Hypohydration During Prolonged Exercise in the Heat
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COPYRIGHT AND USE OF THIS THESIS This thesis must be used in accordance with the provisions of the Copyright Act 1968. Reproduction of material protected by copyright may be an infringement of copyright and copyright owners may be entitled to take legal action against persons who infringe their copyright. Section 51 (2) of the Copyright Act permits an authorized officer of a university library or archives to provide a copy (by communication or otherwise) of an unpublished thesis kept in the library or archives, to a person who satisfies the authorized officer that he or she requires the reproduction for the purposes of research or study. The Copyright Act grants the creator of a work a number of moral rights, specifically the right of attribution, the right against false attribution and the right of integrity. You may infringe the author’s moral rights if you: - fail to acknowledge the author of this thesis if you quote sections from the work - attribute this thesis to another author - subject this thesis to derogatory treatment which may prejudice the author’s reputation For further information contact the University’s Director of Copyright Services sydney.edu.au/copyright CHAPTER 1 INTRODUCTION 1 1.1 INTRODUCTION Hypohydration (>2% body weight deficit) has been reported to increase cardiovascular and thermoregulatory strain and thus impair endurance exercise performance especially in hot conditions (Convertino et al ., 2000; Cheuvront et al ., 2003a; Murray, 2007; Sawka et al ., 2007). To avoid excessive dehydration (>2%), the American College of Sports Medicine (ACSM) Position Stand on Exercise and Fluid Replacement recommends endurance runners to drink ad libitum 0.4 to 0.8 L.hr-1 during exercise (Sawka et al ., 2007). However, drinking fluid ad libitum is likely to lead to a body weight deficit of at least 2% (Fudge et al ., 2007). Fudge et al . (2007) further reported that “in conditions of modest dehydration (up to 3 L), fluid intake does not alter plasma volume, sweat rates or cardiovascular function”. This finding supports the earlier work of Cheuvront and Haymes (2001a) who reviewed the effect of ad libitum fluid ingestion on thermoregulatory responses and concluded that fluid replacement between 60-70% of sweat losses is effective in maintaining thermoregulation. However a systematic study of the effects of graded dehydration (1-5% BW loss) has shown increasing levels of thermal and circulatory strain with graduated fluid restriction (Figure 1.1) (Montain & Coyle, 1992b). 2 Figure 1.1: Influence of dehydration, as assessed by percent reduction in body weight after 2 hours of exercise, on change in cardiac output, heart rate, stroke volume, forearm blood flow during exercise (Montain & Coyle, 1992b) Recent field studies have contradicted previous findings and suggest that maintaining fluid balance is not essential in athletic field settings such as marathon races (Noakes, 2007a; Noakes, 2007b). In reality, elite marathon runners only ingest approximately 200 mL.h -1 of fluid during distance running races because they encounter difficulty in ingesting fluid whilst maintaining a fast speed of running (~85% ͐ʖ O2max ) (Noakes & Maharam, 2003). A number of studies have reported that greater body mass losses occurred in successful athletes who drank ad libitum during exercise (Cheuvront & Haymes, 2001a; Speedy et al ., 2001; Kao et al ., 2008) without affecting the 3 physiological markers of hydration status (Nolte et al ., 2010a; Nolte et al ., 2010b). It is thus not uncommon for elite marathon runners to experience large bodyweight changes and fluid loses as shown by the current marathon world record holder (now deceased) who lost 10% of body mass while establishing that record (Fudge & Pitsiladis, 2009). More recently, Zouhal et al . (2010) found that there was an inverse relationship between body weight change and finishing time in 643 marathon runners. These inconsistencies between laboratory and recent field studies have resulted in uncertainty among coaches, athletes and sport scientist regarding fluid replacement for athletes. Therefore, two questions arise (i) “ Does moderate hypohydration impair endurance performance?” (ii) “What is the critical threshold level of hypohydration to affect the circulatory and thermoregulatory responses during prolonged running performance?” Kenyan runners have dominated the middle and long distance running events at international level since the 1960s. Their outstanding success has captivated research’s seeking to discover the “secret” for the success of Kenyan elite distance running. There have been studies that have reported on the nutritional practices and daily lifestyle of Kenyan runners (Onywera et al ., 2004; Fudge