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How to hydrate for a marathon

Dehydration can have a serious negative effect on performance. As little as 2% dehydration causes:

  • 8% loss of speed
  • 10% loss of strength
  • 20% loss of cognitive function

Dehydration occurs through loss of sweat from the body, mainly in the form of sweat that is evaporated from the skin as the body tries to prevent overheating. Sweat volume and electrolyte loss varies from individual to individual and depending on the ambient temperature. Some individuals can produce large volumes of sweat during exercise but may be very good at retaining electrolytes preventing camps and decreased performance, while others may be “salty sweaters” and lose large amounts of electrolytes leading to cramping and decreased performance.

There are different ways of monitoring hydration levels before, during and after exercise – the easiest and most accessible way is urine colour testing.

Urine colour – basing your urine colour against a chart of different colours has been used for some time to determine hydration status (see chart). A dark yellow / brownish colour indicates dehydration, whereas a clear light yellow colour indicates hydration. This form of hydration assessment can be inaccurate if you take a multivitamin as your urine will be bright yellow regardless of whether you are hydrated or not.

Another way to assess your hydration status is to use pre and post run weighing. Weigh yourself naked before a 30 minute run, don’t drink anything or consume any fluid. Weigh yourself naked again after the run. For every 1 kg of body weight lost drink 1.5 litres of fluid. These beverages should contain electrolytes and a small amount of carbohydrate (about 8% of the solution) and can be consumed gradually after activity.

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