With the Perth weather taking a noticeable turn for the heat
we are accustomed to throughout our Summer months, I feel it is important to
highlight a massive safety and performance variable that I feel is often over
looked. Today I am going to dive into hydration, and more importantly,
dehydration; in an attempt to shed some light on the topic to help keep you
safe and performing at your best whilst exercising. I tie heat and exercise
together as it’s a pretty noticeable trend than when the weather heats up,
people are more inclined to start chasing down their body goals for the Summer.
As a lot of these exercisers are currently a little out of shape coming off the
cooler months, once you couple this with dehydration associated with an increase
in environmental temperature, you have a recipe for disaster.
What is classified as dehydration?
Murray (2007) states that dehydration is classified as a decrease in euhydrated
body mass of 2% or more of total weight. For example, a person who is hydrated
and weighing 100 kilograms would need to lose 2 kilograms of their weight
through sweat and electrolytes to be considered dehydrated. It should be noted,
and whilst rates do vary, each litre of sweat can contain between 920 and 1150
mg/l of sodium. Whilst these numbers might sound like a lot, Murray (2007) also
states that during any physical activity, be it exercise or work, a person will
lose an average of between 400ml to 3000ml of sweat per hour. So, a 100
kilogram person could in fact be considered dehydrated within 1 hour of
exercise if they were to sweat to the upper limit of 3 kilograms per hour. Varying
factors come into play when exercising and have a direct result on rate of
perspiration. These factors include environmental temperate and humidity level,
exercise intensity and duration, fitness level and heat acclimation (Murray,
2007). With all of these factors in mind, my original point of an increase in
exercisers as the weather gets warmer comes with obvious consideration which
need to be addressed pre training.
How risky is dehydration?
Murray (2007) wrote that as levels of dehydration increased, important measures
of physiological performance and health continued to decrease. He stated that
at a decrease in body mass of 4.9%, heart rate, core temperature and perceived exertion
continually rise during a 2 hour cycling bout at 65% of Vo2 max. He also stated
that blood volume, stroke volume, cardiac output and skin blood flow all
decreased. Some take home points from this paper regarding the risk to health dehydration
proposes were as follows; dehydration causes an increase in thermoregulatory
stress, an increase in cardiovascular strain, induces negative changes to
muscle metabolism including an accelerated rate of glycogen depletion and
alterations to central nervous system function leading to increased rates of
fatigue and decreased motivation (Murray, 2007).
Take home message:
It is reported that the average intake of approximately 1 litre of fluid during
physical activity can help mitigate the risk of dehydration and premature
fatigue. Fluids should be a combination of water and electrolytes as to combat
the loss of sodium of up to 1150 mg per litre. Maughan and Shirreffs (2010)
propose that an individual learn their bodies hydration needs by monitoring
urine output prior to exercise and ensuring adequate hydration, or what is
known as euhydration. The authors state that pre exercise weighing as well as
an increase in fluids and electrolytes prior to the commencement of exercise is
a good way to prepare for a potentially dehydrated state during exercise
(Maughan & Sherriffs, 2010).
References:
Maughan, R.J., & Sherriffs, S.M. (2010). Development of hydration
strategies to optimise performance for athletes in high-intensity sports and in
sports with repeated intense efforts. Scandanavian
Journal of Medicine & Science in Sports, 20(2), 59-69.
Murray, B. (2007). Hydration and physical performance. Journal of the American College of Nutrition, 26, 5425-5485.