Rawstone Learn · The Science

The Science of Salt and Hydration

Why the amount of sodium in your glass, not the number printed on the pack, is what decides how much water your body actually keeps.

Rawstone Learn · general education, not medical advice

Plain water is easy to drink and easy to lose. A lot of it passes straight through and out again before it does much for you. The mineral that changes this is sodium. Sodium is the signal that tells your body to hold on to the fluid you have just taken in, and it is the reason a well-built drink can keep you topped up in a way that water alone struggles to.

This is the idea most drink labels skip past, so it is worth slowing down on. When researchers measure how much fluid is still in the body a few hours after people drink, the biggest single factor is not the sugar or the extras a brand likes to advertise. It is the electrolyte content, and mainly the sodium. In one controlled study a milligram of sodium did more for fluid retention than a gram of carbohydrate.

What you lose when you sweat

Sweat is not just water. It carries salt out with it, which is the reason your skin tastes salty after a hard session and why a sweat-soaked shirt can dry with white marks. When you replace only the water and none of the salt, you are refilling half of what left. Sodium is the part of sweat worth thinking about most, because it is the part that governs how well the fluid you drink afterwards stays put.

Concentration beats milligrams

Here is the part that matters for every electrolyte product on the shelf. The milligrams on the pack do not mean much on their own. What counts is the concentration in your glass: how much sodium there is per litre of water once it is mixed. That is what your tongue tastes, and it is what your gut works with.

The research points to a threshold. Below roughly 500mg of sodium per litre, the fluid-holding effect is weak. Many everyday sports drinks sit under that line, which is a handy thing to know when you read a label. Above it, the benefit climbs as the sodium climbs, with the useful working range sitting around 900 to 1,400mg of sodium per litre. That is the band Rawstone is built around.

The Otago heat trial

One of the clearest studies on this comes from the University of Otago in New Zealand. Researchers ran a double-blind, crossover trial: eleven people cycled for three hours in the heat, around 34 degrees, drinking either a higher-sodium drink (about 1,380mg per litre) or a lower-sodium one. The higher-sodium drink held their plasma volume steady, which is the amount of fluid in the liquid part of the blood, a good marker of staying hydrated. On the lower-sodium drink, plasma volume dropped.

We want to be honest about where this evidence is strong and where it is not. The high-sodium advantage shows up most clearly while you are actively sweating in the heat. In a separate study on people who were already dehydrated before they started drinking, a high-sodium rehydration solution and a lower-sodium sports drink both beat plain water, but they came out roughly equal to each other. So sodium's edge is real, and it is biggest during and around heavy sweating, rather than in every situation.

Potassium, and the right ratio

Sodium does the heavy lifting. Potassium works alongside it and rounds out the minerals you lose in sweat. The well-established rehydration formulas run a sodium-to-potassium ratio of roughly 3 to 1 or 4 to 1, and that is where Rawstone sits, with 1000mg of sodium to 250mg of potassium.

The form matters too. We use potassium citrate rather than potassium chloride. Chloride is the cheaper option and it tends to taste metallic, which is a large part of why some electrolyte drinks have that off note. Citrate is gentler on the tongue.

Magnesium, honestly dosed

Magnesium is often linked with muscle cramps, so it is worth knowing what the evidence actually shows before you count on it. We looked hard at it, and the honest answer is that the clinical support for magnesium preventing cramps is not strong. Large independent reviews of the trials have found it does not deliver a meaningful effect, and there are no randomised trials at all on exercise-related cramps.

So we do not build the pitch on it. Rawstone includes 75mg of magnesium, a source of magnesium that contributes to your daily intake, because it is a mineral you lose in sweat and worth replacing, not because we are going to promise you something the science cannot back. It is always worth asking to see the studies behind any big claim like that.

What "premium mineral forms" actually means

Minerals come in different chemical forms, and the form changes how much your body can take up. The magnesium in cheaper tubs is often magnesium oxide, which absorption studies place well behind magnesium citrate. Rawstone uses the citrate forms for both magnesium and potassium. It costs more to make. The payoff is a drink your body absorbs more readily and one that tastes clean instead of metallic. When you see a brand talk about premium mineral forms, that is what it comes down to: better uptake and better taste, from a more expensive raw ingredient.

How to drink it

Stir one serve into 750ml to 1 litre of water and adjust to taste. Lighter, more water. Training hard in the heat and want it stronger, use less. Every batch will be independently lab-tested, and the full mineral amounts are printed on the pack so you always know the concentration you are drinking.

This article is general education about hydration physiology, not medical advice. Rawstone is a food supplement, not a medicine, and is not intended to diagnose, treat, cure or prevent any disease. If you have a condition that means you need to manage your sodium intake, talk to your doctor before using any electrolyte supplement.