One of the first things I did upon my move to Southern Ontario in July 2007 was to coordinate a film shoot for Discovery Channel on heat stress in firefighters. The setting was perfect, with the shoot occurring outdoors on a ridiculously hot and humid day. Actually, the worst thing was my having to wear the exact same clothes for both days (my washing machine, along with my family, was still back in Halifax with our unsold house!).
Not only were the hosts hot and bothered doing firefighting simulations on black asphalt while wearing full gear, but the crew and myself went through coolers after coolers of water to stay hydrated. Throughout the shoot, I constantly impressed upon everybody the importance of staying cool and hydrated, as the last thing I needed was to be doing a filming on heat stress and having the crew collapse from heat exhaustion!
1996 versus 2007
As we discussed last summer in Toolbox, the American College of Sports Medicine (ACSM) Position Stand on Fluid Replacement has undergone a dramatic revision between 1996 and 2007. In 1996, the focus was on drinking as much as tolerable, and the recommendation was to replace all the fluid that is sweated out throughout the course of exercise. As a result, we have the proliferation of water bottles everywhere in gyms and shopping malls, as people became paranoid about not drinking enough water.
Since 1996, scientific research, especially from Professor Tim Noakes’s group in South Africa, has questioned the need for such thorough rehydration. In summary, Noakes proposes that: 1) moderate dehydration during exercise does not necessarily impair performance, and 2) human behaviour alone is sufficient to determine the amount of rehydration during exercise rather than a pre-determined enforced rehydration schedule. Partly based on this these arguments, the 2007 ACSM Position Stand has been modified significantly to a range of 400 – 800 mL/h, with guidelines to accommodate a wide range of individual variability.
Old Habits Die Hard
Science is deliberately built on a foundation of shifting ideas, but human behaviour and habits can be tough to change. One of the fundamental driving factors behind Noakes’s campaign for voluntary thirst being the guide is the observation that many elite marathoners and ultramarathoners drink only minimally during competitions in the heat. Despite such reports, a lot of resistance remains to abandoning hard numbers for rehydration guidelines and simply leaving it up to individual thirst to determine fluid replacement.
To further test the ability of voluntary thirst in maintaining hydration status, Noakes’s group tracked the hydration status of a cohort of elite Kenyan distance runners throughout a major block of training (Fudge et al. 2008). Previous studies reported that these runners tended to drink relatively minimal amounts before, during, and after training, at levels much below the 1996 guidelines of full fluid replacement during exercise. Does this apply over a prolonged period, and does this have longer-term consequences?
This extension of hydration monitoring over a prolonged period is a new and interesting addition to the existing literature, which has primarily focused on laboratory tests and simulations, and also tracking drinking patterns only during the course of an individual test or competition. The testing of elite athletes during intense training also adds a high degree of realism and applicability to the study.
Fudge et al. 2008
The basic details of the study:
• 14 elite Kenyan runners (800 m to marathoners, all heat- and altitude-acclimatized prior to the study and based at altitude) preparing for the national trials prior to the 2005 IAAF World Championships. Considering the dominance of Kenyans in distance running, the trials almost serve as the informal world champs!
• Subjects were tracked over 5 days of training one week prior to the Trials, during a taper phase consisting of decreased volume but maintained intensity.
• Training generally occurred in discipline-specific groups, and typically consisted of two interval sessions per week and also 1-2 variable-distance runs daily.
• Subjects did not receive any dietary advice or instruction from the coaches or experimenters to minimize bias. However, subjects were required to weigh and record all food and drinks consumed, which may serve to subconsciously make subjects more aware of their diet than normal. In turn, this may serve to either increase or decrease dietary intake.
• Multiple indices of hydration status were recorded, including body weight, body composition and fluid compartments using bioimpedance, fluid consumption, diet logs, dietary sodium intake, urine osmolality (electrolyte concentration), urine and sweat sodium concentrations. Core temperature and daily training were also monitored.
Water Water Everywhere
In general, the findings supported the hypothesis that voluntary (ad libitum) drinking was a suitable strategy throughout a period of training:
• Environmental conditions were moderate to warm (21.1oC during afternoon sessions) but not extreme.
• Body mass losses during training were significant but relatively mild, ranging from 0.5 – 1.1 kg depending on time of day and also intensity of training. With the combination of a morning run and an afternoon interval session, subjects ended the second session 1.5 kg lighter (dehydration of 2.7% body weight).
• Drinking and eating during training did not occur at all in this group.
• Daily fluid consumption generally consisted of water (0.7 L), milky tea (1.2 L), other fluids including soft drinks and milk (0.4 L). Fluid consumption through food averaged 1 L, and metabolic generation of water averaged 0.5 L, for a total of 3.8 L daily.
• No changes were reported in day-to-day hydration status, body weights, or response to training.
Summary and Caveat
Fudge et al.’s findings would seem to suggest that the human organism is indeed fairly capable of maintaining hydration status over a prolonged period by subconsciously monitoring and without the need for an enforced drinking program. However, a significant caveat must be raised.
The environmental temperatures were warm but cannot be considered hot, especially not the hot and humid conditions that typify many summer climates. This may have led to the fairly low sweat rates from these runners. Here in the sweltering heat and humidity of southern Ontario, I would rather err on the side of caution and institute a moderate program of regular drinking before, during, and following activity to supplement normal thirst sensations. At the very least, the ACSM 2007 guidelines of 400-800 mL/h of fluid replacement during exercise during training does not appear counter-productive to quality training, and therefore remains a good guideline for us to follow.
Fudge, B.W., C. Easton, D. Kingsmore, F.K. Kiplamai, V.O. Onywera, K.R. Westerterp, B. Kayser, T.D. Noakes, and Y.P. Pitsiladis. Elite Kenyan endurance runners are hydrated day-to-day with ad libitum fluid intake. Medicine and Science in Sports and Exercise, 40:1171-1179, 2008.
Stephen Cheung is a Canada Research Chair at Brock University, with a research specialization in the effects of thermal stress on human physiology and performance. He can be reached for comments at firstname.lastname@example.org .