The Science of Hydration

Sweat rates and electrolyte losses vary dramatically between individuals.

Up to 15 fold in fact.1.2.

So, a 'one-size-fits-all' approach to hydration just doesn't cut it.

But, just why is staying hydrated so important?


Your blood has multiple roles to play when you’re exercising hard…

  1. It has to be directed to active muscles to supply them with oxygen and energy; and to remove toxic byproducts that would otherwise accumulate.

  2. More of it than usual has to be sent to the skin to help dissipate heat and keep your core body temperature in check.

  3. You need to sweat to increase the effectiveness of this heat transfer from your skin to the environment, and sweat is drawn directly from your blood plasma.

Dehydration increases the stress your body is under during these processes so hampers performance.3 That’s why mitigating large losses in body fluid and electrolyte during exercise is critical if you want to keep performing for hours on end. This is especially true in hot conditions, where heat dissipation is more difficult and fluid/sodium losses can dramatically increases.

The Science of Hydration

It's harder to maintain a balance in a sun-baked desert than it is in rainy NYC during winter.

Sounds obvious, but it's so important.


How worried should I be about being dehydrated?

Whilst hydration is clearly important for maintaining performance, there are a few things to bear in mind…

1.  Getting a bit dehydrated is inevitable for most people when they push themselves during physical activity. Your sweat rate can be as high as 2-4 litres per hour, whereas your maximal fluid absorption rate is likely to be only 800ml-1 litre per hour. You don’t need a maths degree to realise you’re likely to be fighting a losing battle when it comes to fluid loss.

2.  A certain degree of acute dehydration is not necessarily as bad as you might think. Whilst some widely reported studies in the past suggested that losing as little as 2% bodyweight via dehydration can hamper performance significantly, more recent analysis of athletes in competition settings has shown that many can continue to perform well at 4-6% dehydration, with some elite athletes (such as marathoner Haile Gebrselassie) finishing races in world class times whilst nearly 10% dehydrated! (Read here)

This is not to say that dehydration won’t derail you if you don’t pay attention to the risk, it absolutely has the potential to do so. Just that, when mild dehydration occurs acutely during exercise it might not be quite as detrimental to performance as was once thought, assuming you started exercising normally hydrated in the first place of course.

So, should I just drink as much as possible before, during and after exercise?

NO! Forcing down large amounts of fluid even if you really don’t feel like it is not going to help and it can even be dangerous. But, how much is enough?

It’s reassuring to remember that you have a pretty powerful, innate ally in the fight against dehydration; your thirst instinct. If you pay attention to your body’s own signals and respond to the early signs of thirst you’re unlikely to get horribly dehydrated in a hurry.

The reason to avoid mindless drinking is that excessive fluid consumption can have some pretty nasty effects on your performance and health. You may well have heard of hyponatremia4 as the condition has got more and more press coverage in the last few years. The word literally means “low (‘hypo’) blood sodium (‘natremia’)” and most often results from consumption of fluids to an extent that you actually dilute the concentration of sodium in your blood stream to dangerously low levels.

Sodium is an electrolyte critical for a variety of functions in the body, including maintenance of:

  • Cell membrane potentials
  • Nerve impulses
  • Muscle contractions
  • Fluid balance

If you drink far more than you need (and especially without replacing sodium adequately through the foods you eat, or the fluids you take in) it can have very serious side effects. In an effort to maintain blood sodium levels in the face of water overload, the body shifts fluid from the blood stream into its own cells, causing them to swell up.

At first this might just result in some slightly swollen fingers, ankles and a general feeling of malaise and lethargy. But, if allowed to progress, can also result in swelling of the brain, headaches, coma and even death in extreme cases.

Taking in adequate sodium along with the fluids you consume is helpful in maintaining sodium levels in your blood stream.5

The key is to…

  1. Keep your fluid levels reasonably topped up during exercise, whilst…
  2. Being realistic that a good proportion of your drinking will have to be done before and after bouts of intense activity.

What about electrolytes?

