All guides

Fasting, and not sure what you can drink?

Electrolytes While Fasting, and What Breaks a Fast

Reviewed · 6 sources · By Anthony Falcone

The short answer: What breaks a fast depends on the fast: experts define a total fast as no calories at all, while clinical fluid-only fasts allow broth and juice up to 500 kcal a day. On a daily eating window such as 16:8, a year-long trial allowed water, black coffee and herbal tea in the fasting hours and nothing more. Sodium and potassium matter mainly on fasts that last several days, which in the large studies were run under medical supervision.1,4,5,2

Before you start

  • If you take insulin or other medicine that lowers blood sugar, talk to your doctor before fasting: low blood sugar was one of the adverse effects recorded even in a closely supervised fasting clinic. The same applies if you take blood thinners, because blood clotting values changed during longer fasts and the clinic advised monitoring.2
  • If you take blood-pressure medicine, have high blood pressure, heart or kidney disease, or have been told to limit salt, ask your doctor before fasting or adding salt. Blood pressure fell during supervised fasting and doctors adjusted medication, and extra salt raises blood pressure in a dose-dependent way.2,6
  • Don't fast if you are pregnant or breastfeeding, severely underweight, have anorexia nervosa, or have advanced kidney or liver disease; these were reasons to exclude people from the large fasting study. If you have had any eating disorder, talk to your doctor first. Fasts longer than a day or two belong under medical supervision.2
  • Don't use potassium supplements or potassium-based salt substitutes without asking your doctor if you have kidney disease or take medicines that raise potassium. Palpitations, fainting or confusion during a fast are reasons to stop, eat and get checked.
  • 0 kcal on a total fast; a fluid-only fast allows up to 500 kcal a day of broth and juice1
  • 14 h minimum daily fasting window in the expert definition of time-restricted eating1
  • 23 trials of 16:8 eating: slight, low-certainty drops in fasting glucose and insulin3
  • 140.1 → 138.7 average blood sodium (mmol/L) in 1,422 adults fasting 4–21 days, still within normal range2
  • 3 g of sodium a day as bouillon in Phinney's supervised very-low-calorie study5
  • 2 g of sodium a day: the EU's safe and adequate intake for adults (5 g of salt)6

What counts as fasting

There was no shared definition of fasting until 2024, when 38 researchers and clinicians went through five survey rounds and an online conference and agreed on 24 terms. Their umbrella definition is broad: fasting is choosing to go without some or all foods, or foods and drinks, for health, religious, cultural or other reasons.1

Inside that umbrella the panel drew clear lines. A total or complete fast means no calories at all for a set time, and can be equated with water-only fasting. Dry fasting means no food and no water. Modified fasting caps energy at about 25% of what you need.1

Fluid-only fasting is a type of modified fasting: water and unsweetened herbal tea as much as you like, plus clear vegetable broth and vegetable or fruit juice up to 500 kcal a day in total. The panel said ultra-processed drinks don't belong in it.1

So what breaks a fast?

By the strict definition, anything with calories ends a total fast. Under the broader clinical definitions, small amounts of broth or juice are part of the fast itself, as long as the day stays within the 500 kcal limit.1

Small amounts didn't stop the metabolic switch in practice. In a German fasting clinic, 1,422 adults had 250 ml of fresh juice at noon and 250 ml of vegetable soup in the evening, with optional honey: about 200–250 kcal and 25–35 g of carbohydrate a day. Ketones in the urine still rose and seemed to plateau after about 5 days.2

Black coffee is the grey zone. The consensus panel raised it, along with the type of water to drink, and then dropped those questions because there was too little scientific data to settle them. The authors also note that experts don't agree on whether there is a calorie threshold that switches fasting responses on.1

16:8 and other daily eating windows

The panel defined time-restricted eating as a daily fasting window of at least 14 hours with no set limit on calories in the eating hours. Some leading researchers would have accepted 12 hours; one reason the panel chose 14 is that the typical overnight fast in people today is already about 12 hours. A 16:8 schedule, 16 hours without food and an 8-hour eating window, fits easily within that.1

A 2025 meta-analysis pooled 23 randomised trials of 16:8 with 1,280 participants. Compared with control diets, 16:8 slightly lowered fasting glucose and insulin, with low certainty. A small rise in HDL cholesterol reached statistical significance, but the authors call that evidence very uncertain. There was no clear difference in LDL cholesterol or triglycerides, also very low certainty.3

A 12-month trial of 247 adults used a 9-hour window ending by 19:00. Outside it, participants could have water, black coffee or herbal tea. They lost about 2.5 kg in 4 months and kept it off at 12, the same as a group given personalised dietitian advice. The authors put it down to eating roughly 400 kJ (about 95 kcal) less a day; the eating day had shrunk by about 2.6 hours.4

Do you need electrolytes on a daily fast?

