The energy trap

Cyclists have a term for it - the Bonk -that sudden loss of energy and mental awareness. Doctor TONY MATTHEWS explains why it happens and how to tackle it

Most cyclists have bonked at one time or another. It happened to me quite recently, on a very hot Sunday. I'd been turning the pedals all day, and everything resembling a shop or cafe was closed.

Thoughts of a drink obsessed me. My pedalling was getting weaker and weaker. Was I hallucinating? Suddenly, in a field, I seemed to see a shimmering white marquee, with ladies in aprons doling out tea. "Six cups, please, and some cake," I heard myself say.

It was the real stuff, and I revived in time to see a big man with a black Labrador slide silently to the ground clutching at my rear mudguard as he fell, pulling it round the wheel. Dead drunk. The real and the surreal were hard to tell apart.

In practice there are two forms of the bonk, resulting from quite different mechanisms. The distinction is very important as they need to be treated in different ways.

The more common form of the bonk results from water and salt loss through sweating during strenuous activity. Sweat contains a large amount of salt. People exerting themselves in hot weather sometimes consider that they are not sweating if there is enough air movement to dry the sweat as it arrives on the surface of the skin. Proof of sweating is the taste of salt on the skin. The higher the humidity the greater the sweating for the given amount of exercise. The loss of enough sweat to decrease the body weight by only three per cent can significantly diminish a person's performance.

The early symptoms of salt depletion are a feeling of fatigue, sometimes with headache, loss of appetite, mild nausea and fleeting muscle cramps, which maybe accompanied by mental vagueness or confusion, less noticeable to the victim than to his or her companions. Often the earliest sign is pain in the muscles of the legs on restarting cycling after a brief rest. The pain wears off, only to recur more severely after the next rest. In its extreme form the Bonk leads to collapse and loss of consciousness and is known as heat stroke.

Dehydration and thirst are also symptomatic. Moreover the thirst is only briefly slaked by drinking pure water, the greater part of which is soon passed in the urine. But if you drink a saline solution which is nearly isotonic (ie equal in concentration) with the body fluids the water is retained, and thirst is quenched. Drinking pure water when you are salt deficient is likely to bring on severe muscle cramps.

These events are more likely to occur during the early period of training or strenuous exercise. One of the effects of training and acclimatisation is to reduce the salt content of sweat. After as little as seven to 14 days' training in the heat the exerciser begins to sweat much more rapidly and profusely, producing a more diluted sweat. The fully trained individual, who is acclimatised to hot conditions, is much less likely to need salt supplementation, unless, of course, he or she transfers to tropical conditions of heat and humidity.

The thirst mechanism is not enough to stimulate complete rehydration after heavy fluid losses during exercise. This means that people who exercise in the heat will not voluntarily drink enough water to replenish body fluid stores after exercise, in which case the body fluids will be only gradually restored over a period of one or two days. Because of this, strenuous cycling over a number of consecutive days can lead to chronic dehydration, which greatly impairs performance. Athletes and ambitious cycle tourists must teach themselves to drink.

During continuous exertion lasting more than half an hour in hot weather it is advisable to drink 250ml of water every half an hour. If acclimatisation to exertion in the heat is incomplete the drink should consist of isotonic saline. This way the Bonk will be avoided.

The aim is not to increase salt intake before an event in the hot weather but to replace it as it is lost in the sweat.

Excessive salt intake can cause nausea and vomiting, too much salt can do more harm than good. Avoid salt tablets - their salt content varies and their absorption from the gut is unreliable.

The standard bicycle bottle holds 530ml. The tall ones hold 750ml. Two large pinches of salt in half a bottle of water (250ml) is not too saline to drink and is a safe means of replacing the salt that is lost in sweat. Many cycles have only one water bottle carrier. For events or strenuous touring in hot weather the amount of fluid contained is usually insufficient.

Rather than carrying made-up saline solutions take a small amount of table salt in a wide-mouthed container. When salt is required put a large pinch of salt in the palm of your hand and lick this while drinking. This is quite a pleasant way of taking salt, but make sure you do not take less than the volume of water indicated above. Taking pinches of salt sounds unsatisfactory but it is surprising how consistent different people's large pinches turn out to be: around half a gram.

The second form of the Bonk is caused by a sudden lowering of the blood sugar, and results in weakness, exhaustion, mental vagueness or confusion, pounding of the heart, sweating and, in smokers, an intense desire to smoke. It can also lead to unconsciousness, but this is rare.

This form of hypoglycaemia is brought on, usually, and paradoxically, when sugar is eaten by people whose sugar regulating mechanism overreacts to the sudden intake of sugar by itself. There is often a history of having consumed sweets or a glucose drink an hour or so beforehand, perhaps in response to similar symptoms experienced then. Clearly the immediate treatment is further sugar or glucose, sufficient to make the victim feel normal. This, however, must be followed up by food consisting, ideally, of all three major components of our diet: complex carbohydrates (as opposed to simple sugars), proteins and fats.

The object is to achieve a further slow release of glucose into the blood stream to meet the inevitable output of insulin (resulting from 'the treatment') and to give it something to work on - ie the storage of carbohydrate as glycogen in the muscles and the liver.

All complex carbohydrates are broken down into the simple sugars: glucose, fructose and galactose. This is done by digestion, so carbohydrates can be absorbed into the blood stream. It all takes time - only glucose, water and alcohol are absorbed directly from the stomach.

Without this follow-up the insulin will merely store the glucose normally circulating in the blood and thus reproduce the symptoms of hypoglycaemia. The running fuel of the brain consists of glucose and oxygen. Depletion of either eventually leads to loss of consciousness.

What food to take depends on availability. Biscuits such as wheatmeals or shortbreads, with their high fat content are useful. Milk is an ideal food in such circumstances, as it is digested slowly and contains carbohydrate, protein and fat in the ratio 1.5:1:1 Sandwiches containing cheese, meat, or egg would serve as well. Beer is to be avoided as it lowers the blood sugar. The traditional cyclist's 'bonk-bag' encourages this form of the Bonk if it contains only sweets.

The more common form of the Bonk is due to the loss of water and salt through sweating as a result of strenuous physical activity during hot weather, especially if the humidity is high.

As a result of research on troops exercising in the desert, one other electrolyte problem has come to the fore recently - loss of potassium in urine and sweat. Potassium is present in the body in much smaller amounts than sodium and it is kept, normally, and naturally within very narrow limits.

But beware! Too much potassium and too little can stop the heart. The safe way to deal with possible potassium loss is to drink one or two tomato juices as served in bars. Orange juice and other fruit juices contain roughly half the amount of potassium that tomato juice does. The normal dietary requirements are taken care of in fruits, including bananas and in vegetables and meats.