How much sodium during a 24-hour fast?

A 24-hour fast is the point at which sodium starts coming up in the questions. Twelve or sixteen hours is short enough that most people coast through on what is already on board. A full day without food is long enough that the fasting forums, keto books, and electrolyte brands all start naming numbers — usually in milligrams, usually with a confident ring to them. The numbers are worth taking seriously, but they are worth attributing honestly too.

Before any of it: if you have high blood pressure, kidney disease, heart failure, diabetes, an eating disorder history, or you take medication that affects fluid or sodium (diuretics, lithium, SSRIs, blood-pressure drugs), this is a question to settle with your clinician, not a magazine. Nothing below is a prescription.

01 Where the popular numbers come from

Three ranges show up over and over.

The “1 to 2 grams of sodium per day” keto-flu number. This is the one most often attributed to Stephen Phinney and Jeff Volek, the low-carb researchers behind The Art and Science of Low Carbohydrate Living. They suggest that people transitioning into ketosis often need to consciously add 1 to 2 grams of sodium daily (roughly 2.5 to 5 grams of salt) on top of what they get from food, to compensate for the kidneys dumping more sodium when insulin falls. It is a clinician’s working recommendation built on physiology and clinical observation, not an output from a randomized trial designed to test that exact dose. It has nonetheless become the de facto reference range for low-carb and fasting communities.

The “1,000 mg per stick” packet-brand framing. LMNT, the brand that did the most to popularize concentrated sodium sticks, ships 1,000 mg of sodium per packet and openly suggests one to three packets a day for many users. That landed the “1,000 mg per dose” anchor in the popular conversation. It is a product spec rebadged as a serving size, not a clinical recommendation.

Clinical sodium recommendations. General population guidance from bodies like the American Heart Association suggests an upper limit of about 2,300 mg of sodium per day, with an “ideal” target closer to 1,500 mg. Those numbers were built from cardiovascular-risk modeling in fed people eating a typical mixed diet — they were not designed for someone deliberately running their insulin low through a fast. They are still the relevant ceiling to keep in view, especially for anyone with blood-pressure or kidney considerations.

Taken together, the practical “fasting sodium” conversation sits in a band that runs from roughly the AHA ideal target (around 1,500 mg) up to the low-carb working range (around 2,000 to 3,000 mg on a 24-hour fast for some people). None of these are precise prescriptions. They are working anchors that have been road-tested by a lot of fasters and surfaced repeatedly in popular literature.

02 Why a fasted body wants more salt than a fed one

The physiology behind the higher band is consistent and uncontroversial in outline.

When you stop eating, insulin falls. Lower insulin tells the kidneys to release sodium and water — the well-described natriuresis of fasting. The body’s main extracellular electrolyte starts leaving in the urine, and there is no incoming food to replace it. Plasma volume drops a little, which is part of why people feel lightheaded standing up on day one of a fast, and why the classic “keto flu” overlaps so heavily with the symptoms of a first long fast.

That is the mechanism people are gesturing at when they say a faster needs more salt than a sedentary office worker eating three meals. It is a reasonably well-understood renal response to low insulin, not a cleanse story.

What that physiology does not license is a one-size dose. Sweat rate, climate, baseline diet, training load, body size, medications, and underlying kidney and heart status all change the answer.

03 A sample dosing pattern across 24 hours

Here is one pattern that lines up with the popular ranges, framed as an illustration rather than a prescription. It assumes a generally healthy adult on a one-day fast with no relevant conditions and no clinician-supplied plan that says otherwise.

• Morning, hours 0 to 8. Around 500 to 1,000 mg of sodium with water, sometime after waking. This covers the early fasted hours, when natriuresis is already underway and many people are also dropping their usual coffee-and-breakfast salt load. If you tolerate caffeine, black coffee fits here too.
• Mid-fast, hours 8 to 16. Another 500 to 1,000 mg of sodium with water in the early-to-mid afternoon. This is often the window where light-headedness on standing first shows up, and where adding sodium tends to take the edge off without overshooting.
• Evening, hours 16 to 24. A final 500 mg or so, ideally earlier rather than late, paired with magnesium if you usually take it. Loading a big sodium dose right before bed can crank up overnight bathroom trips and disturb sleep, which defeats the point.

That is a total in the rough neighborhood of 1,500 to 2,500 mg of sodium across the day — sitting between the AHA ideal target and the upper end of the low-carb working range. Some people will need less. Some will need more, especially if it is hot, if they exercised, or if their normal diet was already salty. A smaller person at rest is not the same as a larger person who ran ten kilometers fasted.

The honest signal is not the spreadsheet. It is your body. Thirst, mild dizziness when you stand up quickly, and a dull headache that creeps in are the cues people most often describe before they top up sodium. Severe dizziness, fainting, an irregular heartbeat, confusion, or vomiting are not cues to add salt; they are cues to break the fast and seek care.

04 The bottom line

The “how much sodium during a 24-hour fast” question has no single right number, but it has a defensible range: somewhere between roughly 1,500 mg and 2,500 mg of sodium across the day for many generally healthy adults, spread into two or three doses rather than one. The popular anchors — the Volek and Phinney 1 to 2 grams per day, the 1,000 mg per packet brand framing, and the AHA ceiling — are useful reference points, not RCT-derived prescriptions, and worth naming as such. Thirst and standing-up symptoms are the real-time signals. And anyone with blood-pressure, kidney, heart, or medication considerations should be setting their fasting sodium plan with a clinician, not a stranger on the internet.