Overtraining syndrome in endurance athletes
Issue: Volume 27 Number 1
Definition
Overtraining has been defined as, ‘an increased training volume and/or intensity, that results in physical performance decrements’. It has been noted that overtraining in strength athletes is different to overtraining in endurance athletes. This article will focus on endurance athletes.
Overtraining has been demonstrated to be a dysfunction of the nervous and endocrine systems.
Much of the endocrine (hormonal) system is regulated by the hypothalamic-pituitary-adrenal(HPA) axis. Studies have shown that theHPA axis is dysfunctional or down-regulated in overtrained endurance athletes. These studies show that overtrained athletes secrete less cortisol, adrenocorticotropic hormone, growth hormone and prolactin during exercise than an athlete who is not overtrained.
It has been noted by myself and others that two distinct patterns of nervous system response appear due to overtraining. These two states have been termed ‘sympathetic dominant’ and ‘parasympathetic dominant’ overtraining. These terms refer to the state of the nervous system at rest. Unfortunately the states are generalised and do not have an exact set of symptoms.
The sympathetic nervous system is the voluntary nervous system that controls movement and the fight or flight response. The sympathetic nervous system uses noradrenaline as its primary mediator. Sympathetic dominant symptoms include raised resting heart rate, increased heart rate for the same power out, getting the shakes and being easily irritated. These symptoms are associated with increased noradrenaline activity.
The parasympathetic nervous system is also called the autonomic nervous system and controls vegetative functions such as making your heart beat, controlling breathing and digestive functions. Parasympathetic dominant symptoms include the resting heart rate ‘appearing’ normal but failing to rise easily during an effort. When the effort is reduced the heart rate falls very quickly. Many athletes mistakenly interpret this as a sign of being very fit, but their performance is poor. Other symptoms include feeling spacey, fast gastric transit time, incomplete evacuation of the stool and disturbed sleep patterns.
The exact cause of overtraining is unknown. An emerging area of research may give some of the answers. This area of research may provide the mechanism by which the nervous and endocrine systems are inhibited. The new area of research is called hypoglycemia-associated autonomic failure.
Hypoglycaemia is low blood-glucose levels. Hypo = low, glyco = glucose and aemia = of the blood. Normal blood-glucose levels are regulated by nervous and endocrine system responses. These responses are mediated by the hypothalamic–pituitary–adrenalaxis. As stated earlier in the article the hypothalamic–pituitary–adrenalaxis is dysfunctional in overtrained athletes.
These studies were designed to look at the effects of repeated hypoglycaemia in healthy human subjects. A group of study subjects were sent hypoglycaemic on two consecutive days using controlled insulin injections. The subjects’ nervous and endocrine (hormonal) system responses were then assessed.
The study demonstrated that normal blood-glucose regulation to hypoglycemia is diminished with consecutive bouts of hypoglycemia.
During the second bout of hypoglycemia, the study noted that there was impaired secretion of all major hormones that maintain normal blood-glucose levels. This reduction in hormone secretion was attributed to a failure in the autonomic nervous system. This altered pattern of nervous system response and hormone secretion is identical to the pattern displayed by overtrained athletes.
It is the author’s view that overtraining in endurance athletes is caused by repeated hypoglycemia. The progression into overtraining may be by different paths as outlined in the simple and complex models (see the next article in this series 'Overtraining syndrome'). The severity and frequency (number of repeated events) of the hypoglycemia will determine the severity of the overtraining.
Whether the athlete presents with the sympathetic dominant or parasympathetic dominant overtraining depends on the severity of the first bout of hypoglycaemia. If the first bout of hypoglycaemia is mild, the body only shuts down the adrenal response and reduces sympathetic nervous system activity. If the first bout of hyoglycaemia is severe, the body shuts down both the adrenal response and sympathetic nervous system activity. This is when the parasympathetic dominant symptoms appear.
Simple model
The simple model is two or more consecutive bouts of hypoglycaemia that lead to a failure of the autonomic nervous system.
An athlete can induce autonomic failure by two or more consecutive bouts of endurance training lasting longer than 90 minutes without the use of a carbohydrate source.
Ninety minutes of exercise is required to deplete glycogen stores, which will in turn cause hypoglycaemia. This simple model can be exacerbated by a low carbohydrate diet, which does not adequately replenish glycogen stores between consecutive days of training. This is the simplest cause of overtraining in endurance athletes.
Preventing hypoglycaemia during training
Use 60 grams per hour of a quality carbohydrate source. The carbohydrate source should be polymerised glucose which is usually made of hydrolysed corn starch. My favourite carbohydrate source is Maxim Original. Most sports drinks contain little polymerised glucose and a lot of simple sugar.
The benefits of using complex carbohydrate during training include training 25 per cent further and a reduction in the secretion of a stress hormone call cortisol. Cortisol is an important hormone and is a key component in the progression into overtraining. The association with cortisol will be expanded in the next article in this series.
Prevention of hypoglycaemia while training is the first step in preventing overtraining syndrome. In the next article in this series, we will look at the complex model of overtraining and make more suggestions on the prevention of overtraining.
The study also demonstrated that the subsequent reduction in cortisol secretion leads to a reduction of hepatic glucose production via gluconeogenesis. The significance of this work is that it was conducted on normal human volunteers and is thereby a standard human response to repeated hypoglycaemia.
