Fluid is a vital part of exercise. In fact, fluid is vital for life and makes up about 60% of body weight. Good hydration helps to maintain an efficient cooling system and keeps the kidneys, respiratory system and cardiovascular system working. However when exercise is added into the equation, fluid becomes more important and more complicated to calculate.
During exercise, muscles only use about 25% of the energy for work, with the rest released as heat – which is why exercise makes you hot! Heat from the working muscles is transferred to the blood. The blood flow to the skin is increased and heat is lost by evaporation – sweating. Sweat comes from water in the blood, so you need to replace this vital fluid to prevent dehydration.
How much do you need?
You can work out your basic daily fluid needs by multiplying your body weight in kilos by 35mls, for example, an athlete who weighs 75kg will need just over 2 ½ litres of fluid a day to stay well hydrated (75 x 35 = 2635mls). On top of this, you will need to take more in to deal with the amount of sweat you lose during an exercise session – and this varies a lot between athletes.
The fluid needed for a specific training session will depend on sweat rate, session length, surrounding temperature and humidity and intensity. Everyone sweats but some sweat more than others. The easiest way to get an estimate of how much sweat you lose is to weigh yourself before and after exercise. Each kilo of weight loss is equal to a litre of fluid lost. However, you will also lose fluid as urine, so you should drink at 1 ½ litres of fluid for every kilo of weight lost.
So, to work your sweat losses and your fluid requirements for exercise, follow these guidelines:
Weigh yourself just before the start of exercise and just after going to the toilet
Weigh yourself in a minimum of clothing remove socks and trainers. After the exercise session, weigh yourself before going to the toilet Run down the body with a towel to remove sweat.
Keep a record of how much fluid you drank during the session.
To calculate fluid loss, take the final weight from the initial weight. Here’s an example to show you how it works:
These are the figures from an athlete who worked hard for 1 hour in the gym.
|Fluids drunk:||500mls||(= 0.5kg)|
|Sweat rate:||weight loss (0.5kg)||+ fluids drunk (0.5kg)||= 1kg||= 1litre fluid|
So this athlete’s sweat loss is about 1 litre an hour. He drank 500mls of fluid during the session which means he needs to aim to drink another 500mls during similar sessions to match his sweat losses.
What to drink? Which fluid you opt for depends on how hard you exercise, and for how long. It is important that you choose a flavour that you like to encourage you to drink more. If you’re exercising at low-to-moderate intensity for less than an hour, then water is fine. If you are working out for more than an hour, then a fluid with some carbohydrate for fuel, and sodium (salt) for improved absorption is recommended. There are a whole range of sports drinks available that are generally divided into 3 types:
Hypotonic – these contain very small amounts of carbohydrate – about 2 to 5g per 100mls – and electrolytes such as sodium and potassium. They are useful for athletes who need to watch their weight. An example available in the UK is Lucozade hydro active; other countries may have other products that fall into this category.
Isotonic – these contain around 5 to 8g of carbohydrate per 100mls and sodium and potassium. They provide fuel and can be used before, during and after exercise.
100mls fruit squash 250mls fruit juice
900mls water 750mls water pinch of salt pinch of salt
200mls fruit squash
500mls fruit juice
800mls water 500mls water
pinch of salt pinch of salt
400mls fruit squash
1 litre water
pinch of salt.
Is it possible to drink too much?
Technically, yes it is. There is a problem called hyponatraemia which occurs when the concentration of sodium in the blood falls to an abnormally low level. This can be caused by drinking excessive amounts of fluid that contain no sodium. This is only likely to be a potential problem in endurance events of longer than 4 hours, particularly when undertaken in hot conditions where sweat rates are very high, and water is taken in large volumes.
However it is good practice to use a drink that contains some sodium (either home made or commercial) when exercising for long periods, especially in the heat.
A useful way to check your hydration quickly is the PEE test. Quite simply looking at the colour of urine shows how well or poorly hydrated an athlete is. See chart. To show good hydration, the urine should be a pale straw colour; the darker the colour, the more dehydrated you are.
Dehydration –the ugly facts
The effects of dehydration are most noticeable when exercise is done in a hot and humid environment
Dehydration reduces mental functioning and skill co-ordination, so dehydration will have an extra impact on sports involving skill and decision making.
High levels of dehydration increase the risk of nausea, vomiting and diarrhoea during exercise and slows down the rate you can absorb fluids.
It is impossible to ‘train’ or ‘toughen up’ your body to handle dehydration so don’t bother trying.
