Marathon: How much hydration is needed?

19:21, May 23 2013

In just over a week's time, thousands of Cantabrians will be taking part in the annual Christchurch Marathon event.

To help potential participants prepare, Good Living has enlisted local endurance sports coaching company Complete Performance to provide some weekly advice.

Today, performance nutritionist Rita Trotman offers tips on hydration.

Hydration is an important factor in supporting exercise performance. Whilst exercising, fluids are lost; largely through sweating. If you fail to replace these lost fluids, up to an 8 per cent loss in performance can occur.

Outside of a laboratory, where scientific methods can calculate sweat loss and rates, the most practical way for people to monitor their sweat loss is to measure changes in body weight.

This means accounting for fluid and food intake, as well as fluid loss through urination.


To calculate fluid loss you should weigh yourself prior to training in minimal clothing (or nude).

Immediately after training, you should towel dry, urinate if necessary and weigh yourself again in the same clothing (or nude) as before the exercise - and on the same set of scales.

It's important that you weigh yourself either naked or in different clothing to your exercise gear, as used exercise clothing holds sweat which is fluid that is no longer available to use.

The same is true for any fluid held as urine in the bladder. The weight of any food/fluid consumed between weigh-ins should then be added in to the equation, this will provide you with an estimate of sweat losses over the whole exercise session.

Dividing the total sweat loss by the duration of exercise will provide an estimation of the rate of sweat loss i.e. how much fluid you use per hour (or whatever measure of time is the best measure of your training time).

Each 100g of weight loss is approximately equal to 100ml of fluid loss; 1kg of weight loss equals 1 litre of fluid loss.

Repeat this sweat loss test several times, and you'll soon end up with a good guide to how much water you should be taking on board to keep your fluid level balanced.

For a more comprehensive understanding of your body and its hydration requirements, keep a training diary and make notes regarding the weather conditions (temperature is very important), and the type of run - flat, hilly.

You'll soon understand how to look after your body better regarding hydration, in all conditions. This will allow you to work out with good accuracy how much water you need to carry with you through different sessions in different conditions.

You'll know when you've done a great job as the variation between start and finish body weights will be very similar meaning you've taken on just enough water to replace your fluid losses.

Training schedule

With just 10 sleeps to go, you should be starting to taper off ahead of the big day.

- Try to complete at least three short, quality running sessions of around 60, 45 and 30 minutes each focusing on technique, aerobic fitness and speed.

- Some sprint/jog sessions are a good option for the 30 minute session.

- Following a warmup of 10 minutes, go hard for 45 seconds then jog for 45 seconds and repeat this five times.

- While you are doing this, pick a point ahead of you that you think that you might get to and run hard at it, if you make it, find another point and try to get to that within the 45-second time limit.

- Complete a five-minute warm down easy jog and stretch.

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