Exercising and
fluid replacement
WHY GOOD HYDRATION IS IMPORTANT DURING EXERCISE Good hydration at any level of exercise participation is important in preventing unnecessary stress to the cardiovascular system as well as many of the other side effects of dehydration such as nausea, vomiting, headaches and sluggishness. It is particularly important in hot and humid weather when sweat loss is much greater.
Guidance n The overall aim during exercise is to consume fluids at a rate sufficient to
replace all the water lost through sweating and breathing
n If conditions are hot and/or humid and/or you are already dehydrated, it is important to increase fluid uptake
n These are only general guidelines and do not take account of individual characteristics and physiology – drink as much as is comfortable, it is difficult to ‘hyperhydrate’ as excess fluid is excreted through urine
n It is recommended that to improve the palatability and desire to drink fluid during exercise, it is preferable for the drink to be cool (15-22o
C).
THE SPORTS DRINK COMPOSITION Research has shown that ingestion of certain concentrations of carbohydrate and sodium can aid rehydration. However the percentage concentrations are relatively precise if they are to achieve their goals of rehydration. The optimal drink should contain: n no more than 4-8% of carbohydrate (40- 80g/l or 4-8g/100ml). For example eg. sucrose, glucose polymers or starch. Amounts exceeding this concentration will slow digestion which may lead to stomach discomfort, nausea and vomiting. In many popular drinks additional water is required in order to dilute the carbohydrate concentration to the level for absorption – too much carbohydrate may actually increase the risk of dehydration and reduce performance. Coca-Cola and Pepsi contain carbohydrates at a concentration of 10-11%, and ready diluted bottles of Ribena contain concentrations of 14%
EXERCISE SITUATION 1 hour – low to
moderate intensity PRE-EXERCISE
Drink around 500ml of water 2 hours before
exercise and another 125- 250ml just prior to beginning exercise
> 1 hour – moderate intensity
As above – although a sports drink may be
beneficial in place of water – particularly if you are
already dehydrated or have failed to eat sufficiently
> 1 hour high intensity and for multiple exercise sessions per day
particularly where a high level of performance is required ie. back to back tennis matches
As above – substituting water for a sports drink to provide sufficient fuel and fluid to optimise longer duration performance
DURING EXERCISE
Drink small amounts of water frequently – a few sips every 15 minutes should be sufficient. The aim is to match intake of fluid with the amount of fluid lost during the activity. The key is to start drinking early and frequently. Drink as much as possible
Guidelines recommend 125-250ml every 15 minutes. The more intense or
prolonged the activity, the more important this becomes. Water is sufficient but a sports drink may help sustainperformance and replace lost fluid more quickly
For intense exercise it is recommended that you consume around 30-60g of carbohydrates an hour. This can be achieved by drinking 600-1200ml per hour (150-300ml every 15 mins) of a solution containing 4-8% carbohydrates (ie. suitable sports drinks)
NUTRITION
n a small amount of sodium (0.5-0.7g/litre of water – this promotes fluid retention and prevents the premature ‘switch off’ of the thirst mechanism
n NO caffeine – newer, trendy energy drinks such as Red Bull and Red Devil contain caffeine which acts as a stimulant giving a sudden boost to energy levels – however while caffeine has been shown to improve athletic performance it is also a diuretic promoting excretion of fluid (thereby hastening dehydration). People taking part in high level sport who might be subject to drug testing should note that caffeine is a restricted substance. Contract the sport’s governing body medical officer for further information
n no carbonation (fizz) – this can cause stomach discomfort and result in you feeling bloated.
The table will help guide you on the amount of fluid you should consume before, during and after activity at various intensities
TOP TIPS n Pale to very pale yellow urine indicates being within 1% of optimal hydration
n There is unlikely to be a decrease in performance if at least 80% of sweat loss can be replaced during exercise
n To calculate the percentage of contents in a drink divide the amount in grams in one serving (in this case 4g) by the amount of fluid in millilitres in one serving (100ml) and multiply by 100 to give a percentage ie. 4g/100ml x 100 = 4%.
POST-EXERCISE
The general aim is to replace 150% of the fluid lost. At this level of exercise aim to consume one litre of water for every 1,000 kcal burnt. Or weigh yourself before and after the exercise and aim to replace every pound lost with 500-750 millilitres of water
Follow the same principles as above. Any weight lost during exercise will be due to loss of water which then needs to be replaced. Water is adequate as the type of fluid replacement as long as enough is consumed
As above it is important to try and replace 150% of the fluid lost. It is advisable to rehydrate using a fluid containing a small amount of sodium per litre. This encourages you to keep
drinking when water may have ‘switched off’ your thirst prematurely
www.exerciseregister.org
The information contained in this article is intended as general guidance and information only and should not be relied upon as a basis for planning individual medical care or as a substitute for specialist medical advice in each individual case. To the extent permissible by law, the publisher, editors and contributors accept no liability for any loss, injury or damage howsoever incurred (including negligence) as a consequence, whether directly or indirectly, of the use by any person of the contents of this article.
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