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1 Institute of Food, Nutrition and Human Health, Massey University, Palmerston North, New Zealand Email: t.mundel{at}massey.ac.nz
We appreciate the comments and insight provided by Dr Marino in response to our article (Mündel et al. 2006) and thank the Editor for allowing us to reply. The comments are both interesting and challenging and therefore we would like to address certain key points.
First, the difference in fluid consumption between the cold (4°C) and control (19°C) fluids in our study was in fact 
250 ml, not 160 ml as commented upon. Dr Marino emphasized that ‘it would seem unlikely that the amount of extra [cold] fluid could explain any differences in temperature response.’ This is in fact not the case, since we have taken the same equation proposed by Candas et al. (1986) and implemented by Kay & Marino (2000), and applied it to our own data. Using this method, we determined that ingestion of the cold fluid accounted for an additional 0.3°C difference in the core temperature response during exercise when compared with the control fluid (control, 0.3 ± 0.1°C versus cold, 0.6 ± 0.1°C, P
= 0.004). This calculated 0.3°C difference in core temperature is very similar to the measured difference observed in our study (0.25°C). It is also important to acknowledge that neither we nor Marino and colleagues (Kay & Marino, 2000) are the first to hypothesize the notion of a ‘heat sink’ as a consequence of cold fluid ingestion. There have been other groups that have demonstrated this effect in a warm environment, both at rest (e.g. Pinson & Adolph, 1942) and during steady-state exercise (e.g. Gisolfi & Copping, 1974). Taken together, the data above strongly support the concept of a heat sink, whereby cold fluid attenuates the rise in core temperature and modifies the effects of heat stress imposed on the body.
In his letter, Dr Marino also raises the issue that fluid ingestion does not produce any significant differences in exercise performance when subjects are restricted from adjusting their workload (Kay & Marino, 2003). It should be noted that one must take caution when comparing exercise regimes that are self-paced in nature (closed-loop) with that of a fixed intensity (open-loop). For example, although fluid ingestion appears to have no effect on self-paced exercise lasting 1 h (Kay & Marino, 2003), the same authors have reported that fluid ingestion delays time to fatigue by 25% during fixed-intensity exercise lasting > 1 h when compared with no fluid (Marino et al. 2004).
We agree that there is now a growing body of evidence to suggest the involvement of anticipatory mechanisms during self-paced exercise in hyperthermic conditions. During this type of exercise, the subject is able to alter their power output and cadence, thus being able to adjust the time taken to achieve a set amount of work; therefore, the duration of exercise is seen as a behavioural response. However, demonstrating this during exercise at a fixed intensity is more difficult, since the subject cannot adjust their power output. Rather, (behavioural) alterations can only be made to their cadence and point of volitional exhaustion. Subjects in our study demonstrated similar cadence and no difference in ratings of perceived exertion, whilst the only other behavioural marker – time to fatigue – was different. On this basis, it is not evident that any anticipatory mechanisms were involved. Nevertheless, even during fixed-intensity exercise in the heat, there appears to be a more complex regulatory system including, but not exclusively, initial and terminal body temperatures as well as rates of increase (González-Alonso et al. 1999).
It would appear that the evidence for an anticipatory regulation mediated by drink temperature during fixed intensity exercise in the heat is, at best equivocal.
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González-Alonso J, Teller C, Andersen SL, Jensen FB, Hyldig T & Nielsen B (1999). Influence of body temperature on the development of fatigue during prolonged exercise in the heat. J Appl Physiol 86, 1032–1039.
Kay D & Marino FE (2000). Fluid ingestion and exercise hyperthermia: implications for performance, thermoregulation, metabolism and the development of fatigue. J Sports Sci 18, 71–82.[CrossRef][Medline]
Kay D & Marino FE (2003). Failure of fluid ingestion to improve self-paced exercise performance in moderate-to-warm humid environments. J Therm Biol 28, 29–34.[CrossRef]
Marino FE, Kay D & Serwach N (2004). Exercise time to fatigue and the critical limiting temperature: effect of hydration. J Therm Biol 29, 21–29.[CrossRef]
Mündel T, King J, Collacott E & Jones DA (2006). Drink temperature influences fluid intake and endurance capacity during exercise in a hot, dry environment. Exp Physiol 91, 925–933.
Pinson EA & Adolph EF (1942). Heat exchanges during recovery from experimental deficit of body heat. Am J Physiol 136, 105–114.
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