Available evidence shows that topping up with extra carbohydrates 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 (e.g. if you're participating in a tournament taking place over the course of a week) and/or the exercise lasts 90 minutes or longer, pre-exercise carbohydrates may improve your performance.
The type of food and its consumption timing is crucial to whether it's helpful or harmful. Some basic physiology should help explain this. Exercising muscles burns 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 they don't dip too low (causing hypoglycemia, 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 (e.g. after eating food containing carbohydrates) 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 lower. If you compete in a morning event without having eaten anything since the night before, you start 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 amount of carbohydrates should be lower the nearer the competition to avoid gut problems.
('Carbohydrates' in Berning, J. and Nelson-Steen, S., 'Sports Nutrition for the 90s', Aspen, Maryland, 1991).
Avoiding The Hypoglycemic 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 categorized according to their 'glycemic index'. A food's glycemic index (GI) gives an indication of the degree of blood-glucose surge to expect.
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High-GI foods (which include glucose, bread and bananas) bring about a marked and immediate rush of glucose into the bloodstream, whereas low GI foods (ex: 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 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, which may help 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 hypoglycemic backlash. Several early studies found that runtime to exhaustion was shorter by about 20-25 percent after athletes consumed 2-3 oz 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 percent sucrose/glucose solution or a 20 percent maltodextrin/glucose solution) did not result in hypoglycaemia, or adversely affect sensory or physiological responses during 50 minutes of moderate-intensity cycling.
Exercise 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 found that the best food to take an hour before activity was carbohydrates, which have a low glycemic index.
Scientists At The University Of Sydney Tested Out Four Different Pre-Race Feeds:
- Boiled lentils
- Baked potatoes
- A sports drink comprising glucose dissolved in water
- 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 percent 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:
- You're going to be active for more than an hour
- You think you suffer from reactive hypoglycemia
- 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 hypoglycemia, is to take in carbohydrate 5-10 minutes before exercise of an hour or more.
If you exercise at an intensity greater than 50% V02 max, 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!).
Calculate your VO2 Max here.
Recommendations For Specific Sports
You won't benefit from extra carbs before competing. Glycogen stores aren't a limiting factor. However, it might be worth taking some sodium bicarbonate before racing. The research is split down the middle - about half the studies have found a benefit, the other half no effect. No serious detrimental effects have been found, however, apart from nausea in susceptible individuals.
A dose of 300mg per kg of body weight has been found effective in some studies for exercise lasting between 30-120 seconds (eg, 800m track). It's probably best to take this 1-3 hours before racing.
('Bicarbonate Loading', Heigenhauser, G. and Jones, N. In Lamb, D. and Williams (eds), 'Ergogenics, Enhancement of Performance in Exercise and Sport,' Brown & Benchmark, lowa, 1991).
Depending on your level of fitness and the terrain involved (ex: hills vs flat running), glycogen may or may not be a limiting factor. Make sure you've had a high-carb meal 2-3 hours beforehand if possible. Taking extra carbohydrates on board in the 10 minutes before racing probably won't hinder your performance - but it's doubtful it will have any positive effect either!
Caffeine before running may improve your capacity to 'kick' at the end of a race - a study found that two cups of strong coffee brought about a significant improvement over 1500m.
For peak absorption, drink about an hour before exercise.
('Effect of Caffeinated Coffee on Running Speed, Respiratory Factors, Blood Lactate and Perceived Exertion During 1500m Treadmill Running', British Journal of Sports Medicine, 26(2), pp 116-120,1992).
If you run for more than an hour, extra carbs before and during a run may help your performance. The amount of carbohydrates ingested four hours prior to performance should be based on body weight. Several studies have used 4-5g/kg with good results.
For an athlete who weighs 60kg (132 pounds), the recommended amount would be 240-300g. The carbohydrates could be consumed in any of several forms, including fluids such as juices or glucose polymer solutions, or solid carbohydrates such as fruits or starches. The fiber content should be minimized to prevent possible intestinal problems during exercise.
If carbohydrates are consumed approximately one hour before competing, 1-2g/kg has been found to enhance performance in several studies. Both glucose polymers and foods with a low glycemic index have been used successfully. If carbohydrates are consumed immediately before exercise, ex: within 10 minutes of the start, about 50-60g of a glucose polymer in a 40-50 percent solution has been used effectively in some studies.
Gut and bowel problems are a blight to many distance runners. If you suffer from this, choose your foods carefully. It's probably wise to avoid food high in fiber, and you should certainly avoid fatty foods. Try liquid meals - choose between those formulated for athletes or convalescents (check for a high carbohydrate content) or make your own by blending milk, skimmed milk powder and fruit.
The advice is similar to that for equivalent distances in running. However, cyclists are less prone to gut problems, so will be able to tolerate solid food before competing. Caffeine has been shown to enhance cycle sprint ability as well as endurance capacity (the evidence is more convincing than for distance running). Individuals vary in their response and toleration for caffeine, so test it out in training.
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
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 percent 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 (ex: football, hockey)
These sports involve a lot of running although the participants may not identify as 'runners'. Investigations found that soccer players cover at least 9,000-11,000 meters 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 use 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.
At competition level, pre-race nutrition may be dictated mainly by trying to compensate for drastic dehydration regimens undertaken to make weight. A typical regimen is severe fluid restriction combined with reduced food intake and heavy exercise in the days before an event. Don't do it! It's not possible in the time remaining to normalize your physiology and restore full blood volume.
A study simulating these conditions found that only half of the lost blood plasma was restored during fluid intake after weighing. This put a significant downer on performance - over a 2,000m course, those who dehydrated and attempted to rehydrate were 15 metres behind.
It's far better to go for a long-term weight control plan, and to start the race fully hydrated.
('Rowing Performance, Fluid Balance and Metabolic Function following Dehydration and Rehydration', Medicine and Science in Sports and Exercise, 25(12), pp 1358-1364).
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 carbohydrates before competing can enhance endurance performance, the optimum regimen 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.