Andrew Hamilton looks at optimum fueling strategies for a half marathon, and explains why one size doesn’t fit all MORE
How what you drink when exercising (and after) affects how much you drink
It’s well established that dehydration can have detrimental effects on sports performance. For example, during endurance events such as the 10K run, 1% bodyweight loss through dehydration would result in a 2% decrease in pace. For the 30-35 minute PB athlete, this would reduce performance by around 30-40 seconds. Even on shorter distances dehydration has been shown to be crucial. Armstrong et al (1985) showed that a 2% decrease in bodyweight would result in a 3% decrease in performance over 1500 metres, which is over six seconds at world-class pace! More extreme levels of dehydration of around 7% loss of bodyweight will have even more dire consequences, drastically affecting coordination and reducing performance by over 30% in endurance events.
Why is performance impaired? Probably because of the reduction in blood volume that accompanies dehydration. During exercise, a good blood flow to the working muscles is needed to provide oxygen and nutrients, and, in addition, especially if it is warm or humid, a good blood flow to the skin is required for cooling. However, as blood volume drops due to loss of water from sweating, heart rate rises to maintain blood flow to the muscles. This means the athlete has reduced economy and generally less tolerance to exercise. At the same time, blood flow to the skin is reduced, probably because it’s more important to maintain circulatory blood volume. Thus cooling is impaired and body temperature rises. This increased body temperature may also contribute to the weary feeling during endurance exercise in the heat, and can even be dangerous if dehydration is severe.
How much do you need to drink?
At rest, water loss is around 100 ml per hour. However, during exercise, as the body generates heat, sweating increases dramatically to prevent overheating. Sweating provides cooling as the water evaporates off the body. The problem for the exercise physiologist and coach is that sweating rates vary massively depending on environmental conditions and individual differences. For example, a study investigating sweating rates during one hour’s running at 70% VO2max found that subjects lost anything between 400-1600 ml, which is a huge range. However, it seems there is less variation among trained individuals, with a ball-park figure for them of 1.5 litres per hour for intense exercise, eg, marathon pace and upwards.This means that a footballer may lose around 3% of bodyweight by the end of the 90 minutes if fluids are not replaced during the match. Losses can be even more extreme in hot and especially in humid conditions, where sweating rates can exceed 2 litres per hour. For a 2:30 marathon runner weighing 70 kg, this could mean a 7% reduction in bodyweight if no fluids were taken during the run, and even if fluids are taken it may be impossible to replace all that is lost until after the event.
From these figures, it should be clear that for intense exercise quite large amounts of fluid are required. A pre-hydration drink of 0.5 L one hour before would be a good strategy, with 250 ml of fluids consumed every 15 minutes from the start of exercise. This means one litre of fluids per hour will be consumed, an amount which should go a long way to minimising bodyweight losses. In the heat, however, even more fluids may be needed. Post-exercise, the athlete should continue to consume fluids at a higher-than-normal rate, possibly another litre in the first hour after exercise, or more in hot conditions. This strategy should guarantee complete rehydration and maintenance of normal bodyweight.
Note that these recommendations are not just for competitions; they should be followed all the time, during and after training, for any workout lasting longer than 30 minutes (in the following article, Jim Bledsoe discusses the validity of the so-called ’60-minute rule’). Ensuring full hydration means getting the best out of each workout. In general life, athletes should drink around 1.5 litres of water a day to help maintain hydration status. This is IN ADDITION TO the fluid intake guidelines given above for training and competition. This baseline amount must be increased in hot conditions. Athletes should also avoid alcohol, tea, coffee and cola drinks, because these have a diuretic effect which increases dehydration.
These are all good habits to get into, so that when it comes to competition or exposure to hot condition the athlete is accustomed to drinking enough fluids. Unluckily, thirst is a very poor indicator of hydration status. The human body will lose half-a-litre of water before getting thirsty, and only a small amount of fluid is needed to quench thirst, compared to the amount lost through sweating. This is why you must be aware of the actual quantities of fluid needed to avoid dehydration and drink accordingly.
What to drink
The electrolyte content of sweat is much less than the electrolyte content in the blood. Thus even after prolonged sweating electrolyte loss is not significant and replacing water is always the main priority. Confusingly, however, this does not mean that one should drink plain water. In fact, sports drinks composed of dilute carbohydrate (CHO) solutions and sodium (Na) are best. The obvious advantage of the CHO content is that it boosts blood glucose levels, helps to preserve muscle glycogen and improve endurance performance. This has been shown in studies where subjects can maintain a fixed pace significantly longer when they consume a CHO drink compared to plain water.
