Sweat, Hydration and Running Performance

Maintaining Hydration, The 2% Body Weight Loss

By Andy Garlick, director of Sports Science at SOS Hydration.

The American College of Sports Medicine recommends maintaining hydration to no more than 2% body weight loss during training or competition. This recommendation, arrived at by a panel of experts relates to physiological wellbeing and performance.

The basics

During exercise we produce heat and our core temperature increases. In order to maintain homeostasis we produce sweat which evaporates on the skins surface removing the heat from the body.The message
Sweat is good. Sweat is necessary. Continued sweating allows the body to regulate body temperature and not overheat.

So what?

When we sweat we lose body fluid. If we do not replace this fluid, we become dehydrated and a number of things happen physiologically which can be characterized in to either cardiac or thermoregulatory responses.

Cardiac Stress

Decreased blood plasma volume – liquid part of the blood
Decreased blood pressure – less blood volume reduces blood pressure
Decreased stroke volume – less blood and oxygen pumped with each beat resulting in
Increased heart rate

Ultimately there are greater cardiac demands on the body for a given work rate when dehydrated. It has been suggested the heart rate increases 3-5 bpm (beats per minute) for every 1% dehydration. Therefore if dehydrated whilst competing you are wasting those precious heart beats, looking at your heart rate monitor and thinking you are working maximally, but not performing at your best.

Thermoregulatory disruption

Our ability to continue to sweat effectively relies on adequate body fluids moving into the blood stream and to the skin to produce sweat and dissipate heat. This inability to lower core body temperature results in heat stress. For every 1% dehydration body core temperature rises 0.15 – 0.2 degrees Celsius.

Take away message

Adequate fluid intake before, during and after activity and effective cooling methods can help to avoid the negative effects of dehydration and aid performance.

To calculate your sweat losses weigh yourself before and after exercise, adding the weight of any fluid consumed during the session. The aim is to maintain your body weight through fluid replacement to maintain hydration. Any loss of fluid over 2% of your body weight is likely to compromise your performance, wellbeing and recovery.