Persistent dehydration MYTHS: Prof Tim Noakes comments on BJSM’s reader poll

BJSM reader poll results

By Tim Noakes

Only 12% of the BJSM readers who answered the poll were correct – this speaks to the power of the prevailing dogma and marketing messages.

Readers have clearly been influenced by the “Science of Hydration.” This mythical concept developed by the sports drink industry during the late 1980s was designed to increase the consumption of sports drinks.

Heatstroke and indeed all heat illnesses are unrelated to measures of fluid balance. Weight loss during exercise includes the weight lost due to the irreversible oxidation of fuels. Moreover, fluid loss during exercise has only a marginal effect on the core body temperature during exercise.  Thus the third answer,  is the only correct answer.

Interestingly, the small rise in body temperature that occurs with “dehydration” is a biological adaptation found in many desert dwelling mammals. Two to three million years ago our evolutionary ancestors developed this adaptation on the arid plains of South and East Africa. When there is inadequate fluid for ingestion, slightly raising the body temperature during exercise in the heat increases these mammals capacity to lose heat without requiring increased sweating. Hence, it is a water-conserving adaptation.  All these mammals could increase their sweat rates to lower their body temperatures, but their brains’ chose not to do this. This shows that this adaptation is:

  1. A biological adaptation of value and;
  2. Is not simply due to a “failure of sweating” caused by “dehydration”.

The latter was naturally the interpretation used by advocates of the “Science of Hydration” further to advance the commercial success of the sports drink industry.
These ideas are covered in my two books, Challenging Beliefs (Struik/Random House, 2nd Edition, 2012) and Waterlogged (Human Kinetics, 2012).

*November’s BJSM carried this key review by well known US primary care physicians Chad Asplund and Professor Fran O’Connor along with Tim Noakes.

Related BJSM Blogs

EVIDENCE-BASED considerations for the Prevention of Heat related illness in Marathon Training (part 1)

EVIDENCE-BASED considerations for the Prevention of Heat related illness in Marathon Training (part 2)


Dr. Timothy Noakes is a Sports Physician, Exercise Physiologist and Discovery Health Professor of Exercise and Sports Science at the University of Cape Town and Sports Science Institute of South Africa.

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