A drop of drink
Post competition celebrations often include alcohol … be sensible and look after fluid needs first as alcohol not only affects co-ordination and decision making, but can also encourage dehydration and slow down recovery. Remember that alcohol is also high in calories and cannot be used as a fuel source for the working muscle. The result is that the calories are stored as fat.
Calling all coaches.
Drinking during exercise does not come naturally to athletes.
You can help athletes by:
Recognising the importance of fluid replacement during exercise
Arranging for athletes to be well educated regarding hydration
Helping athletes prepare a fluid replacement plan for training and competition
Including drink opportunities during training
Monitoring fluid balance from time to time to create awareness of whether your athletes are meeting their fluid intake goals.
Ruth Wood-Martin Accredited Sports Dietitian Top of Page
Nutrition and Hydration during RecoveryRecovery includes a range of processes which include:
- Refuelling the muscles and liver of their expended energy
- Replacing the fluid and electrolytes lost in sweat
- Allowing the immune system to deal with any damage caused by the exercise bout
- Making new proteins, red blood cells and other cellular components
Replacement of body water and electrolytes after exercise is crucial when repeated bouts of exercise are planned within a limited timescale. Athletes have to work hard to take enough fluids to fully rehydrate, especially when exercising in the heat. They must not rely on being thirsty as a sign to drink.
Weight change monitoring (weight before minus weight after training or competition) provides a guide to fluid needs – a loss of 1kg equals a fluid deficit of 1 litre. As fluid losses continue during recovery, athletes will need to consume a volume 1½ times the deficit (ie 1½ litres for 1kg lost) to restore fluid balance.Practice guidelines
Make sure an adequate supply of cool palatable drinks is available
Rehydration should start immediately with half or full strength isotonic drink. A fluid that contains some sodium (salt) and carbohydrate provide faster body water replacement than plain water, is more palatable and helps is assisting refuelling drink to a plan; do not rely on thirst to determine needs remember that fluid continues to be lost during recovery through urine losses and ongoing sweating.
Caffeine containing drinks and alcohol should not be taken in the recovery period as they can increase urine losses
Where possible, post-exercise activities that promote sweat losses eg hot spas, saunas and exposure to the sun should be avoided
Poor refuelling means that muscle glycogen stores will not be re-stocked before the next session, and will lead to poor quality training. The most important dietary factor affecting muscle glycogen refuelling is the amount of carbohydrate consumed; this is particularly important when there is limited time between exercise bouts for recovery.Practice guidelines
A high carbohydrate snack or meal giving 50 – 100g carbohydrate (1g/kg body weight) should be consumed within 30 minutes of stopping exercise, and be repeated after 2 hours or until normal meal patterns are resumed.
Each of the following selections provide approximately 50g carbohydrate and athletes should eat one or two portions as soon as possible after exercise.
- 800 – 100mls of isotonic sports drink
- 1200mls sugar-containing fruit squash
- 500mls fruit juice drink or fresh orange juice
- 2 handfuls of sultanas
- 2 handfuls of jelly babies, wine gums, fruit pastilles
- Standard bar of Turkish Delight
- 3 Jaffa cakes and 2 fig rolls
- 2 – 3 cereal bars
- 2 slices white bread with jam or honey
- 2 pancakes with jam, honey or syrup
- Fruit scone with jam or honey
- 2 slices currant or malt bread with jam or honey
- Soft white roll with banana
- 3 rice cakes with jam or honey
- Low fat yogurt and banana
- 1 tub (150g) custard and 3 tablespoons tinned fruit
A daily carbohydrate intake of 7 to 10g per kg body weight (typically 400g – 600g) is needed to maximise glycogen storage.
When appetite is reduced or stomach comfort is a problem, athletes should focus on compact forms of carbohydrate; low fibre, high sugar foods and sports bars carbohydrate containing fluids are also low in bulk and may be appealing to athletes who are fatigued and dehydrated.
Consult the 50g carbohydrate list.
Small frequent meals may help athletes to achieve high carbohydrate intakes without the discomfort of overeating. Routines of meals should be organised to suit individual preferences, timetable of training/competition, appetite and comfort.
Carbohydrate rich foods and drinks may provide other valuable nutrients such as protein, vitamins and minerals that are also important for the recovery process.
Muscle damage slows down glycogen refuelling and the rate of recovery – try to increase carbohydrate intake during the first 24 hours of recovery to offset this.
- 50 – 70g sugar
- One litre of warm water
- Pinch of salt
- 200ml of sugar free squash
- Mix, cool and drink
|Match Meal Chart|