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Can you give your performance an edge by fuelling up before exercise? Here are the guidelines.

The main nutrient to watch in this context is, of course, carbohydrate; glycogen (the body's carbo store) is the limiting fuel for endurance exercise. Eating a diet high in carbohydrates while training should ensure that you have good muscle glycogen stores prior to competing. Available evidence shows that topping up with extra carbohydrate just before the action has a negligible effect on events of short or moderate duration (less than 60 minutes). It's like adding an extra gallon of petrol to a car with a full tank before a short journey. But if your initial glycogen levels are low (eg, if you're participating in a tournament taking place over a week) and/or the exercise lasts 90 minutes or longer, pre-exercise carbohydrate may improve your performance.

The type of food and its timing is crucial to whether it's helpful or harmful. Some basic physiology should help explain why this is. Exercising muscles burn fat and glucose - the glucose is obtained both from the glycogen stores in the muscles, and from glucose circulating in the blood. The liver has the task of masterminding blood glucose levels and trying to ensure that they don't dip too low (causing hypoglycaemia, resulting in weakness, dizziness and nausea). If the liver registers that blood glucose levels are dropping, it can release glucose into the blood from its own glycogen store. If blood glucose levels rise (eg, after eating food containing carbohydrate) the hormone insulin is released, which forces glucose out of the bloodstream and into storage as glycogen.

The relevance of this to pre-exercise nutrition is as follows. During a fasting period (such as overnight), liver glycogen stores will be lowered. If you compete in a morning event without having eaten anything since the night before, you're starting at a disadvantage. Although your muscle glycogen will still start off high, once it begins to be used up (after an hour or so) there's a reduced amount of blood glucose supplied by the liver to turn to. You can avoid this problem by eating a high-carbohydrate meal 1-4 hours before exercise. The available evidence suggests that the optimal amount is somewhere between l-4g of carbohydrate per kg of body weight. The mount of carbohydrate should be less the nearer to competing to avoid gut problems. ('Carbohydrates' in Berning, J. and Nelson-Steen, S., 'Sports Nutrition for the 90s', Aspen, Maryland, 1991).

Avoiding the hypoglycaemic backlash

The insulin response is more difficult to pin down. Some carbohydrates cause a more marked surge in blood glucose (and hence insulin) than others. Measurements have been made and foods categorised according to their 'glycaemic index' (see table). A food's glycaemic index (GI) gives an indication of the degree of blood-glucose surge to expect. High-GI foods (which include glucose itself and bread, bananas) bring about a marked and immediate rush of glucose into the bloodstream, whereas lowGI foods (eg, beans, lentils) release glucose at a slow and steady rate over a much longer time period. There's a potential problem with high-GI foods - the high insulin response can can actually lead to an over-enthusiastic hoarding away of blood glucose, leading in turn to a net blood-sugar drop and its unwelcome effects. Some individuals seem to be more susceptible to this problem than others, however, which may help to explain why research studies have come up with apparently contradictory information in this area.

For some time in sports nutrition it has been accepted wisdom that sugar should not be consumed within the 60-15 minutes prior to exercise for fear of a hypoglycaemic backlash. Several early studies found that runtime to exhaustion was shorter by about 20-25 per cent after athletes consumed 2-3oz of glucose within an hour before an endurance test. However, a number of more recent studies have not found this effect.

For example, research carried out at the Gatorade Sports Science Institute in Illinois found that pre-exercise feedings of carbohydrate (6 per cent sucrose/glucose solution or a 20 per cent maltodextrin/glucose solution) did not result in hypoglycaemia, or adversely affect sensory or physiological responses during 50 minutes of moderate-intensity cycling. Exercise was started at the time of the individual's peak insulin response (usually somewhere between 20-45 minutes after carbohydrate consumption). Although blood sugar did drop when exercise commenced, it did not reach significantly low levels and returned to baseline values after 30 minutes of exercise. The volunteer cyclists were not aware of any subjective problems, such as perceived muscle weakness or nausea, during the exercise ('Glycaemic and Insulinemic Response to Pre-exercise Carbohydrate Feedings', Seifert, J., et al, International Journal of Sport Nutrition, 4, pp 46-53,1994).

