Electrolytes While Fasting: What You Need, When and How

1. Know the three electrolytes that matter on a fast

Electrolytes are minerals that carry an electrical charge in your body fluids. They hold water where it belongs and carry the signals that make nerves fire and muscles contract, your heart included. Three deserve your attention on a fast:

• Sodium keeps fluid in your blood vessels and helps hold your blood pressure steady. Fasting moves it first, because your kidneys let more of it go in the first days without food.
• Potassium sits mostly inside your cells and keeps your heartbeat and muscles working normally. A meal replaces what a daily fast costs you, while losses add up over fasts of several days.
• Magnesium takes part in the work of hundreds of enzymes, many of them in muscle and nerve. Much of it is stored in bone, so a short fast barely touches it.

On a fast you stop taking these minerals in while your kidneys keep making urine. Over 16 hours that matters very little; over two or three days it can matter a lot.

2. Decide whether your fast needs extra electrolytes

Doctors have described the natriuresis of fasting for decades: in the first days without food, the kidneys pass more sodium and water, and eating again reverses it. A 2020 paper notes that the effect is strongest in people with obesity or high blood pressure, and that its exact cause is still debated.

That reversal with food is why most people on a daily 16:8 schedule need nothing extra. You eat twice a day, so the shift resets every evening, and most American adults already eat more sodium than the 2,300 mg daily limit in the Dietary Guidelines. Longer fasts are another matter. In studies of total fasts lasting weeks, people built up a sizeable potassium deficit over the first two weeks before the losses slowed.

Your fast	Extra electrolytes?	What usually works
12 to 16 hours a day, like 16:8	Usually none	Two normal meals with vegetables and your usual salt
18 to 20 hours, or one meal a day	Sometimes	A pinch of salt in water if you feel flat in the last hours
24 to 36 hours, once in a while	Often	Salted water or broth spread over the day
48 hours or more	Yes, planned ahead	Daily sodium, a potassium-rich last meal and a plan made with your doctor
Any length, with kidney, heart or blood pressure problems or a diuretic	Ask first	Your doctor's plan before any salt, powder or potassium

To find your row, count the hours since your last bite. FastTrack starts and stops the fast automatically, so the number is there when a headache shows up at hour 20.

3. Start with sodium from salt, brine or broth

Sodium is the mineral most fasters run short of first, and the easiest to fix. Salt has no calories and does not break a fast, so pick whichever of these suits you:

• Salted water: stir 1/8 to 1/4 teaspoon of salt into a 16-ounce glass of water, about half a liter, and sip it over half an hour. Downed in one go on an empty stomach, it can make you queasy.
• Pickle brine: two or three tablespoons from a jar of dill pickles. Skip the brine of sweet or bread-and-butter pickles, which contains sugar.
• Broth: a mug of chicken bouillon or salted vegetable broth on day two of a longer fast. It has a few calories, a trade most people accept past 36 hours.

Two or three small doses across a 24-hour fast feel better than one big one and keep you from overdoing it. A level teaspoon of salt weighs about 6 grams, more than the 5 grams a day the World Health Organization sets as the upper limit for adults, and two or three quarter teaspoons stay under it.

If you have high blood pressure, take blood pressure medicine or have been told to limit salt, ask your doctor before adding any.

4. Get potassium from food in your eating window

Potassium is where fasting advice online gets careless. On a daily fast, your next meal replaces what you lost, so your plate is the place to get it. Before a longer fast, build the last meal or two around it:

• A medium baked potato or sweet potato, skin on
• A cup of cooked black beans, lentils or chickpeas
• Half an avocado, a banana or a cup of plain Greek yogurt
• A cup of cooked spinach, or a salmon fillet at dinner

Powders and pills are a different matter. Many electrolyte mixes and lite salts use potassium chloride, and too much potassium in the blood, called hyperkalemia, can be life-threatening. A 2021 clinical review names chronic kidney disease, diabetes, heart failure, a past high potassium result and blood pressure drugs that act on the renin-angiotensin system, (ACE inhibitors such as lisinopril, ARBs such as losartan), as reasons to check potassium more often. The authors of a 2024 review of guidelines proposed potassium-enriched salt for people with high blood pressure, except those with advanced kidney disease, those already taking a potassium supplement and those on a potassium-sparing diuretic such as spironolactone.

If any of that describes you, ask your doctor before using a potassium powder or a salt substitute, fasting or not. If none of it does and you are planning two days or more, a sugar-free mix can be part of the plan. Check the label for sugar, maltodextrin and calories; our page on whether electrolytes break a fast covers what to look for.

5. Treat magnesium as optional

Magnesium has a reputation as the cure for fasting cramps, and the evidence is thinner than the reputation. A Cochrane review pooled 11 trials with 735 people. For ordinary night-time leg cramps, mostly in adults in their 60s, magnesium did about as well as a placebo, and the authors concluded it is unlikely to give older adults meaningful protection. Results in pregnancy were mixed, none of the trials involved exercise cramps or fasting, and the main side effects were diarrhea and other stomach upset.

So when a cramp hits during a fast, check your salt and fluids first. Food in your window covers magnesium well for most people:

• An ounce of pumpkin seeds or almonds, a small handful
• A cup of cooked black beans or a bowl of oatmeal
• Spinach, Swiss chard and other leafy greens
• A square or two of dark chocolate after dinner

Some mineral waters are naturally rich in magnesium, with the amount per liter on the label, and they count as water on a fast. If you have kidney disease, ask your doctor before taking any mineral supplement.

6. Watch for signs you need more electrolytes while fasting

Running low on sodium feels vague and easily passes for plain hunger or a missed coffee. Use this as a quick check during a fast:

What you notice	Try first	Stop and eat, or get help, if
Dull headache late in the fast	A glass of water, then salted water	It is sudden or severe, or comes with fever or vision changes
Light-headed when you stand up	Stand up slowly and sip salted water	You faint, nearly faint or it keeps happening
Tired and flat on day two of a longer fast	A mug of salted broth	It has not lifted an hour or two later
Calf cramp at night	Stretch, and check your salt and fluids that day	Cramps come back night after night
Racing or irregular heartbeat, muscle weakness or confusion	Nothing to try at home	Always: end the fast and get medical help

Headaches have several causes besides salt, and our guide to headaches while fasting walks through them in order. Check your water too. If you log your glasses in FastTrack, look back at the days a symptom showed up, since a low-water day often explains it before any mineral does.

7. Ask a doctor first if this is you

Electrolyte advice is written for healthy adults. Talk to your doctor before you fast or add salt, potassium or magnesium if you:

• Have chronic kidney disease, or have been told your kidney function is reduced
• Have heart failure, a heart rhythm problem or high blood pressure
• Take a diuretic (water pill), an ACE inhibitor, an ARB or any other medicine that changes your potassium
• Have diabetes treated with insulin or tablets that lower blood sugar
• Are pregnant or breastfeeding, are under 18 or have a history of an eating disorder

Plan any fast longer than 48 hours with a doctor too, even if you feel healthy. Whatever the length, stop the fast and eat if dizziness does not pass, or if you faint, feel your heart racing or skipping, get confused or notice real weakness in your legs. Get medical help right away if you faint or if those symptoms do not settle soon after eating.

The short version

On a 16:8 schedule, your meals cover your electrolytes and you can skip the powders. On fasts of a day or more, start with sodium, get potassium and magnesium from food in your window, and save potassium products for when your doctor agrees. If you have kidney, heart or blood pressure problems or take a diuretic, have that conversation before your first fast.