Keto & low-carb

How much sodium do you lose on keto?

Cutting carbs quietly changes how your kidneys handle salt, and the flat, foggy feeling people call keto flu is often just sodium that hasn't been replaced.

Rawstone Learn · general education, not medical advice

Most people start a keto or low-carb diet thinking about food. What often catches them out is a mineral: sodium, the main part of ordinary table salt. In the first week or two, a lot of people feel flat, foggy, headachy or low on energy. That cluster of feelings has picked up a nickname, keto flu. A big part of it comes down to something simple that nobody warns you about, which is that keto changes how much sodium your body holds on to.

The insulin and kidney link, in plain terms

Here is the chain, step by step, with no jargon left unexplained.

When you eat carbohydrates, your body releases a hormone called insulin to help manage the sugar in your blood. Insulin does a second, quieter job as well: it tells your kidneys to keep hold of sodium rather than letting it go in your urine.

On keto you eat very few carbs, so your insulin stays low. With less insulin giving that hold-on-to-salt signal, your kidneys do the opposite. They let more sodium pass out in your urine. Sodium pulls water along with it, which is why the scales often drop several kilos in the first week of low-carb eating. That early loss is mostly water and salt leaving together, not fat.

Why the keto-flu feeling and low sodium overlap

Sodium is not just seasoning. Your body uses it to hold on to fluid, to keep your blood volume steady, and to help your nerves and muscles fire properly. When you are quietly losing more of it than you are taking in, the everyday result can feel like the keto-flu list: tiredness, a dull head, light-headedness when you stand up, and workouts that feel harder than the numbers say they should.

It is worth being honest about what this is and is not. Under-replaced sodium is one common reason people feel rough in early keto, not the only one, and this is general physiology rather than a diagnosis. The useful takeaway is straightforward. If your body is flushing out more salt, the sensible response is to put back what you are losing, the same way you would drink more water on a hot day.

How much are we actually talking about?

There is no single magic number, and the amount rises the more you sweat and the hotter it is. Many low-carb guides suggest adding a few grams of salt across the day, on top of what is already in your food, especially in the first couple of weeks while your body adjusts. Some of that can come from simply salting your meals more generously. The rest is where a measured electrolyte powder is handy, because you can see exactly what you are getting instead of guessing.

This is also where concentration matters, not just the milligrams on the label. Sodium only does its fluid-holding job well once there is enough of it in the water you drink. Research on how much fluid a drink helps you keep points to a working range of roughly 900 to 1,400mg of sodium per litre of water, with the effect being weak below about 500mg per litre. It is worth checking the label against that range, as many everyday sports drinks sit under the lower line and behave more like flavoured water. We unpack that threshold science in full in the science of salt and hydration.

Where Rawstone fits

Rawstone is a sugar-free hydration powder. It carries 1000mg of sodium per serve, alongside 250mg of potassium and 75mg of magnesium, with zero sugar and under half a gram of carbohydrate per serve. That matters on keto, because many mainstream electrolyte and sports drinks carry several grams of sugar or other carbohydrate a serve, which is exactly what a low-carb eater is trying to avoid. The quick way to tell is the nutrition panel: check the carbs and sugars per serve.

How you use it is up to your day. Stir one full serve into 750ml to 1 litre of water and it lands in that well-evidenced hydration range while staying pleasant to drink. For a normal, cooler day, half a serve in a regular glass is a fine everyday top-up. Training hard or sweating in the heat is when the full serve earns its place. We use potassium and magnesium as citrate, which dissolves easily and tastes cleaner than the chloride forms, which can carry a metallic edge.

Think of it as a clean, measured way to get a full 1000mg of sodium in one glass, with zero sugar and a smooth Reb M sweetness, so you can see exactly what you are putting back once you cut the carbs.

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.