Meanwhile, other research has found that the best food to take an hour before activity is carbohydrate which has a low glycaemic index. Scientists at the University of Sydney tested out four different pre-race feeds: ( 1 ) boiled lentils (2) baked potatoes (3) a sports drink comprising glucose dissolved in water, and (4) plain water. Volunteers cycling to exhaustion averaged 117 minutes with the lentils, compared with 108 minutes for the sports drink and only 97 minutes with potatoes. There seemed to be a link to blood glucose levels - after 90 minutes, blood glucose was about 20 per cent higher for the lentil eaters compared to those who took the sports drink ('Carbohydrate Feeding before Exercise: Effects of Glycaemic Index', International Journal of Sports Medicine, vol 12 (2), pp 180-186,1991). So there may be a benefit from a pre-exercise meal of lentils or the like if 1) you're going to be active for over an hour, 2) you think you suffer from reactive hypoglycaemia, and 3) you are unable to take in any extra carbs as you go. Can't stand lentils? Check the table for other low-GI foods.

Another strategy which should suit everybody, whether or not you have a tendency to hypoglycaemia, is to take in carbohydrate 5-10 minutes before exercise of an hour or more. If you're exercising at an intensity greater than 50% V02max, the insulin response to glucose ingestion is suppressed. Choose something that will be absorbed quickly either a sports drink or a high-GI food (solid food isn't recommended for runners so close to racing - take the risk only if you know your gut can handle it!).

racket sports (squash, tennis, badminton)

Compared to athletic events such as running and cycling, nutrition research related to racket sports is sparse. However, general guidelines can be put together by looking at the type of exercise involved. Most court games require a combination of strength, endurance and sprinting capacity, taxing both aerobic and anaerobic metabolism. As with any activity, the longer it goes on, the more likely that glycogen will become limiting. This means that it makes sense to eat a high-carb diet, and that taking in extra carbohydrate before playing may bring some benefit. As already mentioned, the timing of this may be crucial if you're a hypoglycaemic reactor.

A recent study on 28 elite tennis players found that blood sugar levels were far better behaved if a muesli bar was eaten 15 minutes before a game, compared to eating the bar 45 minutes before. The research, carried out at the University of Cologne, found that eating the bar 45 minutes before activity led to a swoop in blood-glucose levels to 25 per cent below normal ('Blood Sugar Levels and Carbohydrate Substitution in Tennis', International Journal of Sports Medicine, vol 14, p 163,1993). Given the nature of the activity, it would probably be even better to have a carbohydrate-containing sports drink rather than solid food - less chance of gut gremlins! Field team sports (eg, football, hockey)

These sports involve a lot of running although the participants may not identify as 'runners'. Investigations have found that soccer players cover at least 9000-11000 metres during a typical match, mixing jogging, sprinting and walking. It's a type of exercise guaranteed to use up muscle glycogen rapidly. This will start to bite in the second half of a match - players who've used up their glycogen credit will find it more and more of a struggle to muster anything faster than a walk. Players will put themselves at an advantage by eating a diet that's generally high in carbohydrates; before a match, a high-carb snack 5-10 minutes prior to play may bring some benefit, as will drinking a sports drink (probably an isotonic containing glucose polymers) at half-time.

Final considerations

There's a lot of individual variation in response to different foods. The only person who can really know what's going to work best is you. Use the research findings as a guide, and then try out different strategies in training.

Although taking in some extra carbohydrate before competing can enhance endurance performance, the optimum regime is to do this AND to take in extra carbs while active (probably best in the form of a drink).

No last-minute food will make up for a poor diet in previous weeks. Give yourself a serious head start by eating a high-carb diet during training.

Article was written by Janet Pidcock