Endurance capability depends on diet and rate of work. The companion article on rates of work will have to wait until another time as I am still collecting data. This article outlines dietary requirements for long endurance events.
"What should I eat during long endurance walks?" is a question that has been asked quite often recently, as ExCS members have trudged round the Welsh '3000s' then the Lakes '3000s' and finally the Bob Graham Round, which is a traverse of 42 peaks of the Lake District covering 72 miles and 27000 ft of ascent. Another question often posed is "How should I train?"
Mike Perryman solved the training problem in Holland (where there aren't any hills) by arriving at his office early in the morning, punching the button to send the lift to the top floor, darting out and racing up the stairs to meet it, travelling down in the lift and repeating the process until the building got too crowded to continue. John Bowers noted that since carbohydrate provides energy at about 4 kcal/g and fat provides about 9 kcal/g the best food with which to fill your rucksack was lard. John pointed out that lard was the ideal survival food - if you got cold you could rub it on your body for insulation, and you could burn it for heat and light at night. The Bowers hypothesis has yet to be put to the test, but there are indications that it is not the best diet to choose, even though the government Manual of Nutrition lists lard as being the cheapest source of energy.
Energy requirements for various rates of exercise are frequently quoted. The common unit of energy is the kilocalorie (kcal), the metric unit is the joule. 1 kcal is equivalent to about 42 kJ. Rates of energy consumption can be quoted in kcal/hour or kJ/hour and are also sometimes quoted in Watts. 100W is 360 kJ/hour or 86 kcal/hour. Average energy requirements for a 70 kg man might be quoted as:
|Sleeping/resting||65 kcal/hour to 90 kcal/hour|
|Light to moderate exercise||200 kcal/hour or 3000 to 4500 kcal/day|
|Snow walking, skiing||470 kcal/hour|
|Running, hill climbing||600 kcal/hour|
Energy is obtained from fat and carbohydrate. Protein is not normally metabolised for energy unless there are serious deficiencies in the diet. Carbohydrate is stored in the body as glycogen in the liver and muscles. Typically muscle glycogen amounts to 480 kcal and liver glycogen to 280 kcal. Fat stores can amount to over 140,000 kcal. Energy release from fat is slow. For high intensity exercise (at 70% VO2max) energy is mainly derived from carbohydrate metabolism. The proportion varies with the level of exercise and glycogen depletion is one of the main causes of exhaustion. High protein powder, vitamins and energy bars all claim to aid endurance, but the most effective method is training. One of the characteristic features of an endurance trained person is the increased ability to metabolise fat. This has a glycogen sparing effect allowing exercise to continue for longer before glycogen depletion and hence exhaustion occurs.
There are several energy pathways in the body but the short term ones are not of interest here. There are two long-term aerobic pathways. Typically glycogen metabolism can produce energy at twice the rate of fatty acid metabolism. A 70 kg man fed on a normal mixed diet might have enough glycogen for around 115 min endurance at 75% VO2max. Glycogen levels can be boosted by eating a high carbohydrate diet, and are reduced by eating a high fat/protein diet. In addition, the technique of carbohydrate loading, described later, can 'trick' the body into storing more glycogen than normal.
As noted above, fat provides double the energy per gram than carbohydrate, but cannot be metabolised as fast. The body contains large stores of fat but they are not easily mobilised. One of the characteristic features of an endurance-trained person is the increased ability to metabolise fat. This delays the onset of exhaustion caused by glycogen depletion. If the rate of energy expenditure is consistently greater than energy intake then body fat is metabolised to make up the difference. This causes a potentially dangerous condition known as ketosis where ketones can be detected in the blood and urine. This is also apparent where a high fat diet is eaten without 'acclimatisation' and can be so severe that fitness drops to nothing and acetone can be smelt on the breath.
Adaption to a high fat diet has advantages where the weight of provisions is important. The Footsteps of Scott expedition (who man-hauled 350 lb sledges to the south pole) decided on a diet that was 57% fat, 34% carbohydrate and 9% protein, providing 4800 kcal/day, and reported no ill effects, but they were extremely endurance-trained. For comparison, the figures for a typical Western diet are 38%, 51% and 11% providing 2000 to 3000 kcal.
It is not a good idea to eat fat during heavy exercise. It slows down digestion and delays the absorption of any carbohydrate eaten at the same time. For a stroll on the hills this is not going to matter, but for a serious endurance event you should not eat foods such as cheese and meat during exercise. After exercise a meal high in fat and protein can be satisfying precisely because it lingers in the stomach. If exercise continues over several days then you should, however, concentrate on replenishing the glycogen stores.
A high rate of exercise requires glycogen metabolism. The technique of carbohydrate loading can artificially boost glycogen levels before an event. The procedure is as follows. Heavy exercise to exhaustion for three days depletes glycogen stores. A high carbohydrate/low fat diet is eaten for the next three days, with only light exercise, and the glycogen level can be raised to two or three times its normal value. Water is stored along with glycogen and this can create an uncomfortable feeling, but since it is released during glycogen metabolism it helps to maintain fluid balance. It is important to eat the right sort of carbohydrates. Cereals, bread, biscuits, potatoes, rice, pasta, yoghurt, dried and tinned fruit are all starchy carbohydrates and contain the B vitamins essential for their metabolism. You should avoid eating meat, sausages, pastry, cheese, butter, margarine and fried foods.
