During exercise, the primary loss of water is typically through sweat. Sweat rates can vary hugely and in hot conditions have been reported to be as high as 10 litres per day. Alongside water, we also lose significant amounts of sodium and it can be critical to ensure we are replacing both of these. Getting hydration strategies right can be crucial to exercise performance, especially during events over long distances, or in extreme conditions. This 2nd part of our hydration blog, examines recommendations for staying hydrated on the bike (for advice on hydration off the bike, please read the 1st part).
Hydration and Endurance Sport
Dehydration can be detrimental to aerobic performance (>2-3% dehydration) and cognitive function (>3%). It has also been demonstrated to increase perceived exertion at a given power output. The effects are especially pronounced in high temperatures although can still occur in longer duration exercise in less extreme environments. However, taking on too much fluid can be equally detrimental, not only does it increase the likelihood of needing to stop for a “comfort break”, in extreme cases, hyponatraemia can occur with potentially deadly consequences. Current guidelines advise athletes to limit body mass losses through dehydration to 2% and to avoid drinking so much as to gain weight during exercise.
What Should Drinks Include?
Although specific recommendations vary depending both on the individual and nature of the exercise they are undertaking, there are a number of general principles which underlie advice to all athletes.
Always aim to start exercise well hydrated. This can usually be achieved by drinking approximately a small bottle (500 ml) steadily in the 4 hours prior to exercise.
We recommend always carrying liquids when training (for a short steady ride water or a low sugar soft drink is often adequate), and carrying a “sports drink” when competing in any event longer than one hour. Alongside water, the two key constituents of sports drinks are carbohydrates and sodium and the way in which these two nutrients are absorbed in the small intestine (via the SGLT1 cotransporter) means that consuming glucose and sodium together enhances the uptake of both. Commercial sports drinks are typically 6-8% carbohydrates although lower concentrations may be adequate if also carrying ride food. When riding for more than 2 hours or in hot conditions, it is advisable to include sodium in your drinks. A wide range of sodium concentrations can be effective depending on conditions, and there is typically, 0.2-0.7 grams per litre (equal to 0.5-2.1 g / l of salt) in commercial sports drinks. This reflects both the wide range of sweat losses between riders in different environments and variation in the palatability of salt between individuals. In recent years low calorie salt tablets have gained popularity and whilst we do not recommend these for intense competition (unless added to a carbohydrate containing drink), they can be an effective measure in staying hydrated during training provided adequate carbohydrate intake can be achieved with food alone. They can also form part of an effective recovery routine.
Staying hydrated is of greatest importance in extreme conditions and can be a real challenge for athletes. Having an effective hydration strategy in place can significantly improve performance. This is especially the case when competing in hot environments, where regulating temperature becomes more difficult and sweat rates increase markedly. A more common scenario in the UK is competing in cold and wet weather. Although we are slightly more able to tolerate dehydration in the cold, a combination large sweat losses (especially when wearing winter kit), impaired thirst and cold induced diuresis, make dehydration surprisingly common. Making a conscious effort to drink regularly during rides, and rehydrate afterwards can bring significant performance benefits.
Although we may well be losing greater amounts of sodium and using more carbohydrates than is possible to replace in our drinks, the body has a limited capacity to absorb nutrients. Gastric emptying is limited to a maximum of 1.2 litres per hour and this can be reduced further when other nutrients, such as carbohydrates and sodium, are present. Likewise we are unable to absorb more than 90g carbohydrates per hour whilst sodium concentrations above recommended levels can produce unfavourable concentration gradients leading to loss of body fluids into the digestive tract, away from the vasculature. Effective hydration strategies must therefore take account of the balance that must be struck between providing sufficient amounts of both nutrients and fluids whilst ensuring the drink is palatable and the risk of gastric discomfort is minimised.
After exercise, rehydrating should be a priority. This is best achieved by drinking 100-150% of the lost volume of fluid drunk slowly over the hours following exercise. We should also aim to replace lost sodium either as part of a drink or in the form of food. Furthermore, fluids are better retained if drinks are absorbed more gradually and one effective way of achieving this is through the inclusion of protein and carbohydrates both of which important parts of the recovery routines of endurance athletes. Commercially available recovery drinks are a convenient solution, especially when travelling, but several studies have found cheaper alternatives, such as milk, to be equally effective.
Assessing Hydration Status
During exercise, unlike more sedate activities, the rate of turnover of body fluids is high and urine colour is not a sensitive measure of hydration status. Instead, measuring changes in body mass provide a better means of assessment. This can be done at home using a set of bathroom scales by measuring nude body mass before and after a ride. Ideally we aim to lose less than 2% body mass and not gain weight during endurance exercise. For example a rider who is 70.0 kg at the start of a ride should aim to be 68.6-70kg at the end. It may also be of use to calculate your sweat rate in different environments as a guide to how much you should aim to drink during exercise. This can be done using this calculator: http://www.mysportscience.com/single-post/2017/07/14/How-much-do-you-sweat
• Always aim to start exercise well hydrated by drinking approximately 500 ml slowly in the 4 hours before • Aim to minimise dehydration to less than 2% during exercise • If exercise is over 1 hour, or taking place in a hot environment, include sodium and carbohydrates • Consider using cooled drinks in hot temperatures • Replace 150% of sweat losses after training. Include sodium, protein and carbohydrates which will both enhance recovery and aid rehydration