovery products. Perhaps this is the reason for the plethora of misinformation on what the body needs to recover. The research suggests that all the body needs is plenty of carbohydrates, sources of which do not necessarily need to be marketed directly to athletes.
Next time you start feeling burnt out, just remember this is your body's way of telling you your glycogen reserves are running low. Time to have pasta for dinner!