The general guidelines for both fluid and carbohydrate intake during exercise are presented. As a general rule, carbohydrate ingestion should be a priority during competition and on days you train at a relatively intense effort for longer than 80 to 90 minutes. Remember, however, that carbohydrate may also be helpful for shorter, interval sessions or when you are unable to eat an adequate pre event meal. Currently, the recommendation is to aim for roughly 30 to 60 grams of carbohydrate per hour (approximately 0.7 g carbohydrate/kg body mass) with a focus on high-GI choices.
For best results, you should consume carbohydrate at regular intervals (if allowed by the rules of your sport) and begin shortly after the onset of exercise. Hot-off-the-press evidence, however, suggests that higher carbohydrate intakes—up to 100 grams per hour—may be beneficial during endurance and ultra endurance events such as marathon and ultramarathon runs, century and double century (160 and 320 km) bicycle races, and adventure racing. Athletes striving for higher hourly intakes of carbohydrate, however, need to ensure that the carbohydrate comes from several rather than a single sugar source.
Although scientists used to think that muscle was able to take up and utilize a maximum of one gram of dietary carbohydrate per minute—regardless of body size—this exciting new research has suggested this is not true.The limiting factor apparently lies in the intestines, where different carrier proteins are responsible for escorting different sugars, such as glucose and fructose, across the intestinal wall. If only one type of simple sugar is consumed, we indeed can only absorb about 60 grams of sugar per hour, but if more than one type is consumed, we can recruit different carriers and absorb and oxidize up to 100 grams per hour.
Fortunately, the carbohydrate found naturally in fruits and fruit juices is made up of a variety of sugar sources, as is the carbohydrate provided by many fluid-replacement products such as Gatorade. Like carbohydrate intake, fluid intake during exercise should be closely monitored on days you train or compete for more than an hour. This of course is provided you are well hydrated at the start of exercise; otherwise, fluid intake is even more important. To prevent dehydration, the American College of Sports Medicine recommends that athletes drink enough fluid during exercise to replace what is lost through sweat.
A typical sweat rate is 800 to 1,500 milliliters per hour and is even higher during exercise in hot and humid environments, including the outdoors and non-air-conditioned gymnasiums. It is best, however, to have an idea of your own fluid requirements, which can change throughout your training season and in response to different types of exercise and different environments. Regularly obtaining your weight before and after exercise also helps you determine if you are under- or over drinking. You should never gain weight during exercise because it means you are overhydrating and maybe placing yourself at risk for hyponatremia.
Understanding your own fluid needs during exercise will help you establish a personal fluid intake plan according to the rules and rigors of your sport, and help ensure that you do not go overboard and consume too much fluid. For example,I typically aim to consume about one liter of fluid every hour, which is close to my sweat rate during moderately intense running. When I run a long race, I estimate how much fluid (in small cups) I need to drink at each water station based on the distance between stations and my estimated pace. If the stations are too far apart, such as in a marathon with water stations every five miles (8 km), I have to decide whether to ask a relative to supply water or to carry my own.
During a training run, ride, or cross-country ski event, I might have the option of stashingwater along a predetermined route or planning a route where I can get water at gas stations or from a friend’s outdoor hose. If I played a team sport, I would estimate how much fluid I would need to consume during regularly scheduled breaks, bench time, and time-outs and balance that with my anticipated playing and bench time. This may sound like a bit of work, but improperly hydrating can impair performance and have life or death consequences.