Does dehydration decrease performance? Maybe not!


There has been some robust debate recently in the sport science literature. Some experts are saying a level of dehydration is safe in endurance athletes and that we should rely on thirst to tell us when to drink. In contrast, there are some experts that say drink before we get thirsty and try and maintain your body weight during endurance exercise. This research study on marathon runners suggests marathon performance is better in those that lose more body weight.

The Research

Body weight changes were measured in 643 marathon finishers (560 males and 83 females) before and after the 2009 Mont Saint-Michel Marathon in France.  The runner’s body weight was measured 60-90 minutes before the start of the marathon while they were wearing their running clothing but not shoes. Within 20m of the finish of the event their body weight was measured again by the same researcher on the same scales. Again, no shoes and the same clothing were worn and yep, some sweat may have been in the clothes! Runners were advised to drink 250 mL of fluid (sports drink or water) every 20 minutes during the event. Weather conditions were: 9-16 degrees Celsius, 60-82% humidity and strong winds. For statistical analysis, the runners were divided into three groups – sub 3hr, 3-4 hr, and greater than 4 hour marathoners.

The Results

The average age of the runners was 43±8.8 years and runners lost on average 1.7 kg or 2.3±2.2% of their body weight during the event. Males and females lost similar amounts of fluid (1 kg weight loss equals approximately 1 litre of fluid loss) and their was no effect of age on fluid loss. The weight lost ranging between gaining 5% and losing 8% of pre-race body weight. 55% of runners lost more than 2% of their body weight – the gold standard often mentioned as leading to decreased endurance performance.

Interestingly, and here is the punch line for this research, the runners who finished under 3 hours lost significantly more body weight (3.1±1.9%) and thus fluids than either the 3-4 hr runners (2.5±2.1%) and the 4 hr plus runners (1.8±2.4%). When the scientists plotted the relationship between race time and body weight lost, the relationship was very strong suggesting the greater the weight lost, the faster the run time.

So What?

The bottom line is that these results support a growing body of evidence that questions the laboratory-based findings that a body weight loss of greater than 2% impairs sport performance. In contrast, the results support previous field-based studies that show that successful athletes in marathons, ultra-distance, and triathlon events are those who lose more than 3-4% of body weight during competition. The other intersting observation from this study was that almost 10% of runners gained weight by overdrinking, an observation common in ‘back of the packers’ who read they should drink a lot but often aren’t exercising as hard as faster runners and thus may not lose fluids as much as the books suggest. This overdrinking can lead to potentially fatal condition called hyponatremia (low blood salt levels).

The bottom line again, drink before, during and after training and events and monitor your body by listening to how it responds. For information on the effect of heat (and cold) on masters athletes and, more importantly, how to manage exercising in the heat and cold as an older athlete, see Chapter 11 of my book The Masters Athlete.