Exercise While Pregnant Heart Rate

Fluid Recommendations Daily Intake

Establishing universal fluid recommendations is not possible due to the significant interindividual variability in the amount of fluids needed. There are multiple influences that affect fluid needs, including variability in sweat rates, dietary habits, and environmental factors that also influence sweat rates. The DRIs have established total water recommendations 125 oz for men and 91 oz for women and these can be used as an estimate of fluid needs. Because food contributes about 19 percent of our fluid intake, actual consumption of fluids from beverages is recommended to be about 101 oz for men and 74 oz for women (Institute of Medicine (U.S.). Panel on Macronutrients and Institute of Medicine (U.S.). Standing Committee on the Scientific Evaluation of Dietary Reference Intakes 2005), though there is great variability in these amounts. Active adults, for example, regularly engaging in physical activity and living in warm, humid climates may have fluid needs as high as 6 L/day (about 203 oz), though this varies greatly by individual (Kenefick and Cheuvront 2012).

All fluids count toward meeting one’s needs. Water, juice, milk, and even caffeinated drinks including sodas and coffee contribute towards the DRI fluid recommendations. Caffeine was once thought to result in dehydration, and while it has been shown that in the short term caffeine may result in mild diuresis; in the long term caffeine does not seem to negatively affect hydration status (Tarnopolsky 2010). Caffeine will be discussed in greater detail in Chapter 7.

Controversy Over Fluid Recommendations

Assessing hydration status allows an athlete to tailor one’s fluid replacement practices to maximize performance and avoid detrimental health effects resulting from dehydration. It is well established that dehydration resulting in loss of up to 5 to 10 percent total body water will negatively impact performance in a variety of exercise types and intensities (Maughan 2012), so athletes should be educated on fluid replacement to avoid such losses. Yet, controversy exists over the effects of dehydration occurring to a lesser degree. Up until recently, there was general consensus and relevant research indicating that dehydration resulting in a body weight loss of 2 percent or more has deleterious effects on performance. The ACSM wrote a position stand in 2007 supporting this assertion and providing fluid replacement recommendations in accordance with prevention of this 2 percent body weight loss; these recommendations were further supported by a position statement made by the ACSM, Academy of Nutrition and Dietetics, and the Dietitians of Canada (ACSM et al. 2007) (ACSM, American Dietetic Association, and Dietitians of Canada 2000).

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Recent research has challenged the belief that performance is hindered at a body weight loss of 2 percent. For example, one study examined ultraendurance runners completing a 161 km ultramarathon and found that half of the runners lost more than 2 percent of their body weight, yet this did not seem to have adverse performance outcomes (Hoffman and Stuempfle 2014); another study examined high-intensity intermittent exercise and noted similar findings (Yamashita et al. 2015). Other studies find that athletes who drink in accordance with thirst (“ad libitum”) do not experience negative performance effects even with a body weight loss greater than 2 percent. Yet, there are significant methodological concerns of translating the results of these studies into practical recommendations.

Electrolyte Needs

Electrolytes are minerals in the body that help maintain fluid balance through their electrical charge. Calcium, potassium, phosphorous, chloride, and magnesium are all examples of serum electrolytes. Sodium, however, is especially relevant to athletes because sodium can be lost in considerable amounts in sweat during exercise. This can significantly alter fluid balance in the body. The amount of sodium lost in sweat will vary by individual. The typical American diet contains roughly 3.5 g of sodium, and so overall intake is usually not a concern (Kenefick and Cheuvront 2012). However, individuals exercising in very hot temperatures over long periods of time may benefit from electrolyte replacement during exercise. Electrolyte recommendations are discussed in the following section.

Timing of Intake

Until more research is available to replace the current set of recommendations, the ACSM guidelines for fluid replacement will be provided. It is possible that these guidelines will be revised, but until then these standards will determine recommended intake.

Fluid Recommendations Before Exercise

Individuals should commence exercise euhydrated and ensure that plasma electrolyte levels are normalized. This should be achieved several hours before exercise for appropriate fluid retention; this strategy will also allow urination to return to normal output and prevent athletes from taking unnecessary breaks during exercise. Specifically, athletes should aim for 5 to 7 mL per kg of body weight (mL/kg) at least 4 hours before exercise. If the individual still is not producing urine, or urine is very dark in color, athletes should consume another 3 to 5 mL/kg 2 hours before exercise (ACSM et al. 2007). Again, the focus should be to give oneself plenty of time to reach euhydration and allow urine output to return to normal levels. Consuming small amounts of electrolytes, especially sodium, with fluids or foods prior to exercise can support water retention, balance electrolyte levels, and can help stimulate thirst to encourage drinking behaviors.

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