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When we’re dehydrated, our blood volume will decrease which in turn will cause a decrease in blood flow to your muscles. This can then result in higher body temperature, a reduced sweat rate, increased muscle glycogen use, increased perception of effort and a higher heart rate.

All of these factors on their own or combined can contribute to reduced performance as well as concentration. A reduction as little as 2% of your body weight due to sweat loss can cause dehydration. Therefore, it is vital you begin training or your race in a hydrated state and try to remain hydrated throughout.

If we can calculate our sweat rate alongside the duration of exercise you can then predict your fluid requirements. Why have an individualised approach? Well because as humans, we all sweat differently due to genetics, gender, training status and acclimatization and on top of this, external factors will also have an impact. For example, external temperature, humidity, wind and even your clothing will also impact sweat rate.


Before exercise:

Around 2 hours before exercise, aim to drink 6 to 8 ml of fluid per kg of body weight. This will allow time for absorption and removal of any excess. It is also beneficial to have sodium (electrolytes discussed below) before exercise as this will stimulate thirst whilst also promoting the retention of fluids.

During exercise:

During exercise, aim to drink often to prevent dehydration, ideally trying to avoid reductions in baseline body weight greater than 2%. How much to drink depends on your individual sweat rate.

How to calculate sweat rate

The simplest way to measure sweat rate is to weigh yourself before and after exercise without consuming any food/water. This way any difference in weight is primarily due to sweat loss. Though most of the time its rare we don’t consume any food or fluid during exercise. So how do I measure sweat rate whilst also taking into account those factors?

  1. Empty your bladder and weigh yourself (nude or minimal clothing but make sure it’s the same after) and make a note of your weight so you don’t forget it
  2. Do your training session – make a note of the conditions and/or exercise intensity as well as how much you drank
  3. Weigh yourself after exercise in the same conditions you did in step 1
  4. Calculate ‘weight loss’: Weight before – weight after = weight loss
  5. Calculate ‘total sweat loss’: Weight loss + intake volume = total sweat loss
  6. Finally, calculate sweat rate: Total sweat loss / exercise time = sweat rate

This will give you your sweat rate for the given session you did but keep in mind that external conditions and intensity of exercise will also affect your sweat rate. It could be useful to repeat the test in specific conditions if training for an event where you know the external factors will be different to your normal training environment.


After exercise:

With your sweat rate calculated, you can try and rehydrate effectively but going to the bathroom will also affect your weight. You can try and estimate this in losses as well but understandably not as easy to measure exact amounts.

Therefore, it’s been recommended that after exercise you should aim to consume 1.5 L for each kg of body weight lost as this will take into account the fluid that will be naturally lost from the body via urine. However, this does not need to be consumed all in one go, a proactive approach is to aim for 500ml immediately post-exercise and then at regular intervals after that.


What are Electrolytes and why do we need them?

As well as thinking about our sweat rate and overall fluid loss, it’s also important to consider electrolytes. Most of what our bodies require to function comes from small electrical charges that are sent around the body. These electrical signals contribute to hydration as well as many other factors such as muscle contraction and nerve function.

Our bodies need to maintain electrolytes so it can perform its variety of functions effectively. Vital electrolytes that need topping up regularly include sodium, calcium, potassium, bicarbonate, chloride, phosphate and magnesium. When we sweat during exercise, we predominately lose sodium and potassium, so these are important to think about when trying to stay hydrated.

If our body is low in electrolytes, it can cause fatigue, headaches and dehydration. Long-term, if your body has consistently low levels of electrolytes there can lead to more serious consequences. Think about our bodies as an electric battery, if power is low it needs to be recharged, this same concept can apply to the human body, if our electrolytes are low, our ‘battery’ needs recharging.

How can you replace lost electrolytes?

There are plenty of good food sources that provide electrolytes such as potatoes (phosphorous and magnesium), leafy greens such as kale or spinach (calcium and magnesium) or coconut milk (sodium, potassium, calcium and magnesium) to name just a few examples!

At Science in Sport, we want you to be able to replace electrolytes easily on the go during exercise. Our two main products to aid with hydration are rich in electrolytes!

SiS Hydro

Hydro is an effervescent tablet that dissolves in water, it was first developed to keep the British Sailing team optimally hydrated at the 2008 Beijing Olympics. Each tablet contains:

  • Chloride: 269mg
  • Sodium: 345mg
  • Potassium: 65mg
  • Magnesium: 8.1mg
  • Calcium: 102mg

Directions for use: Dissolve one hyrdo tablet in 500ml of water and consume 3-4 hours prior to your sporting event. Then consume a further 250-500ml in the hour before starting. During shorter exercise, where carbohydrate intake is not necessary, consume 500ml of hydro per hour. Once prepared consume within 24 hours and do not consume more than 4 tablets per day.

A caffeine version which includes 75mg of caffeine, is ideal for those who want an extra boost!

SiS GO Electrolyte power

GO Electrolyte provides an electrolyte solution but with easily digestible carbohydrate. This can maintain endurance performance and protect against cramp. This product is mixed at appropriate concentrations of carbohydrate (36g as a 6% solution), sodium (20mmol/L) and fluid (500ml serving) in order to achieve the combined aims of energy and hydration.

Directions for use: drink 500ml every 24-60min of exercise and consume with 24 hours, simply add 40g of GO Electrolyte (2 scoops = 40g) to 500ml of water in a bottle and shake well.

Take home messages:

  1. Sweat loss as little as 2% of your body weight can have a detrimental impact on performance.
  2. Start training hydrated and try to calculate your individual sweat rate to optimally hydrate during your training.
  3. To optimise hydration replace electrolytes as well as fluid loss.

American College of Sport Medicine. 2009. ACSM position stand: Exercise and fluid replacement. Medicine and Science in Sport and Exercise 39 (2): 377 – 390.

Maughan R. J. and L. M. Burke. 2002. Sport nutrition. Handbook of sports medicine and sciences. Oxford: Black-well science.

Jeukendrup A. and Gleeson M. 2010. Sport nutrition. An Introduction to Energy Production and Performance. Human Kinetics.

Shirreffs, S. M., & Sawka, M. N. (2011). Fluid and electrolyte needs for training, competition, and recovery. Journal of sports sciences, 29(1), 39-46.

Written By

Dr Emily Jevons - PhD in Exercise Physiology & Nutrition, Clean Sport Advisor

Emily has worked with Science in Sport since 2021. With a PhD in Exercise Physiology & Nutrition, she currently provides nutritional advice for endurance athletes. Emily not only understands the science behind performance nutrition solutions, but also the physiological and psychological demands of sport after competing competitively in swimming and triathlon for a number of years.