Have you considered hydration for the upcoming Rottnest Swim

Author: Alison Slevin

Date Published: 19 February, 2019

The tricky thing about hydration when swimming is that you often don't realise how much fluid you are losing. This is more obvious when running, cycling or playing team sport, but is hidden when you are swimming. Whether you are swimming solo, duo or part of a team, planning hydration is a must to ensure optimum performance and a quicker recovery following the race.

If you feel thirsty, you generally have lost 3-4% of body weight in fluid. Dehydration of >2% body mass has shown to negatively influence performance in endurance events so by the time you feel thirsty, your performance has been affected for a while.

To ensure you are off to a smooth start, being adequately hydrated prior to entering the water is crucial. The American College of Sports Medicine (ACSM)(1) recommend consuming between 400-600ml of fluid in the two hours prior to endurance events.

In order to plan your nutritional needs we must first consider what we lose during exercise: water and sodium from sweating and carbohydrate, fat and protein from energy consumption.

In addition to your food intake throughout the swim, a fluid intake comprising of water, electrolytes and carbohydrate will assist in restoring this energy and hydration level. Usually within the first hour, water alone is recommended as you use your energy stores - provided your nutrition has been well planned in the days and hours leading up to the swim.

Over the following hours, an intake of between 600-1200 ml/hr of a solution containing carbohydrate and sodium has been recommended.(2) This works out to be somewhere in the range of a cupful every twenty minutes. Carbohydrates depletion is considered to be directly associated with fatigue and during events of greater than two hours, a carbohydrate intake of up to 90g/hr is recommended during the event.(3) These guidelines can assist planning whether your preference is for sports drinks containing carbohydrates and sodium all in one or using electrolyte drinks and carbohydrate gels. It is always advisable to experiment with food and fluid intake during training sessions to prevent any mishaps with nausea or gut discomfort on the day.

Some readily available sports drinks in Australia are listed below with additional information on their nutritional values to assist the planning process.

CHO Carbohydrate, Na Sodium, K Potassium

Due to the large variability in individual sweat rates reported, the ACSM recommends using changes in bodyweight as a reflection of sweat loss during exercise to calculate individual fluid replacement requirements. Weighing yourself before and after a training swim, taking into account the fluid intake during the session can give an estimation of your per hour sweat rate (1 Litre = 1 Kg). Of course urination must be considered here also if relevant.


Weight before 2-hour swim 60kg plus fluid intake of 1 litre = 61kg

Minus weight after swim 58kg = 3kg or 3 Litres lost over 2 hours

1.5L lost per hour therefore aim for restoring 1.5 litres per hour of event

Seeing as there is continued fluid loss after finishing endurance exercise due to sweat loss and urination, it is imperative to continue with a hydration plan in the ensuing hours. Aim to replace a further 120-150% of fluid lost in the first 2-4 hours after the swim.

Other nutritional considerations in the hours following the swim include:

  • Carbohydrates: 1-1.2 gram/kg of body mass immediately upon finishing. 0.8g/kg body mass is sufficient when taken with high quality protein.
  • Protein: 0.3 grams/kg body mass of high quality protein (Approx. 20-25g) within an hour of finishing.

Key points to maximise your performance in the swim and your recovery after:

  • Don't wait until you are thirsty before you top up your fluids
  • Throughout the swim, drink combined solutions of water, electrolytes and carbohydrate to assist in restoring energy and hydration levels, somewhere in the range of a cupful every twenty minutes to replace fluids
  • Experiment with food and fluid intake during training sessions to prevent any mishaps with nausea or gut discomfort on the day of the Rottnest swim.
  • After the swim, aim to replace a further 120-150% of fluid lost in the first 2-4 hours after the swim.
  • It is also important to consume protein and carbohydrates in the first hour after the swim to maximise uptake and recovery. This could be a meat and salad roll or a 600mL flavoured milk.

Best of luck to everyone taking part!


(1) American College of Sports Medicine, Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. American College of Sports Medicine position stand. Exercise and fluid replacement. Med Sci Sports Exerc. 2007 Feb;39(2):377-90. doi: 10.1249/mss.0b013e31802ca597.

(2) Von Duvillard SP, Braun WA, Markofski M, Beneke R, Leithauser R. Fluids and hydration in prolonged endurance performance. Nutrition. 2004 Jul-Aug;20(7-8):651-6.

(3) Stellingwerff T, Cox GR. Systematic review: Carbohydrate supplementation on exercise performance or capacity of varying durations. 2014. 39(9):998-1011. doi: 10.1139/apnm-2014-0027.

Photo Credit www.rottnestchannelswim.com.au

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