Being more than two per cent dehydrated causes a decline in performance. During running, water and electrolytes (calcium, potassium, magnesium and sodium) are lost when you sweat. While this is influenced by a range of factors, such as weather, individual sweat rates, running intensity and efficiency, on average runners can expect average sweat rates of one to two-and-a-half litres per hour. Dehydration causes your blood volume to drop, which lowers your body’s ability to transfer heat and forces your heart to beat faster, making it difficult for your body to meet aerobic demands.
To perform at your best, it’s recommended that you start a run in a hydrated state and stay less than two per cent dehydrated. One of the easiest ways to monitor your hydration status is by checking your bodyweight on a daily basis and weighing yourself before and after long runs. You should never gain weight after a run. Another way is to pay attention to the colour and frequency of your urine. It should be a pale straw or pale yellow colour.
Over hydrating can actually be more dangerous than not drinking enough. Hyponatremia occurs when your fluid intake exceeds your rate of fluid loss from sweating, which results in low blood-sodium levels. Symptoms include nausea, disorientation and muscle weakness. It can develop in fast runners who are sweating out sodium at such a rapid rate that water or sports drink are not adequately replacing it. It can also occur in runners who are drinking more fluid than they are sweating, thereby diluting their blood sodium. This tends to be more of a problem for slow runners during long runs or races where they may stop at every water station yet are not necessarily needing or using the amount of fluid they are ingesting.
You can avoid the risk of hyponatremia by monitoring your fluid intake before, during and after runs (see below) and making use of electrolyte drinks which contain sodium and potassium, or adding hydration sports sachets to your water bottle. As a general guideline, in hot weather if you are running longer than 30 minutes, choose a sports drink that includes electrolytes.
One of the best guides to maintaining an optimal hydration state is actually thirst! The concept of drinking according to thirst may seem too simple to be an accurate barometer of fluid needs. However, there is now increasing scientific evidence to support the notion that thirst is actually the ideal way to gauge hydration needs. To determine your own hydration needs, it is important to listen to your own body as quantities will vary, but here are some general guidelines:
Ideally, aim to drink 400-500ml one to two hours before a run. If you’re short of time or run first thing in the morning, then try to consume 100-200ml 15 to 30 minutes before going out.
Depending on how long your run is and what the weather is like, you may need to take a water bottle with you. Aim to drink 100-200ml every 20 minutes. As the stomach can only empty 800-1,000ml of fluid/hour, your own needs may be less than this. One of the easiest ways is to set your watch to beep every 15 to 20 minutes to remind you to take a drink. Drinking smaller amounts at regular intervals can help you absorb fluid more effectively without gastric upset.
Weigh yourself and consume around one litre of hypotonic or isotonic drink for every 1kg total weight loss. Look for a drink that contains around 50 mmol sodium per litre, as well as potassium and magnesium.