What can athletes learn from astronauts about hydration?

By Andy Blow | 4 Minute Read

A good bit of what we know about salt and fluid retention in the body comes from work that scientists at NASA did towards the end of the last century.

NASA regularly observed their astronauts having all sorts of trouble with low blood pressure when they came back down into the earth’s gravitational field after completing missions in space. 

When they looked at their performance in different phases of a mission it was noted that astronauts were commonly found to be suffering with low blood pressure because they were losing bodily fluids (and therefore blood volume) during their time in microgravity. One NASA paper I read suggested that astronauts live with as much as a 3-4% deficit in total body fluid levels during a typical mission. 

Whilst this level of fluid loss seemed to be tolerable to them whilst they remained in a weightless environment, it often proved very problematic as they returned to earth. It was causing them to feel weak, light headed and even to black out on re-entry or once they landed back on terra firma. That's not something you want to be dealing with when you’re trying to land a rather expensive space craft! 

So NASA focused considerable attention on figuring out what kind of drinks they could use with astronauts to boost their hydration status as aggressively as possible just before re-entry to earth’s atmosphere, in order to help combat the low blood pressure issues. 

They tested a lot of permutations of drinks containing carbohydrates and electrolyte mixtures (as well a few other additives) and in essence found that the more sodium you put in a drink, the more effective the drink would be at being retained in the body and bloodstream and correcting dehydration.

This was not a complete surprise as it's been known for a long time that sodium is a key component of extra cellular fluid (ECF) in the body and that the fastest way to replace ECF lost through sweating was with a saline drip or Oral Rehydration Solution (ORS), a salt/sugar mixture used to combat dehydration during cholera outbreaks in the developing world from the late 1960s onwards.

But what the NASA work did was to put some more solid numbers behind these assumptions and show that, when it came to keeping ingested fluid in the body, sodium was the key ingredient.




So, what does this mean for athletes?

Drinking a strong electrolyte drink to optimise your hydration status before long, hot or really hard training sessions and events can significantly improve your performance.

We call this "preloading" and doing it effectively is all about striking a balance between being aggressive enough to drive some extra fluid retention in your blood stream without this leading to gastro-intestinal issues or excessive fluid build-up making you feel bloated and sluggish.

Typical sports drinks - which generally contain ~200 to 500mg of sodium per litre - simply don’t cut it when it comes to preloading as they're just way too dilute to make a meaningful difference to blood volume. The reality is it’s not vastly different from drinking water.

At the other extreme, most of the scientific studies that have been conducted in this area have looked at using extremely strong electrolyte drinks containing ~3,600mg of sodium per litre. That's like drinking a bag of saline solution that would normally be put into you via an IV!

Whilst this has been shown to be highly effective at boosting blood plasma volume, it’s has a tendency to cause upset stomachs, sickness or diarrhoea - something that is obviously very counterproductive when you're trying to improve your performance!

In the end we settled on a strength of 1,500mg/l (32oz) for our preloading drinks. 1,500mg/l seemed to be the ‘sweet spot’ in that it's very palatable and easy on the gut (we never get comments about stomach upsets from using it) whilst still being effective at boosting your blood plasma volumes and getting you optimally hydrated before your start sweating.

If you want to test whether preloading improves your performance, follow this protocol before your next long/intense training session or B-race.

What to do

  • Drink 1 x PH 1500 with 500ml (16oz) of water the evening before tough, sweaty activities.
  • Drink another 1 x PH 1500 with 500ml (16oz) of water about 90 minutes before you start.
  • Finish your drink at least 45 minutes before you start to give your body time to fully absorb what it needs and pee out any excess.
  • Drink the 1500 in water you’d have drank anyway to ensure you don’t overdo it.
  • DON’T just drink lots of water in the build-up to an event. You can end up diluting your body’s sodium levels before you start, increasing the risk of hyponatremia.


  • Boosting your blood plasma volume before intense exercise is a proven way to enhance your performance, especially in hot conditions.
  • Having more blood makes it easier for your cardiovascular system to meet the competing demands of cooling you down and delivering oxygen to your muscles.
  • 1,500mg/l electrolyte drinks are very effective at increasing your plasma volume as they contain 3x more sodium than a typical sports drink.
  • That extra sodium helps to pull water into your bloodstream and keep it there. This may allow you to get away with drinking considerably less in shorter/harder events where previously they would have had to try to consume more on the move (not easy when you’re flat out!). It can also help reduce the amount of times you need to pee before you start.
  • Preloading with PH 1500 can also help you avoid/alleviate muscle cramps, especially if you’re prone to suffering from them late on in events and especially when it’s hot. 89% of athletes with cramp who try preloading PH 1500 say that it solves their problems.
  • You can’t preload anywhere near as effectively with weaker sports drinks as you’ll lose a large proportion of the fluid as urine. Or it’ll slosh around in your stomach without being properly absorbed. 

If you’re looking for a way to optimise your performance then testing sodium preloading is definitely worth a try. If you have any questions about how to preload effectively or need help optimising your approach, drop us an email.

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