A new study shows that weight loss is not equal to water loss during exercise.

Written by: Matt Fitzgerald

A team of exercise scientists at the University of Cape Town, South Africa, recently studied hydration and dehydration in the real-world context of a running race, and obtained some interesting results. The researchers visited the 2009 Two Oceans Marathon and recruited 21 runners participating in the half-marathon event and another 12 participating in the 56-mile event to participate in the study.

Measurements of body mass, total body water, and blood osmolality were taken before and after the races. All of the runners participating in the study were advised to drink according to their thirst while running. Both the half-marathon runners and the 56-mile runners lost weight. The 56-mile runners lost body water, while the half-marathon runners did not. Both groups lost more total body weight than water weight, and the blood osmolality of both groups increased.

What do these results mean? They mean, first of all, that some of the weight lost during running is not body water. In the past, exercise scientists believed that all weight lost during running contributed to dehydration. Timothy Noakes, one of the scientists involved in this study, has been arguing for years that the advice to drink enough to completely offset weight loss during exercise causes people to drink too much, precisely because not all of the weight loss that occurs during exercise is body water. Whereas exercise scientists have traditionally told athletes that they can’t trust their thirst to set their drinking rate during exercise, because athletes who drink by thirst lose weight, Noakes believes that thirst does an excellent job of motivating athletes to replace only the body water they lose during exercise, and not non-water body weight.

Noakes started down this path of research after making the simple observation that, if  sweating and the loss of body water that occurred through it were so bad, it was awful strange that human beings were so good at it. As he got farther along this path, Noakes arrived at the belief that moderate dehydration is not nearly as dangerous to the human animal as hyponatremia, or a decrease in the blood sodium concentration. Sweating and moderate dehydration help the body defend its blood sodium concentration during exercise, and that’s why we sweat so much and why we don’t drink enough to fully offset body weight losses during exercise.

So, what is the practical upshot of all this? The old method of weighing yourself before and after workouts and using the difference in body weight to calculate an ideal drinking rate during exercise is not a good idea. It will cause you to drink too much, because some of that weight-loss you’re seeing is non-water body weight. Your best bet is to drink by thirst, as this will offset your actual water losses and help you maintain a stable blood sodium level, which is more important than maintaining full hydration during exercise.


Check out Matt’s latest book, Racing Weight Quick Start Guide: A 4-Week Weight-Loss Plan for Endurance Athletes.