Hydrating for endurance exercise

Hydrating for endurance exercise

September 20, 2019

Hydrating for endurance exercise

by our Sports Scientist Stephen Morehen

Although it isn’t a lab-developed, glossy-packaged sports supplement, good old H2O is nevertheless critical for performance. Water is used in digestion (gastric secretions), waste disposal (urine), heat dissipation (sweat) and nutrient transportation to and from cells (blood). Without sufficient levels of water, the everyday functions of your body will be compromised, fatigue will begin to develop, and performance will ultimately decline [1].

Although this summer may not have been the it’s-coming-home heatwave of last year, if you’ve exercised outside over the summer months you have probably noticed an increase in the amount you’ve sweated. The average person can lose 0.8-1.4L of fluid per hour of exercise through sweat (and even more lost as water vapour in your breath). Dehydration in exercise is commonly defined as losing >2% of your bodyweight in fluids (e.g. an 80kg person losing over 1.6L) without replacement of those fluids [2]. Your sweat rate is also greatly affected by other factors such as clothing, temperature, humidity, body mass, wind and peripheral blood flow [3]. The best starting point to prevent the onset of dehydration is to ensure that you are well hydrated before you even begin a workout.

The maximum gastric-emptying rate of your stomach is around 1000-1300ml/hour [4]. In simple terms, this is how much fluid your stomach can effectively deal with per hour. Exceeding this will result in excess fluid sloshing around in your stomach and can lead to gastrointestinal discomfort. You should be able to handle workouts of up to an hour without a strict hydration strategy, providing you are well hydrated to begin with. For workouts lasting beyond the sixty-minute mark, consider implementing a hydration routine to prevent premature fatigue. A meta-analysis by Holland et al (2017) concluded that for moderate continuous exercise lasting 1-2hrs, aiming to consume 0.15-0.2ml/kg body mass/minute can see performance improvements of 2% [5]. If you are liable to forgetting when to drink you should consider setting alarms on your watch or designating landmarks along the route as “drink points”.  Temperature will affect how often you will need to drink too. You will find yourself drinking more on hot days and less on cold ones. Listen to your body, if you feel thirsty then take a drink – simple!

When it comes to rehydrating during exercise, you generally have 3 options…

  1. Water – whilst water is great for hydrating throughout the day, it may not be adequate for intense or long duration exercise. This is because exercise causes sweating to increase. Electrolytes such as potassium, sodium, magnesium and calcium are lost in sweat, with sodium being the most important of these. If you replace lost fluids with water then the remaining sodium in the body can become very diluted, this can result in hyponatremia (hypo = low, natremia = presence of sodium) which in turn may cause headaches, loss of cognitive function and nausea.
  2. Electrolytes (drink or effervescent tablet) – these provide you with electrolytes usually in an isotonic concentration. This means they are delivered in the same concentration as they are found in the body and are therefore absorbed quickly and efficiently. Restoring the electrolytic balance within your cells aids fluid retention and subsequent hydration.
  3. Carbohydrate drinks – these drinks will have high levels of carbohydrate in them allowing you to top up your blood glucose without having to eat anything, as eating food during an event may not be desirable or possible for everyone. It should be noted that your stomach can only absorb 60g of glucose per hour and so consuming carbohydrate drinks alongside bars and gels is essentially a waste as no more than 60g of the good stuff will be absorbed. To counter this, some carbohydrate products will also contain fructose as this is transported in a different pathway to glucose, allowing you to absorb up to 90g of carbohydrate an hour [6]. Many sport specific drinks will combine carbohydrates with electrolytes and sometimes even caffeine to deliver everything you need in one beverage.

Everyone is different, and it is worth experimenting with what hydration strategy works best for you. Just ensure you are replacing lost electrolytes during workouts longer than 60 minutes to offset premature fatigue.

References

  1. Trangmar, S.J.; Gonzalez-Alonso, J. Heat, Hydration and the Human Brain, Heart and Skeletal Muscles. Sports Med 2019, 49, 69-85, doi:10.1007/s40279-018-1033-y.
  2. Cheuvront, S.N.; Kenefick, R.W. Dehydration: physiology, assessment, and performance effects. Compr Physiol 2014, 4, 257-285, doi:10.1002/cphy.c130017.
  3. Baker, L.B. Sweating Rate and Sweat Sodium Concentration in Athletes: A Review of Methodology and Intra/Interindividual Variability. Sports Med 2017, 47, 111-128, doi:10.1007/s40279-017-0691-5.
  4. Leiper, J.B. Fate of ingested fluids: factors affecting gastric emptying and intestinal absorption of beverages in humans. Nutr Rev 2015, 73 Suppl 2, 57-72, doi:10.1093/nutrit/nuv032.
  5. Holland, J.J.; Skinner, T.L.; Irwin, C.G.; Leveritt, M.D.; Goulet, E.D.B. The Influence of Drinking Fluid on Endurance Cycling Performance: A Meta-Analysis. Sports Med 2017, 47, 2269-2284, doi:10.1007/s40279-017-0739-6.
  6. Rowlands, D.S.; Houltham, S.; Musa-Veloso, K.; Brown, F.; Paulionis, L.; Bailey, D. Fructose-Glucose Composite Carbohydrates and Endurance Performance: Critical Review and Future Perspectives. Sports Med 2015, 45, 1561-1576, doi:10.1007/s40279-015-0381-0.

 

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