RAWSTONE

The Chemistry of Electrolytes · Episode 01

How dehydrated is too dehydrated? The 2% line, explained

Feel flat, foggy or a bit headachy by mid-morning? It might not be your sleep. It might be water, and it takes a lot less than you would think.

Short answer. Lose about 2% of your body weight as water and studies show your attention, reaction time and short-term memory all drop. Mood can slip even earlier, at just over 1%. For an 80 kg person, 2% is about 1.6 litres of sweat: one hot morning of work or a hard session. And thirst usually shows up late, so you are often already there before you feel it.

Rawstone Learn · general education, not medical advice

What 2% does to your brain

Dehydration researchers usually measure water loss as a share of body weight, because weighing someone before and after is the most accurate way to track it. A 2012 review by Ana Adan pulled together the studies on how water loss affects the way we think. The pattern was consistent: once people had lost about 2% of their body weight as water, their attention, their reaction time and their short-term memory all got worse.

That is not a lot. It is a sweaty morning, not a desert crossing. And it is why the "line" for dehydration sits much closer than most people expect.

Even less hits your mood

A 2012 study led by Lawrence Armstrong looked at young women who were mildly dehydrated, by about 1.36% of their body weight on average. They were not near the 2% line. Even so, they rated their mood as worse, felt more tired, found it harder to concentrate, and reported more headaches than when they were well hydrated.

If you have ever felt flat and short-tempered by the afternoon of a hot day without knowing why, this is one likely reason.

Why it happens

Sweat does not come from nowhere. It is drawn from the fluid in and around your blood vessels, so as you sweat, the volume of fluid in your blood goes down. With less fluid to move, your heart has to beat faster to keep up.

Your brain feels it too. In a 2011 study, Matthew Kempton's team scanned teenagers' brains after they sweated for 90 minutes. They scored about the same on a thinking task as before, but the scans showed their brains working noticeably harder to get there. Same result, more effort. That extra effort is a good description of what "foggy" feels like.

You get there fast

Working or training in the heat, many people sweat a litre or more an hour. Here is what 2% looks like for a few body weights:

On a Queensland summer morning on a roof, in a hot gym or on a long run, that can happen before smoko.

Thirst comes late

Thirst is a useful signal, but it is not an early warning. It tends to switch on once you have already lost about 1 to 2% of your body weight. In other words, by the time you notice you are thirsty, you are often already at the level where the studies above start to see changes.

Two quick checks: look at your urine (pale straw is good, dark yellow means you are behind), and if you train or work in the heat, weigh yourself before and after. Each kilogram you lose is roughly a litre of sweat.

Who should think about this

So just drink more water?

Water is the start, but it is not the whole story. Sweat is salty, and replacing it with plain water alone can leave you short on sodium. That is the next part of the series: why water alone isn't enough. If you are wondering how much sodium is too much, see is 1000mg of sodium too much?

Where Rawstone fits

Rawstone is an Australian-owned, sugar-free hydration powder: 1000mg sodium, 250mg potassium and 75mg magnesium per 7g serve, mixed into 750ml of water, one serve a day. Launching on Amazon Australia this summer.

See the product

Or join the launch list for a launch-day discount code.

How much dehydration affects your focus?
Studies put the line at about 2% of your body weight lost as water. At that level, attention, reaction time and short-term memory all drop. Mood can dip earlier: in a 2012 study, women who lost about 1.4% felt worse, found it harder to concentrate and got more headaches.
How much sweat is 2% of your body weight?
Multiply your weight by 0.02. For an 80 kg person that is 1.6 kg, or about 1.6 litres of sweat. For a 60 kg person it is 1.2 litres. Working or training in the heat, you can sweat a litre or more an hour.
Will I feel thirsty before I am dehydrated?
Usually not. Thirst tends to show up late, often once you have already lost 1 to 2% of your body weight. So by the time you feel thirsty, you are often already at the level where focus and mood start to slip.
How can I tell if I am dehydrated?
Two simple checks: pale straw-coloured urine is a good sign, dark yellow is not. And if you weigh yourself before and after training or a hot shift, each kilogram lost is roughly a litre of sweat.

Sources

  1. Adan A. Cognitive performance and dehydration. Journal of the American College of Nutrition. 2012;31(2):71–78.
  2. Armstrong LE, Ganio MS, Casa DJ, et al. Mild dehydration affects mood in healthy young women. Journal of Nutrition. 2012;142(2):382–388.
  3. Kempton MJ, Ettinger U, Foster R, et al. Dehydration affects brain structure and function in healthy adolescents. Human Brain Mapping. 2011;32(1):71–79.
  4. Sawka MN, Burke LM, Eichner ER, et al. American College of Sports Medicine position stand: exercise and fluid replacement. Medicine & Science in Sports & Exercise. 2007;39(2):377–390.

By Tom Bolger, Rawstone founder. Tom works in beverage manufacturing and trains in the gym most days. He started Rawstone because he wanted a full-strength hydration powder he would actually drink every day.

This article is general education about hydration, not medical advice. Rawstone is a formulated supplementary sports food, not a medicine, and is not intended to diagnose, treat, cure or prevent any disease. Consume no more than one serve a day. If you have a condition that means you need to manage your sodium or fluid intake, talk to your doctor before using any high-sodium product.