However, during and immediately after exercise when fluid replacement is the priority, it is recommended that the CHO should be dilute, around 5%. This means that a 500 ml drink should contain 25 grams of CHO. If concentrations are higher than 8%, gastric emptying is slowed, meaning that it takes longer for the water to leave the stomach, which is a disadvantage. The gastric emptying is slower because the CHO concentration of the drink is greater than the concentration of the body fluids, and so the stomach draws water out of the small intestine. Temporarily, this further attenuates the dehyd-ration. It is the concentration of solution that manufacturers refer to when they say drinks are ‘isotonic’. Isotonic drinks have the same (iso) concentration as body fluids, and so gastric emptying is not impaired. Ideally, for fast rehydration the drink should be slightly less concentrated (hypotonic) than body fluids so that gastric emptying is swift. Isotonic drinks available include Gatorade, Isostar and Lucozade Sport. Hypotonic drinks include Dioralyte and Rehydrat, which are medical drinks marketed for stomach upsets. Coke, normal Lucozade and fruit juices are not suitable for rehydration because their concentration of CHO is too high.
What about sodium?
The value of some Na content in the fluid is that it aids absorption and retention of the fluid. First, the Na acts with the CHO to help active absorption into the blood. Second, and perhaps more important, Na is used by the body to maintain blood water volume. If the Na concentration drops, as would happen if high volumes of plain water were consumed, then the body reacts by inhibiting the anti-diuretic hormone. Thus urination is increased and the thirst drive is switched off until blood volume drops and Na concentration is back to normal. Therefore Na is crucial if the athlete wants to retain any of the fluids consumed after finishing exercise. Studies have proved the positive effect of Na content, speeding hydration time up to 20 minutes compared with 60 minutes when subjects drank only plain water, with the plain-water group drinking less fluid and urinating more post-exercise.
To sum up, fluids for rehydration should comprise a dilute CHO and Na solution. The exact concentration of CHO will depend on which has greater priority, CHO feeding or water replacement. A hypotonic concentration (less than 5%) is best in hot conditions when water replacement is the absolute priority; isotonic (5-8%) is best for CHO along with water replacement. If you use a commercial sports drink, make sure that it contains a reasonable level of Na – some don’t have any since it can have a negative effect on taste.
And speaking of taste…
Having established in theory the best type of drinks for fluid intake during and after exercise, researchers more recently have begun to address more practical questions, such as whether the type of drink or taste influences the fluid intake. For instance, Wilk and Bar-Or looked at fluid intake differences between water, grape-flavoured water and grape-flavoured water with glucose and Na during exercise in the heat. They found that intake was 610 ml, 882 ml, and 1157 ml respectively. This suggests that taste and the type of drink can affect the volume of fluid intake.
Jack Wilmore (the legendary Texan physiologist now working in Australia) and a research team investigated this further in a very well-designed study involving a group of 15 trained male runners. Each athlete had to perform three 90-minute runs on a treadmill at a temperature of 30 deg C, and 50% humidity. The intensity was standardised to 60% of VO2 max for each athlete. During each run, they were randomly assigned to one of three drinks: plain water (H2O), 8% concentration sports drink (A) and a 6% concentration sports drink (B). After each run, the athlete sat in a recovery room, at the same temperature and humidity, for a further 90 minutes. Each drink was made fully available to each runner during each trial and recovery period, although no encouragement to drink was made. The aim of the study was to observe differences in the amount of each drink consumed during exercise and recovery, and how this related to taste preference and hydration status. At the end of each trial, subjects completed a questionnaire about the qualities of the drink, and after the experiment subjects performed a taste test between the two sports drinks.
And the results? Fluid intake during the 90-minute run was 1.02 L for A, l.03 L for B and l.08 L for H2O. The average weight loss after the run was 0.84 kg for A, 0.82 kg for B and 0.79 kg for H2O. These results show two things. First, that during exercise there is no difference in intake between drinks; second, none of the subjects consumed as much fluid as they lost in sweat, reinforcing the idea that a conscious effort must be made to consume enough fluids. During the 90-minutes recovery period, fluid intake was 0.97 L for A, 1.00 L for B and 0.63 L for H2O, and the average regain in weight was 0.6 kg for A, 0.72 kg for B and 0.32 kg for H2O. This is a very interesting result since it shows that after exercise the volume of sports drinks consumed was more than 50% greater than water. This reinforces the earlier recommendations, that CHO and Na solutions are best for rehydration. In addition, and importantly, it supports the recommendations with practical proof that, given the choice, athletes will consume more of a sports drink than plain water.
With regard to taste preference, the athletes consumed slightly more of the sports drink they preferred on the taste test, 1.1 L against 0.97 L during exercise, and 1.02 L against 0.9 L post-exercise. This shows that taste is important, and that athletes should stick to what they like. In this study, drink A was the most preferred, but not universally, which shows that individual taste is variable. Another interesting point to emerge was that plain water was perceived to be less sweet, cleaner and more thirst-quenching than the sports drinks. This may explain why it was consumed equally during the run but not after it, since during the run the clean taste made it as palatable as a flavoured drink, but afterwards its greater thirst-quenching capacity meant that subjects didn’t feel the need to consume as much of it. Again, this underlines the benefit of Na in a sports drink because it helps to maintain the thirst drive and encourage more fluid intake.
This study shows that, with free choice, post-exercise the sports drink will be more effective for rehydration. It also shows that taste preference will influence fluid intake, so athletes should be allowed to choose their favourite sports drink, just so long as it has the recommended properties. With a team or a squad, this may mean that two or more drinks need to be provided.