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From Richard Smaje

I was very interested in Timothy Noakes’s “central governor” idea of protective fatigue discussed in the article “Running on empty” (20 March, p 42). It seems to fit with some of my experiences of steady exercise.

However, it doesn’t seem to square with the condition of sudden exhaustion well known to cyclists (especially myself as a rather unfit example) as “bonking”, in which the victim suddenly, and with little or no prior fatigue, feels completely drained of energy, shaky and almost unable to continue. I’m no physiologist, but I believe this is a hypoglycaemic state, as taking in plenty of glucose and water brings about rapid recovery.

What interests me is that there seems to be no protective fatigue prior to the onset of exhaustion here. I wonder how this would fit with the ideas discussed.

Timothy Noakes replies:

• We are actively researching this question. We have found that there are two responses to reduced liver glucose/glycogen stores – the ultimate cause of the symptoms Smaje describes.

Athletes who are on a high carbohydrate diet and who take in carbohydrate during exercise will usually stop when they experience some of the symptoms that you describe, but with almost normal blood glucose concentrations. The governor can be said to work well in their case, as it terminates the exercise before chemical hypoglycaemia develops.

In contrast, those who do not eat a particularly high-carbohydrate diet, or who do not take in carbohydrates in training or in competition, are likely to have very low blood glucose concentrations when they reach the point of exhaustion. Clearly the governor has failed to stop these exercisers prior to system failure.

Interestingly, the fact that glucose ingestion rapidly reverses these symptoms provides good evidence for the action of a brain controller sensitive to either blood glucose concentrations or liver glycogen stores, or the rate of liver glucose production, or combinations of any of these variables.

Oxford, UK