Think of your body as a finely tuned machine, a bit like a motor car. You drive it to work everyday and it gives you no trouble. Then once or twice a month you decide to drive it up the equivalent of the north face of Everest - by taking it pot-holing. How does your delicate machine manage to cope with such a gear change ? What is going on inside the engine to allow you to survive such an ordeal ? In this article I hope to take you through some of the processes which our bodies undergo to keep themselves in A1 condition.
The general principle for survival is to keep the cells in your body alive. Obviously some cells such as brain cells and heart muscle cells are more important than others and if it came to the crunch these will be kept going at the expense of other cells (such as toe cells !). Cells don't need much - water, energy, nutrients and oxygen - and it's the body's job to deliver these at the right time in the right quantities. Keeping the balance just right is a process called homeostasis and much of your body's physiology is part of a feedback control circuit designed to do just that.
Consider the needs of your body in ticking-over mode. Water is taken to your cells by the circulatory system through blood vessels, and its the kidney which is responsible for regulating your fluid loss. Energy is stored in muscles, liver and fat tissue, and is also kept at a constant level in your blood by the action of hormones . Nutrients such as amino acids are stored in the liver and muscle. Amino acids can also provide energy but are needed as building blocks. Other nutrients such as salts are carried in the blood - these are regulated by the kidney. Finally oxygen is carried by the red blood cells in the circulatory system - the heart and lungs are involved in moving the required amount of oxygen about.
Now what happens when you increase your physical activity or if the external temperature changes ? More energy is required by the muscles to do the work and if its cold, energy is also needed to keep your temperature constant. The way in which the body responds to these changes in conditions and maintains the status quo is essential for survival.
All functions of the body require energy as fuel and this is taken in the form of food. Food is eaten as carbohydrates (sugar and starch), fat and protein, and all of these can be broken down to produce an energy currency called ATP which cells use to live. Energy can be moved from organ to organ and cell to cell by being converted into other forms, mainly gluocose and fat. Different parts of the body prefer to use energy in different forms - eg. the brain requires glucose as its sole source of energy. This means that glucose levels in the blood must be maintained to sustain brain function. Muscles can use a wider range of fuels - glucose, fatty acids and ketone bodies (a breakdown product of fat) but they prefer to use glucose for bursts of activity. So it is carbohydrates which are the most important food for a days caving.
Eating a good breakfast can give blood glucose levels as high as 120mg/100ml. This glucose can be stored in the liver and muscle in the form of glycogen. The average 70kg person has stores of glucose and glycogen in muscle and liver of about 1, 600 kCalories. After a meal (eg. double breakfast with beans at the Fountain) this can be a lot higher. Even at rest, this fuel will get used up - but expenditure of effort will result in it being used up more quickly. The liver responds to the levels of blood glucose dropping by converting its glycogen stores back into glucose. Whether glucose is stored in or released from the liver depends on the action of two hormones - insulin and glucagon. Insulin tells the liver to store glucose because there is plenty in the blood. Glucagon tells the liver to release glucose because there is insufficient in the blood. Insulin is only produced when there is plenty of glucose in the blood eg. after a meal, and so once it has done its job to get blood glucose levels down it in effect turns itself off. Insulin also pushes glucose into muscles where it is used up or converted to glycogen.
One potential problem that cavers face results from the fact that generally isn't customary to take sandwiches down the cave - unless you like them squashed and soggy. So after the huge breakfast, most people eat nothing or take a wee snack with them such as a Mars bar. After you have run down your blood glucose levels you may feel quite hungry but the last thing you should do is to eat very sweet food. The reason for this is that a Mars bar is concentrated glucose. It is readily absorbed into the body and blood glucose levels shoot up over a very short period of time. For this period you will feel quite good as glucose rushes into the muscles. However, high blood glucose leads to insulin being produced which will very rapidly push the glucose into the liver for storage. As the blood glucose levels approach 80 mg/100ml, insulin production will be switched off, but the high amount of insulin already made will be around for longer and will continue to lower the blood glucose levels even further. This means that it is potentially possible for blood glucose levels to drop quite low and you will feel terrible.
So what is the solution ? One way round this - if you really must eat - is to take a snack which has more complex carbohydrates such as starch. Some oat bars contain sufficiently low glucose and high starch that the blood glucose will go up slowly. Dried fruit is also good - prunes, raisins and apricots. Insulin levels will not shoot up as rapidly and the control system will be more finely tuned. If you do feel really tired, and your muscles are aching and you have to get up that pitch, then its likely that no amount of energy will help in this situation. Your muscles may well be bound up with lactic acid due to overactivity and this will only disappear with plenty of rest. Down a cave, this is not the most sensible thing to do because in general the sooner you get out the better. This aspect - stamina - can only be built up slowly over many trips, so take a short break and get someone else to help you get out - this is a team sport after all. .
But do you have to eat at all down a cave? A caving trip will easily use up all your glucose and glycogen stores, so if you don't eat what does the body do to stop you collapsing in a heap? Well other fuel sources are still available. The average person has about 24, 000 kCalories stored in protein and 135, 000 kCalories stored in fat. During an energetic trip you may use up as much as 6, 000 kCalories a day, but you still have enough energy reserves to keep you caving for a month ! The 1,600 kCalories of carbohydrate reserves are used up quickly though - easily within a day trip - so to keep yourself going, the body must start mobilizing the fat reserves. Its essential to keep blood glucose levels above 50mg/100 ml to keep the brain going, so some of the fat reserves must be turned into glucose. The brain and muscles also start using fat as the main fuel. As soon as blood sugar levels drop, fat stored in adipose tissue is mobilized as triacylglycerols, and the liver starts converting energy into glucose - a process called gluconeogenesis. This keeps the blood glucose levels at an acceptable level. In a caving situation, there may be a short lag whilst these metabolic pathways are activated. During this time, you may feel tired and hungry, but it will only be temporary. Once your fat is mobilized, your muscles will be able to use this energy source directly as fuel. Incidentally, many migratory birds can use their fat deposits to fly at 40 km per hour for 60 hours non-stop. Fat is a very efficient fuel and produces 6 times the amount of energy as glycogen/glucose. So the fat caver will have much more reserves than a thin one.
In addition to slow release carbohydrates, its an excellent idea to take some easily used fuel such as glucose (Mars Bars) down a cave just in case you get stuck there for a long period of time. Its not at all important that your blood glucose levels may drop temporarily in this type of situation because chances are you'll be sitting around waiting to be rescued and its not so critical as if you were trying to get up a pitch which requires huge amounts of energy. It is essential to keep your metabolism firing mainly because you need to keep warm. The amount of energy you lose due to heat loss has to be made up from your overall fuel reserves so your body will try and minimise heat loss by shutting down peripheral blood supply - your hands and feet may feel cold. This is not a problem though it may be quite painful. Keeping still and curled up to reduce surface area is the best thing to do. Walking about trying to get your hands and feet warmed up will only increase your heat loss and reduce your energy levels. Layers of body fat will keep you warm - so the fat caver wins again at this stage! To come to the thin cavers rescue, one of the reasons some people have more body fat than others is that they aren't very good at burning off excess energy in the form of heat. Thin people are thin because they can do this, so chances are can feel as warm under cold conditions as long as they have plenty of energy to burn.
Integration of Metabolism, Chapter 23 of Biochemistry by Lubert Stryer (1981) (published by W H Freeman and Company, San Francisco)
Metabolic Adaption to Prolonged Physical Exercise by H Howald and JR Poortmans (eds) (1975) published by Birkhauser Verlag, Basel.
copyright Elizabeth M Ellis, email: mailto:firstname.lastname@example.org