Carbohydrate requirements during exercise are similar. Sandwiches, fruit and fruit juices, dried fruit and cereal bars can be used to top up the glycogen levels and delay the onset of exhaustion. Standard advice is that sugary foods should be avoided since they release energy too quickly. The sudden rise in blood sugar levels leads to over-production of insulin and hypoglycaemia. Whilst this applies to moderate exercise, I am not certain whether it applies to the longer duration, heavy exercise events. Practical observations on eating during exercise are given later.
Manufacturers of breathable fabrics use a figure for water evaporation from the skin that seems to equate to about 0.2 litres per 100 kcal of energy consumed. Total losses in hot climates can be many times this, over 2 litres/hour. Cold climates can have a diuretic effect, and there is some evidence that dehydration is a cause of frostbite.
The concentration of electrolytes in sweat is lower than in body fluids so sweating increases the concentration. The priority is therefore to replace the fluid rather than the electrolytes, so salt tablets are not, in general, a good idea. The electrolyte balance can be restored at the end of the day in a meal.
Lack of water can very quickly degrade performance. Experiments have shown that even drinking water 'ad lib' may not be enough to replace losses. On a hot day you really need to drink to excess.
There are two groups of so-called sports drinks. Energy drinks are most useful after an event and taken during heavy exercise can cause vomiting. Electrolyte drinks contain small quantities of salts in isotonic concentrations. These are not to replace electrolyte losses, but to aid water absorption from the intestine, which is otherwise rather slow. In addition, during heavy exercise the gut does not work very well so large amounts of neat water can pass straight through.
Rather than buy an electrolyte drink you can mix up your own powder. (sodium chloride and glucose oral powder BP is NaCl 500 mg, NaHCO3 750 mg, KCl 750 mg, glucose (dextrose monohydrate) 20 g diluted to 500 ml for use). This is the salt and glucose solution given to babies with diarrhoea. The glucose is not there for energy, but to balance the concentration in the blood and therefore aid absorption. The bicarbonate will effervesce off if acidic fruit juices are added which is a pity as the drink otherwise tastes unpleasantly like sugary salt water.
Unless you are actually body-building you do not need extra protein. The western diet contains plenty of protein, and protein drinks are therefore a waste of money. Protein metabolism can be deduced by measuring the nitrogen content of urine, faeces, sweat and shed skin tissue. Experiments on mountaineers have shown that there is no significant increase in protein metabolism during exercise. Even if you were body-building, simply eating extra protein is not sufficient. Excess protein is not stored, and unless you get the balance of amino-acids right you just waste what you eat.
Vitamin supplements are not normally needed though many people take them. If you are eating a lot of carbohydrate then you need the B group vitamins that are involved in metabolism and energy release. Starchy, unrefined carbohydrates such as brown bread, pasta, rice and potatoes contain B vitamins whereas refined sugar, of course, doesn't. This highlights the importance of a sensible, balanced diet. Vitamin supplements are useful on expeditions where the range of foods available may be limited.
Eat starchy carbohydrate and limited fat. Do not eat fat if you are carbohydrate loading. There is no need for protein supplements. The need for vitamin supplements is questionable. Remember - training does not only improve your muscles, it increases your ability to metabolise fat.
Eat starchy carbohydrates. Do not eat fat or protein. Possibly do not eat refined sugary foods. Drink plenty of fluid - more than you think you need, and in an isotonic form.
The long walks we have been doing recently come outside the scope of most physiological data. Sprint, middle distance and marathon runners know how to train in the way to suit them best. At the other extreme, long expeditions provide physiological data on adaption to particular diets and exercise regimes. A Bob Graham Round is not covered by any of these events, being a 24 hour endurance event at a metabolic rate of perhaps 500 kcal/hour. These very brief observations were made during the Lakes 3000s and Bob Graham Round walks by members of ExCs in '87 and '88.
I noticed several physiological effects. Swelling of the fingers was one symptom, possibly caused by lack of blood circulation in the extremities. Diarrhoea and 'upset stomach' was common, possibly due to lack of blood supply to the intestine, and the gut 'shutting down' Two people took Imodium (2 mg loperamide hydrochloride) prophylactically to reduce gut motility. (If diarrhoea is persistent then isotonic drinks, especially the recipe mentioned above, are beneficial as they replace not only sodium but potassium and bicarbonate).
Everyone had their own food preferences, but dried fruit, bananas, tinned peaches (don't forget to take a fork), rice pudding, flavoured milk and yoghurt drinks were all popular. Chocolate had to be forced down, and cereal bars were not popular. People reported a general disinclination to eat anything. It is known that sweet foods can cause queasiness and vomiting during exercise, and the general opinion was that savoury foods would have been more welcome. Unfortunately most of these contain fat and protein, but even so, tins of spaghetti bolognese were consumed at some rest points. Hot tea cheered people up. The calorific intake was probably well below 500 kcal/hour (the estimated metabolic rate) though no one did any accurate recording of the food consumed.
I should point out at this stage that I have not been trained in physiology and I would welcome any comments on the above information by a real physiologist or nutritional scientist. Even so, it would be interesting to run some tests during these events, though I can not see anyone willing to agree to this. Calorific value of food intake is easy to record. Metabolic rate can be measured using a respiratory monitor and oxygen analyzer. Mobilisation of body fat stores can be detected by measuring the ketones in a blood or urine sample using a simple test. Water loss can be measured by weighing the subject. Rectal temperature is a good indication of how dehydrated a subject has become. Blood sugar, blood pressure and pulse rate might also be interesting to measure. Would any volunteers please step forward?.
The usefulness of the above data is reduced slightly since I have not had time to compile a detailed set of references. The following sources (which in turn refer to others) were used.