et al ., 2006; Fudge et al ., 2008). These studies have shown that a negative energy balance with low fluid intake was practiced by the Kenyan runners prior to competition and during training. However, this practice did not impair the Kenyan runners’ performance as they have maintained their supremacy in endurance running for the past 40-50 years. Thus, Fudge et al . (2007) proposed that as hypohydration necessarily results in a decrease in body weight, there will be a reduced energy cost of running and consequently a reduced metabolic heat load and therefore delayed fatigue and enhanced running performance. However, whether the mechanism or mechanisms responsible for the Kenyan runners’ world class 4 performance is associated with body water loss and running economy is contentious and in need of further investigation. As ambient temperature increases, sweat evaporation is a very effective means to provide a cooling effect for heat dissipation to achieve heat balance. Sweat evaporation limits the rise in core temperature and prevents the progressive development of hyperthermia. However, if fluid intake is less than sweat loss during exercise, it will lead to hypohydration which is associated with impaired endurance exercise performance (Sawka, 1992; Cheuvront et al ., 2003a; Murray, 2007; Maughan, 2010). The adverse effects of hypohydration associated with hyperthermia on prolonged exercise performance have been extensively studied, yet it is unclear whether the impairment to exercise performance is due to hyperthermia per se , or hypohydration per se , or a combination of hypohydration and hyperthermia. Concurrently, the perspective that hypohydration is the primary factor precipitating impaired endurance performance in hot conditions has been challenged with two alternative hypotheses: (i) Critical core temperature hypothesis and (ii) Circulatory strain hypothesis. In a review of long distance running literature, the runners’ core temperatures were in the range of 38 to 40.8 °C as they completed the marathon / distance running races in varied environmental settings from 6 to 32 °C (Cheuvront & Haymes, 2001b). It is unknown why some runners can tolerate such a high core temperature and completed the marathon races, but some runners tend to terminate exercise as core temperature nears 40 °C (González-Alonso et al ., 1999b). Meanwhile, the increase in intracellular temperature will induce a heat shock protein 72 (HSP 72) response (Holtzhausen et al ., 1994). There are several reports documented that HSP 72 affords protection to heatstroke and also acts as a biomarker of multiple organ tissue damage (Huisse et al ., 5 2008; Ruell et al ., 2009; Dehbi et al ., 2010). Ravindran et al . (2005) found that resistance to dehydration damage in some cell lines correlates with the presence of a range of HSPs. The key question is whether these changes are causally linked; does prolonged exercise in hot conditions coupled with hypohydration induce a heat shock protein response ? The ingestion of glutamine supplementation has been shown to induce HSP expression during hyperthermia (Ziegler et al ., 2005) and provide an ergogenic effect during dehydration (Hoffman et al ., 2010). Another question to be answered is whether glutamine ingestion effectively enhances HSPs expression together with the restoration of fluid in dehydrated subjects during exercise in the heat . The Review of Literature in Chapter 2 surveys our current knowledge of the circulatory and thermoregulatory responses during prolonged exercise in the heat coupled with hypohydration and identifies avenues of enquiry that remain to be investigated. To further understand the effects of hypohydration and thermotolerance during prolonged exercise performance in hot climatic conditions, four studies were designed to investigate the underlying physiological mechanisms. Chapter 3 describes the research thesis design and the hypotheses and mechanisms tested as part of this thesis. Chapter 4 describes the common methodology which has been employed in this thesis. Chapter 5 investigates the hydration status of elite Kenyan distance runners competing in hot, humid conditions. Chapter 6 employs a specially constructed vest with pockets to insert sandbags of known weight, which were used to simulate the increase in bodyweight associated with simulated hyperhydration. In the same chapter hypohydration is induced via a prolonged walking protocol to attain a significant body weight deficit as a result of sweating. In Chapter 7 the effect of diuretic-induced dehydration on prolonged running performance in hot and cool climatic conditions is investigated. Chapter 8 investigates the effect of alanyl-glutamine ingestion on prolonged running performance 6 in hot and hypohydrated conditions. Finally, the key findings and recommendations