Fluid Recommendations During Exercise
Most athletes do not experience significant fluid losses until the threshold of about 1 hour of high intensity exercise, or 60 to 90 minutes of steady state exercise (or more). Fluid replacement during exercise below these time frames typically is not warranted; exceptions are for athletes starting exercise in a dehydrated state or exercising in extremely hot and humid climates, or at high altitude. There may be individual variation in sweat rate and sport type that may also warrant fluid replacement within 60 to 90 minutes. A 140 lb male distance runner, for example, may lose up to 2 L of fluids during a 1 hour run due to high metabolic heat production. This emphasizes the importance of knowing an estimate of one’s individual sweat rate (such as using pre- and postexercise weight as a measurement tool) as will be described.
Though not uncontroversial, sufficient research supports decrements in performance with a body weight loss of 2 percent or more (Kenefick and Cheuvront 2012). The goal of consuming fluids during exercise is to prevent such body weight losses, as well as to maintain electrolyte balance so that performance is not impaired. There are multiple variables affecting the amount of fluids lost during exercise, including the duration of exercise, intensity, clothing, environmental conditions, individual sweat rates, training status, and acclimatization. Accordingly, there is no singular fluid replacement strategy.
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Ideally, athletes get a sense of their fluid losses by tracking weight loss during exercise over various conditions and exercise types (intensity and duration). One way to achieve this is to have athletes weigh themselves in minimal clothing before and after exercise. Assuming that 1 mL of sweat equates to a 1 g loss of body weight, athletes subtract their post-exercise weight from their pre-exercise weight, correcting for urine losses and fluids consumed. See Table 4.6 for an example. As athletes perform these calculations over a variety of contexts (high-intensity versus moderate-intensity training, hot versus cool climates, indoors versus outdoors, etc.) they can start to get an estimate of what their typical sweat rates are in various situations. This becomes a very practical and useful tool that athletes can calculate by themselves. While the goal is not to replace 100 percent of fluids lost, as this can lead to feelings of fullness, sloshing, and even
Keeping in mind that there is no “one size fits all” fluid recommendation, a starting point for fluid replacement for exercise greater than 1 hour is approximately 0.4 to 0.8 liters per hour (L/hr). This may be inadequate for individuals who sweat up to 2 L/hr, but in general will be appropriate for most individuals. Within this range, petite female marathon runners who run at a relatively slow pace may be at the lower end of the recommended range (about 0.4 L/hr), and larger-framed male athletes engaging in interval work may benefit from a higher fluid intake (about 0.8 L/hr). Customization of fluid intake is required, and knowing one’s individual sweat rate becomes quite useful. Fluids are optimally absorbed when they are ingested at regular intervals, such as every 10 to 15 minutes, though practical considerations of the sport type determine what is most feasible. Avoiding consumption of very large volumes in a short amount of time will also help to avoid gastric distress.
Electrolyte replacement during exercise depends upon exercise type and duration, as well as environmental factors and acclimatization. Sodium replacement is recommended in hot climates when individuals are exercising for several hours, such as in marathon running and other ultraendurance events lasting more than several hours; it may also benefit individuals who are not heat acclimated. An elite marathon runner finishing in less than 3 hours may not need to worry about sodium replacement, for example, but the recreational runner finishing in 6 hours likely will benefit from sodium replacement. Other athletes, including football players practicing for several hours at a time and who are wearing heavy equipment, may lose considerable amounts of sodium during the workout. For those requiring sodium replacement, about 20 to 30 meq/L is a good starting point (ACSM et al. 2007) and can be fine-tuned as needed on an individual basis. This amount can be consumed from most sports drink and in most cases salt tablets are not needed.
A risk of inadequate sodium intake is hyponatremia, or when plasma sodium drops below 130 mmol/L. See “Practical Implications: Hyponatremia” for additional information.
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