When I’m working out, I always take a sports drink with me, but I’ve started wondering do I really need to? And do our kids need to take a drink with them when they go to practice or play outside? The latest research shows that when exercising for relatively long periods of time (more than an hour), dehydration can be a concern. The goal of hydrating during a workout is to avoid losing more than 2% of your body weight in liquid, at which point negative results have been reported and dehydration is typically defined. The amount that you sweat varies from person to person, so the best way to determine whether you should bring along a drink is to actually weigh yourself both before and after a workout. If your weight has changed by more than 2% during that time, you should consider drinking fluids during your workout.
What the research says:
The American College of Sports Medicine published a Position Stand Document titled “Exercise and Fluid Replacement” that addresses the athlete’s need for hydration and fluids before, during and after exercise . Although that document was published more than five years ago, it succintly reviews of all the relavent data available related to hydration and exercise. The summary below combines the reccomendations in the ACSM position paper with a review of the latest publications.
How is dehydration defined and measured?
The Merriam-Webster dictionary defines dehydration as the process of losing water. All of us sweat when exercising but the sweat rate depends on multiple variables, including the duration and intensity of exercise, the environmental conditions and the type of clothing/equipment worn, body weight, genetic predisposition, and the fitness level . Consequently, we will all suffer a degree of dehyration during activity, but the degree will vary from person to person.
Perhaps more important for this discussion, though, is defining how much dehyd
ration is detrimental when exercising. Dehydration levels of more than 2% (meaning a weight loss of more than 2% of body weight) have been shown to decrease aerobic exercise performance and cognitive/mental performance in warm or hot weather (1, 2, 4), and greater levels of dehydration have been shown to further degrade aerobic exercise performance. So dehydration during exercise is typically defined as a loss of more than 2% of body weight during exercise, at which point results generally show performance will be affected. However, this number will vary depending on the individuals sweat rate and other unique characteristics, as well as weather, intensity of exercise and duration of exercise.
There are several ways to measure dehydration. These methods include changes in plasma osmolality, plasma volume, urine volume, urine color, urine osmolality, and body weight, as well as bioelectrical impedance measurements . For the average person, changes in total body weight are the easiest to measure. It involves two simple steps. Firstly, each individual should acquire baseline weight measurements by measuring themselves several consecutive mornings at the same time. Afterwards, weight loss during exercise can be determined by measuring total body weight both before and after a workout and determining how much weight is lost. When the amount of weight loss during exercise is normalized to the baseline average weight, the percent weight loss is determined. It’s that simple.
What effect does dehydration have on athletes during exercise?
Based on recent publications, “being dehydrated by just 2% impairs performance in tasks that require attention, psychomotor, and immediate memory skills, as well as assessment of the subjective state ”. In addition, “cognitive/mental performance, which is important where concentration, skilled tasks and tactical issues are involved, is also degraded by dehydration and hyperthermia .” So, recent results point to a link between dehydration and impaired cognitive ability.
In addition, physical performance is impaired when dehydrated due to increased body core temperature, increased strain on the cardiovascular system, and impairment of metabolic function, and perhaps altered central nervous system function .
How prevalent is dehydration during sports?
A study published in early 2012 looked at illnesses suffered by track and field athletes during the 13th International Association of Athletics Federations World Championships in Athletics 2011 in Daegu, Korea . That study reported that exercise-induced dehydration constituted 12% of all illnesses diagnosed. Similarly, a study conducted in 2007 found that experienced runners who ran an 8-mile course in 20°C weather and consumed fluid voluntarily experienced a weight loss of approximately 1.9% . In fact, dehydration is often considered the most common medical emergency incurred by athletes competing in endurance events . But how relevant is it to the average athlete?
Studies compiled by ACSM have found that changes in body weight during exercise can vary from 0% in training swimmers to 3.5% in Ironman triathletes; from 0.36% in waterpolo athletes to 1.59% in soccer players; from 1.0% in basketball players to 1.5% in football players . So the degree and incidence of dehydration will depend on the intensity and duration of the activity, external conditions, and the individual.
Studies have been done to measure the sweat rates and average level of dehydrations for athletes participating in severl different sports and at varying levels of competition . Based on these results, athletes are known to achieve sweating rates from 0.5 to 2.0 Liters per hour with athletes competing in high-intensity exercises over extended periods in the weather (such as tennis and football in the summer) having the highest sweat rates on average. In the same vein, dehydration rates were higher than 2% for athletes competing in endurance events in the weather, such as half-marathon running and triathlons .
Does this apply to children playing sports?
A retrospective study in the US emergency departments during the period 1997-2006 found that patients aged 19 years or younger accounted for the largest proportion of exertional heat-related illness, the majority of which were associated with performing a sport or exercising . The root cause of this phenomenon is unknown, though. In years past, studies concluded that children were more susceptible to heat-related illnesses during exercise because of differences in surface area-to-body mass ratio for heat absorption, a greater metabolic heat production and a lower sweating rates . However, recent studies have shown conclusively that “there exists no convincing evidence that the risk of exercising in warm weather is any greater than that of adults” . So biologically, children are not predisposed to suffer heat-related incidences more than adults. What causes the high incidence of heat-related illnesses? The incidences of heat-related illness may be elevated in children because of their reluctance to drink while at play. A study published in 1980 found that exercising children progressively dehydrate when not forced to drink . In fact, a similar study found that only approximately 4% of students at play stopped to drink water . In that same study, students at play for approximately 30 minuets were found to have lost approximately 0.63% of their body weight. So, although the change in weight during activity was not at the level of 2%, they had lost on average 0.63% of their weight in only 30 minutes. So perhaps the incidence of heat-related illnesses in children is related to their lack of proper hydration.
1. American College of Sports Medicine position stand. Exercise and fluid replacement. American College of Sports Medicine, Sawka MN, Burke LM, Eichner ER, Maughan RJ, Montain SJ, Stachenfeld NS. Med Sci Sports Exerc. 2007 Feb;39(2):377-90.
Heat illness surveillance in schoolboys participating in physical education class in tropical climate: an analytical prospective descriptive study. Somboonwong J, Sanguanrungsirikul S, Pitayanon C. BMJ Open. 2012 Jul 7;2(4).
10. Determination of future prevention strategies in elite track and field: analysis of Daegu 2011 IAAF Championships injuries and illnesses surveillance. Alonso JM, Edouard P, Fischetto G, Adams B, Depiesse F, Mountjoy M. Br J Sports Med. 2012 Jun;46(7):505-14. Epub 2012 Apr 20.