Our body is over 70% water. The water is in our blood circulation (our hearts, arteries and veins), the fluid between our body cells (interstitial fluid) and the fluid inside our body cells (intracellular fluid). Crucially, the fluid that surrounds our brain (CSF or cerebrospinal fluid) is almost all water.

We gain water from the fluids we drink and the fluid content of the food we eat and we lose water in urine, sweat and faecal matter. If we lose more than 2% of our body fluid, our physical and mental performance deteriorates, the more we lose, the greater the impairment. Very rapid, severe fluid loss will induce life threatening physiological shock that shuts down major organs including kidney function.

We sweat almost all the time, we only become aware of it when it runs on our skin surface, usually our face and trunk. Sweating is an important part of our body’s heat regulation systems.

As sweat evaporates, heat is removed from our skin together with the water. Wiping sweat off our skin helps to remove the increased body heat generated by exercise. So we need to keep wiping sweat off our face and trunk with a towel to minimise body heat. Higher humidity dramatically reduces the amount of sweat that can evaporate, so wiping sweat off becomes even more important.

The symptoms of inadequate hydration vary with the individual, their age, fitness body fat content and their degree of acclimatisation. The symptoms include:

• Thirst
• Muscle cramps
• Muscle soreness
• Poor muscle recovery
• Muscle weakness
• Headache
• Reduced or no urine output
• Physiological stress, heat stroke

All of these are avoidable and should be avoided (including actually feeling thirsty). We can lose up to 2% of our body fluid without significantly affecting our capacity to exercise If the room temperature and humidity are moderate (moderate is usually given as 18-24°C and humidity 20-30%). Once the temperature and humidity start to get higher, our performance is affected even at fluid losses below 2%. Acclimatisation to high temperature and humidity helps to develop tolerance, but for all general purposes 2% is the maximum fluid loss we can tolerate. It’s also worth noting that even in acclimatised individuals, coping with increased heat and humidity always results in significant physiological stress to the body, this reduces or negates the benefits of exercise.

Bottom line, raised humidity has the same physiological effect as raising the temperature, so training in 40°C at 60% humidity has the same physical effects as training in 62°C at 10% humidity.

The fluid we lose must be replaced. Sweat is mostly water, so water is what we need to rehydrate. Along with the water in sweat, we also lose electrolytes and other nutrients. Electrolytes are salts of all kinds dissolved in the water in our bodies. We need to replace the electrolytes and nutrients lost in sweat during heavy exercise. Magnesium is the electrolyte lost in sweat that has most effect on muscle performance.

How do we know when we have lost 2% of our body fluid? Most fluid loss is water which conveniently weighs 1Kg per litre. So if you weigh 80Kg before exercise and 78Kg after exercise, you have lost 2 litres of water. To avoid the effects of dehydration, you would need to drink an extra 2 litres of water during and immediately after exercise. You need to record your pre and post-exercise weight to make sure you adequately rehydrate, especially if you are training hard in conditions of higher temperature and or humidity. Aim to maintain normal hydration throughout exercise, this will minimise exercise-induced physiological stress. Bottom Line, you should not be thirsty before a workout and you should drink enough to weigh the same before and after a workout.

Really important stuff
Please note that if you are losing more than 2Kg in a workout, you may well be overtraining or you are taking on an advanced training schedule that really requires close supervision and monitoring by qualified exercise physiologists.

Most sports drinks contain water, sugar (usually glucose) and common table salt. This does help absorb water but it also adds calories and may lack other essential nutrients. Our recommendation is that you rehydrate with water mixed with an electrolyte and nutrient supplement that has sufficient magnesium in a bioavailable form. The wrong form of magnesium will give you diarrhoea, and diarrhoea will dehydrate you very rapidly. The best rehydration supplements currently available are Vitascience Bio-Enhanced Magnesium Powder or UltraMuscleze. These are mixed as follows: One teaspoon of powder (6 grams) to 1 litre of cold water. Both of these supplements effectively replace the electrolytes and nutrients lost by sweating.

For optimum benefit you should take, but NOT EXCEED the maximum daily intake of 2 teaspoons of hydration supplement in 2 litres of cold water. Taking other supplements containing selenium, chromium or magnesium in other products in addition to these hydration supplements will result in overdose. Optimum is good, more is toxic not better.

Cool, or better still, chilled water helps to reduce the increased body heat generated by exercise. Keep your rehydrating mixture in a chilled water container with some ice and/or keep it in a fridge.

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