Water makes up 70-75% of your muscle tissue, and dehydration during exercise is commonly recognised as a major contributor to fatigue. For example, a 1994 study on trained cyclists demonstrated that reductions in body weight of 1-2% as a result of deliberate dehydration resulted in a 44% reduction in performance. 1
However, this should not be taken as an instruction to drink as much fluid as you can tolerate during races or training. From 1996 until 2007, this was in fact the advice of the American College of Sports Medicine. However, the number of instances that occurred during that time of marathon runners, in particular, who collapsed and sometimes died during races, together with the pioneering studies on hydration in exercise by Professor Tim Noakes, resulted in recognition by the ACSM that their advice was contributing to the development of the life-threatening condition hyponatremia. This is where levels of sodium in the blood fall dangerously low.as a result of excess fluid intakes. Hyponatremia seems to particularly affect females and those running more slowly in races due to their smaller size and greater time available to drink respectively.
In 2007, the American College of Sports Medicine updated its position statement on fluid replacement in exercise to state that the goal of drinking during exercise is to prevent excessive dehydration (>2% body weight loss from water deficit) and excessive changes in electrolyte balance to avert compromised performance, and this advice remains in place today. It was also suggested that as sweat rates, and the electrolyte content of sweat, vary widely between individuals, athletes should estimate their sweat rate by weighing themselves before and after exercise, and devise a customised hydration programme to replace the fluid losses indicated by the weight loss. 2 No specific recommendations are made about how much to drink.
Tim Noakes and others argue, based on various studies, that dehydration of up to 3-4% can be well tolerated and does not affect performance adversely in elite athletes. Noakes also believes that few runners are in danger of dehydration or heat exhaustion in races, as their thirst dictates how much they should drink. It is thought that the body also warns against over-drinking through as part of temperature regulation process. The real danger is from excessive fluid consumption. These topics are explored in depth in his 2012 book “Waterlogged” 3. Noakes recommends using your thirst as your guide to how much to drink during and after endurance sports, and this is becoming increasingly recognised as good hydration practice.
Starting your exercise in a well hydrated state is important to prevent fatigue. The ACSM 2007 position statement recommends drinking 5-7ml of fluid per kg of body weight slowly in the 3-4 hours prior to exercise, and to allow time to excrete any excess. 2 That’s 350-490ml for a 70kg athlete. Water, coffee, tea, fruit juice, milk and sports drinks can all contribute to this fluid intake. It’s a good idea to check your urine shortly before starting to exercise; if it is a colour other than pale yellow, you need to consume more fluid.
Once you are exercising, the requirement changes to replacing the fluid lost in sweat. The amount required is very individual, depending on the intensity of exercise, the outside temperature and humidity, and your own sweat rate. Use your thirst as your guide. If you are exercising very intensively or for longer than an hour, use a sports drink both to replenish lost fluid and to maintain adequate blood glucose levels. The carbohydrate content should be 6-8g/100ml for optimal fluid absorption. More highly concentrated fluids like fruit juice impair absorption, so dilute these half and half with water if you use these to make a sports drink. If you have a high sweat rate, you may also need to replace sodium by adding electrolyte tablets or solution to water or home-made sports drinks. Electrolytes are generally included in commercial sports drink. If you drink too little your rate of gastric emptying slows and you may experience gastro-intestinal problems as well as fatigue. 4
After exercise, continue to drink according to thirst and do not drink too much fluid too quickly. Review the colour of your urine regularly, drinking more if it is remains dark, and stay focused on drinking regularly throughout the rest of the day, particularly if your next exercise session is in less than 24 hours.
Jo Scott-Dalgleish BSc (Hons) is a BANT Registered Nutritionist, writing and giving talks about nutrition for endurance sport. Based in London, she also works as a Registered Nutritional Therapist, conducting one–to–one consultations with triathletes, distance runners and cyclists to help them eat well, be healthy and perform better through the creation of an individual nutritional plan. To learn more about these consultations, please visit www.nutritionforendurancesports.co.uk
1 Walsh et al. Impaired high-intensity cycling performance time at low levels of dehydration. Int J Sports Med. 1994; 15(7): 392-8, cited in in A Jeukendrup, R Jentjens & L Moseley. Nutritional Considerations in Triathlon. Sports Med 2005; 35(2): 163-181
2 Sawka et al. American College of Sports Medicine position stand: Exercise and fluid replacement. Med Sci Sports Exerc. 2007; 39(2):377-90
3 Noakes TD. Waterlogged: the Serious Problem of Overhydration in Endurance Sports. Human Kinetics (2012).
4 Rehrer et al. Effects of dehydration on gastric emptying and gastrointestinal distress while running. Med Sci Sports Exerc.1990; 22(6):790-5, cited in A Jeukendrup, R Jentjens & L Moseley. Nutritional Considerations in Triathlon. Sports Med 2005; 35(2): 163-181