When you fast or eat low-carb, your body holds on to less sodium and water, so you lose electrolytes more quickly - which is why many people on intermittent fasting or keto routines add electrolytes. Sodium, potassium and magnesium are the ones that matter most. A sugar-free, calorie-free electrolyte drink supports hydration without breaking a fast. Always listen to your body and check with a GP if you have a health condition.
Why fasting and low-carb change your electrolyte needs - the science
This is the part most people miss, and it is genuinely physiological. When you eat fewer carbohydrates or go without food for a while, your insulin levels fall - research indicates that on a ketogenic diet, daily insulin secretion can drop by more than half. Insulin normally has a sodium-conserving effect on the kidneys: it tells them to hold on to sodium. So when insulin falls, that signal eases, and studies indicate that your kidneys release more sodium and water than usual.
So it is not that fasting "uses up" electrolytes - it is that your body holds on to less of them, and you flush more out. On top of that, when you are not eating, you are also not getting the sodium, potassium and magnesium that food normally provides. The result is that people who fast or eat low-carb can run low on electrolytes faster than they expect, which is why electrolytes are such a common topic in fasting and keto circles.
The electrolytes that matter most when fasting
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Sodium. The big one. With lower insulin you excrete more sodium, and you are not salting food, so sodium is the electrolyte people most often need to top up when fasting or eating low-carb.
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Potassium. Works with sodium in fluid balance. It normally comes from fruit, vegetables, legumes and starchy foods - exactly the foods low-carb and keto diets cut back, so intake tends to drop.
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Magnesium. Involved in normal muscle function and energy metabolism. Its main dietary sources are green leafy vegetables, wholegrains, nuts, seeds and legumes - again, foods that are limited on low-carb, so it is easy to fall short.
Calcium and chloride matter too, but sodium, potassium and magnesium are the focus for most fasting and low-carb routines.
"Keto flu" and how some people feel
In the first days of a low-carb diet or during longer fasts, some people report feeling flat, low on energy, or getting headaches - commonly called "keto flu."
Topping up electrolytes - especially sodium - and drinking enough water is the usual way people support their hydration through that adjustment. To be clear, this is general information about hydration, not medical advice, and an electrolyte drink is not a treatment for any condition. If symptoms are severe or you feel unwell, see a doctor.
Will electrolytes break a fast?
This is the most-asked fasting question, so here is the honest version. A fast is defined by restricting food and calorie intake. Plain electrolytes - sodium, potassium and magnesium - have essentially no calories, so a sugar-free, zero-calorie electrolyte drink will not break a fast.
Where it gets nuanced: some electrolyte products add carbohydrates or sugar for flavour or fuel, and a drink with a meaningful number of calories would technically interrupt a true fast. If staying fasted matters to you, choose a sugar-free electrolyte with no added sugar. For most people doing intermittent fasting for general routine, a few flavouring calories are not a concern, but a zero-sugar option keeps it simple.
How much, and how to use them
There is no single number, and it is worth being honest about that: despite all the advice online, there is no solid evidence base for exact salt targets while fasting or on keto - not all recommendations have the required high level of evidence to support them (as pointed out in this study).
Be wary of sources quoting precise daily milligram figures. The practical approach most people use:
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Add a sugar-free electrolyte to water during your fasting window or across a low-carb day.
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Lead with sodium - it is the one most affected by fasting and low-carb eating.
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Drink to thirst rather than forcing large volumes of plain water, which can dilute your sodium further.
Follow the directions on your product, and if you have a heart, kidney or blood-pressure condition, or are on a sodium-restricted diet, talk to your healthcare professional first. This is a supplementary product and is not intended to diagnose, treat, cure, or prevent any disease.
Who this is for
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Intermittent fasting (16:8, OMAD and similar) where you go long stretches without food.
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Low-carb and keto routines, especially in the first couple of weeks of adapting.
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Anyone eating less than usual who wants to support their hydration through it.
Frequently Asked Questions
What electrolytes should you take while fasting?
Sodium, potassium and magnesium are the three that matter most when fasting. With lower insulin your body holds less sodium and water, and you are not getting electrolytes from food, so a sugar-free electrolyte that leads with sodium is the usual choice to support hydration.
Will electrolytes break a fast?
Plain electrolytes - sodium, potassium and magnesium - have essentially no calories and will not break a fast. A sugar-free, zero-calorie electrolyte drink is safe if you want to stay strictly fasted. Products with added sugar or carbohydrates add calories, so choose a sugar-free option if staying fasted matters to you.
Will a 20-calorie electrolyte drink break a fast?
A small number of calories is unlikely to matter for most people doing intermittent fasting for general routine, but technically any meaningful calories interrupt a true fast. If you want to be strict, choose a sugar-free electrolyte with no added sugar and essentially no calories.
Why do I need electrolytes on keto?
On a low-carb or keto diet, lower insulin makes your kidneys release more sodium and water, so you lose electrolytes faster - and you are eating fewer of the foods that normally provide them. Adding electrolytes, especially sodium, is how many people support hydration while adapting, and it is commonly linked to easing "keto flu" symptoms.
What salts do I need for fasting?
The key ones are sodium (the main focus), potassium and magnesium. A balanced sugar-free electrolyte blend usually covers all three, which is simpler than mixing individual salts. There is no reliable exact dose, so lead with sodium and drink to thirst.
Can I have electrolytes every day while fasting or on keto?
For most healthy adults, a sugar-free electrolyte as directed is fine for daily use alongside water. There is no need to overdo it. If you have a heart, kidney or blood-pressure condition, or are on a sodium-restricted diet, check with your healthcare professional first.
