‘Loading up’ or ‘hyper-hydrating’ with fluid before an event seems advantageous for those competing in ultra-endurance events, activities during which there is little opportunity to drink, or in hot humid conditions. Unfortunately, you cannot achieve hyperhydration by simply consuming large volumes of water or sports drinks before the event. The body simply excretes surplus fluid and you will end up paying frequent visits to the toilet or bushes. However, there is a method of hyperhydration that involves the consumption of glycerol along with fluid 2 hours before exercise. Glycerol is a hyperhydrating agent, which, through its strong osmotic activity, drags water into both the extra-cellular and intra- cellular fluid. This results in an increase in total body fluid. In theory, you will be able to maintain blood volume, increase sweating and reduce the rise in core body temperature that occurs during exercise. Studies at the Australian Institute of Sport found that by doing this, athletes retained an extra 600 ml of fluid and improved performance in a time trial by 2.4% (Hitchins et al., 1999). A study at the University of Glasgow found that hyperhydrating with a combination of creatine and glycerol resulted in increased total body water but did not improve performance in a 16 km time trial compared with normal hydration (Easton et al., 2007). The potential performance benefits should be weighed up against the possible side effects, which include gastrointestinal upsets and headaches.
limit dehydration to less than 2% of your body weight (IOC, 2004; Coyle, 2007). For example, this would mean 1 kg for a 50 kg person, 1.5 kg for a 75 kg person and 2 kg for a 100 kg person.
Clearly, the more you sweat, the more you need to drink. Studies have shown that you can maintain optimal performance if you replace at least 80% of your sweat loss during exercise (Montain & Coyle, 1992). But previous advice from the American College of Sports Medicine (ACSM, 1996; ACSM, 2000) to drink ‘as much as possible’ during exercise or ‘to replace the weight lost during exercise’ or ‘ad libitum’ has been replaced with advice to drink according to thirst because of the risk of hyponatraemia (water intoxication), particularly during prolonged exercise (Noakes, 2007). The new advice for regular exercisers and athletes is: don’t force yourself to drink. The IAAF advises drinking when you’re thirsty or drinking only to the point at which you’re maintaining your weight, not gaining weight. Drink less if you begin to have a queasy sloshy feeling in your stomach. Constantly drinking water (and that means more than 4–6 litres) over a relatively short time may dilute your blood so that your sodium levels fall. Although this is quite rare it is potentially fatal.
If you plan to run a marathon or exercise for more than four hours in warm weather drink no more than 800 ml per hour, be guided by thirst and sip a sports drink containing sugar and salt instead of plain water.
From a practical point of view, the ACSM recommend cool drinks (15–22°C). You will also be inclined to drink more if the drink is palatable and in a container that makes it easy to drink. Studies have shown that during exercise athletes voluntarily drink more of a flavoured sweetened drink than water, be it a sports drink, diluted fruit juice, or fruit squash (Passe et al., 2004; Wilk & Bar-Or, 1996; Minehan, 2002). Drinks bottles with sports caps
are probably the most popular containers. It is also important to make drinks readily accessible; for example, for swim training have drinks bottles at the poolside, for games played on a pitch or court (soccer, hockey, rugby, netball, tennis) have the bottles available adjacent to the pitch or court.
During low or moderate-intensity activities such as ‘easy pace’ swimming, cycling, or power walking carried out for less than an hour, fluid losses are likely to be relatively small and can be replaced fast enough with plain water. There is little benefit to be gained from drinking sports drinks compared with water during these types of activities.
During high-intensity exercise lasting less than an hour, drinking a sports drink containing up to 8 g sugar/100 ml rather than water may benefit your performance (Wagenmakers et al., 1996; Ball et al., 1995). Examples of these activities include a 10 km run, tennis, squash, cycling, sprint training, circuit training and weight training.
During high-intensity exercise lasting longer than an hour (e.g. half-marathon, football match), you require rapid fluid replacement, as well as fuel replacement. In other words, you need to avoid early glycogen depletion and low blood sugar, as well as dehydration, as all three can result in fatigue.
The IOC (2004) and IAAF (2007) recommend consuming between 20–60 g carbohydrate/hour to maintain blood sugar levels and delay fatigue. Most commercial sports drinks contain this level, which corresponds to the maximum rate at which fluid can be emptied from the stomach. More concentrated fluids take longer to absorb (ACSM, 1996). During hot and humid conditions you may be losing more than 1 litre of sweat per hour. Therefore, you should increase your drink volume, if possible, and use a more dilute drink (around 20–40 g/L).
Sports drinks based on glucose polymers may be a good choice if your sweat rate is low (e.g. during cold conditions) yet you are exercising hard, because they can provide more fuel than fluid replacers as well as reasonable amounts of fluid. In practice, many athletes find that glucose polymer drinks cause stomach discomfort and that sports drinks containing 4–8 g carbohydrate/100 ml do an equally good job.
The key to choosing the right drink during exercise is to experiment with different drinks in training to find one that suits you best (see Table 7.1).
THE COMPLETE GUIDE TO SPORTS NUTRITION
Choosing the right type of drink
Exercise conditions Drink
Exercise lasting <30 minutes Nothing; water
Low–moderate intensity exercise lasting less Water than 1 hour
High-intensity exercise lasting less than 1 hour Hypotonic or isotonic sports drink High-intensity exercise lasting more than 1 hour Hypotonic or isotonic sports drink or
glucose polymer drink
Table 7.1