  • Electrolytes are a big part of the hydration equation simply because you can lose a lot of them in your sweat.
  • The main electrolyte you lose in your sweat is sodium. It accounts for around 90% of the ions lost because it’s prevalent in your blood plasma, the pool from which sweat is drawn.
  • It is true that you do also lose some calcium, magnesium and potassium, but in relatively trivial amounts, so sodium replacement should be your main focus when it comes to electrolyte replenishment.
  • Sodium replacement helps you maintain blood volume, which in turn helps with management of your core temperature, delivery of blood to working muscles and the skin and your overall performance.5
  • Because you lose a relatively large amount of sodium in your sweat, when you’re going at it hard for several hours at a time total sodium loss can be really high. Therefore an appropriate level of supplementation is required alongside the fluids you consume in order to keep the body balanced. 6
  • The interesting thing about sodium loss in sweat is that it varies dramatically from person to person, with some people losing as little as 0.2g per litre of sweat and others losing near 2.0g per litre!** This means that different athletes need to take in very different amounts of sodium when sweat output is high. This variance in sweat sodium loss is largely down to genetics.**7 Because of the huge variability that exists in individual losses Whatever the cause, it’s clear that a ‘one-size-fits-all’ approach to electrolyte supplementation doesn’t work.

**Our own data on over 2500 athletes sweat tested. Published data elsewhere suggests similar ranges in non-athletic populations

How can I maintain electrolyte levels during exercise?

You can replace the sodium you lose in a variety of ways…

  1. Through food. More and more sports specific gels, bars and chews are incorporating sodium into their formulations now so check the labels of what you use for info on that. If you're doing longer, ultra distance events you can also probably snack on salty 'real foods' foods rather than just surviving on sweet stuff as well.
  2. Using hydration tabs or capsules with your water. The advantage of this approach is that you can more accurately measure the amount of sodium you’re taking in, meaning you can tailor your intake to your personal needs and experience. Basing your intake on the levels that have worked for you in key training sessions and build up events is the most effective way to dial in a strategy that works for you. 

What about hydration and electrolyte intake before and after exercise?

Pre-loading with sodium rich fluids (such as H2Pro Hydrate 1500) has been shown to be a good idea before strenuous or prolonged exercise, especially in the heat. It works by expanding blood plasma volume before you get started thereby increasing the pool of blood available to pump round the body and to be excreted as sweat, assisting with temperature regulation without compromising cardiovascular function. 8, 9

Post exercise, when you’re trying to recover and potentially playing catch up from some mild dehydration adding sodium to drinks is proven to increase the volume of water you can hold onto, without peeing it out.10 This helps restore normal fluid balance more quickly so you’re ready to go again the next day, or for the next session.

All H2Pro Hydrate tablets and Sweat Salt Capsules contain close to zero calories. They're specifically designed to provide electrolytes and assist with fluid replacement, not deliver a source of carbohydrate energy. This allows you to fine tune intake to your needs, without adding unwanted calories.

Take our Sweat Test to get your plan now!

If you have any questions about hydration, drop us an email. 


  1. Godek at al. Sweat Rates, Sweat Sodium Concentrations, and Sodium Losses in 3 Groups of Professional Football Players. J Athl Train. 2010 Jul-Aug; 45(4): 364–371
  2. Maughan RJ et al. Fluid and electrolyte intake and loss in elite soccer players during training. Int J Sport Nutr Exerc Metab. 2004; 14:333-46
  3. Barr SI. Effects of dehydration on exercise performance. Can J Appl Physiol. 1999 Apr;24(2):164-72
  4. Hew-Butler,T.D., PhD, Hew-Butler,T.D., PhD, Fowkes-Godek,S.P., ATC, Dugas,J.P.P., Hoffman,M.D.M., et al (July 2015) 'Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015', Clinical Journal of Sport Medicine:, Volume 25 (Issue 4), pp. p 303–320. 
  5. Del Coso, J. et al. Effects of oral salt supplementation on physical performance during a half-ironman: A randomized controlled trial. Scand J Med Sci Sports. 2015 Feb 14. doi: 10.1111/sms.12427
  6. Rehrer NJ. Fluid and electrolyte balance in ultra-endurance sport. Sports Med. 2001;31(10):701-15
  7. Beth Brown et al. Low abundance of sweat duct Cl− channel CFTR in both healthy and cystic fibrosis athletes with exceptionally salty sweat during exercise. American Journal of Physiology - Regulatory, Integrative and Comparative Physiology 1 March 2011 Vol. 300 no. 3, R605-R615
  8. Sims ST, van Vilet L, Cotter JD, Rehrer NJ. Sodium loading aids fluid balance and reduces physiological strain of trained men exercising in the heat. Med Sci Sports Exerc 39: 123–130, 2007
  9. Berger NJA, Campbell IT, Wilkerson DP, Jones AM. Influence of acute plasma volume expansion on V̇O2 kinetics, V̇O2 peak, and performance during high-intensity cycle exercise. J Appl Physiol 101: 707–714, 2006.
  10. Maughan RJ and Leiper JB. Sodium intake and post-exercise rehydration in man. Eur J Appl Physiol Occup Physiol. 1995;71(4):311-9