Nothing in these sources calls for electrolyte drinks on a daily eating window. The year-long trial allowed only water, black coffee and herbal tea outside the window, and its adverse events were minor and no more common than with dietitian advice.4

If you train hard in the heat, sweat changes the picture. EFSA cites sodium concentrations in adult sweat from 0.23 g to 4.20 g per litre, depending on sweat rate, hydration, how much sodium you eat and how used to the heat you are. In that case, salting your meals inside the eating window is a simple first step, unless you have been told to limit salt.6

Longer fasts: why sodium and potassium matter

Fasting makes the kidneys excrete more sodium. Phinney explains that this can shrink circulating blood volume and push the kidneys to lose potassium as well. That's why, in a supervised 6-week very-low-calorie ketogenic study, participants were given 3 g of sodium as bouillon and 1 g of potassium every day.5

In the clinic study of 1,422 adults fasting for 4 to 21 days, people drank about 3 litres of water or herbal tea a day alongside the juice and soup. Average blood sodium fell from 140.1 to 138.7 mmol/L, potassium fell slightly and magnesium stayed stable; all averages stayed within the normal range.2

Problems were rare but real. Doctors recorded low blood sodium in 3 people (0.21%), low potassium in 1 and heart rhythm disturbances in 3, all short-lived. These people were seen by nurses every day and by doctors several times a week, who also adjusted medicines. That supervision is part of why the results looked safe.2

How much sodium is sensible

The European Food Safety Authority considers 2.0 g of sodium a day a safe and adequate intake for adults. Labels often list salt instead: one gram of salt contains 0.4 g of sodium, so 2 g of sodium is 2 ÷ 0.4 = 5 g of salt.6

More is not automatically better. In EFSA's analysis of trials, systolic blood pressure rose by an average of 5.3 mmHg for every extra 2.3 g of sodium a day. The 3 g of supplemental sodium in Phinney's study came with daily medical oversight, so treat it as context, not a target.6,5

What to do with this

  • Decide which fast you're doing. A total fast means no calories; a fluid-only fast allows water and herbal tea freely plus broth or juice up to 500 kcal a day.1
  • On 16:8 or a similar daily window, keep the fasting hours to water, black coffee or unsweetened herbal tea, as the 12-month trial did.4,1
  • Expect modest effects from the window itself: about 2.5 kg, kept off for a year, in the long trial, and only slight changes in blood sugar markers across 23 trials.4,3
  • For fasts of several days, sodium and potassium need attention and so does supervision; in the large clinic study, broth, juice and daily medical checks were part of the plan.5,2
  • Keep sodium in proportion: the EU's safe and adequate intake is 2 g a day (5 g of salt), and more raises blood pressure.6

How sure is this?

  • We found no trial that compared electrolytes with no electrolytes during fasting. What we know comes from a supervised metabolic study described in a review and from an observational clinic cohort.5,2
  • The 1,422-person fasting study had no control group and was run at the clinic where several authors work or consult, which is an industry tie. Most participants came from upper social classes, and all were supervised daily.2
  • The fasting definitions are expert opinion, not trial results. Several panel members declared ties to fasting clinics, a fasting app or supplement companies, and the panel left questions such as black coffee unresolved.1
  • The 16:8 evidence is mostly low or very low certainty: 12 of the 23 trials lasted under 12 weeks, and only 4 included women alone.3

Common questions

Do electrolytes break a fast?

Under the expert definition, a total fast only rules out calories, so a calorie-free drink doesn't break it while a sugary sports drink does. No study we found tested electrolytes during fasting directly.1

Does black coffee break a fast?

A 12-month eating-window trial allowed black coffee in the fasting hours. An international expert panel discussed it and left it open because there was too little scientific data.4,1

Is 16:8 intermittent fasting good for you?

Across 23 trials, 16:8 slightly lowered fasting glucose and insulin compared with control diets, with low certainty, and didn't clearly change LDL cholesterol. In a year-long trial a 9-hour window gave about the same weight loss as dietitian advice.3,4

Can you drink broth while fasting?

Not on a total fast, which allows no calories. Clear broth is part of fluid-only fasting up to 500 kcal a day, and the large clinic study served 250 ml of vegetable soup each evening.1,2

How much salt should I have while fasting?

We found no fasting-specific target. The EU's safe and adequate intake for adults is 2 g of sodium a day (5 g of salt); a supervised very-low-calorie study gave 3 g of sodium as bouillon. Ask your doctor first if you have been told to limit salt.6,5

Sources

  1. Koppold DA, Breinlinger C, Hanslian E, Kessler C, Cramer H, Khokhar AR, Peterson CM, Tinsley G, et al. International consensus on fasting terminology. Cell Metabolism, 2024. Position stand. doi.org/10.1016/j.cmet.2024.06.013
  2. Wilhelmi de Toledo F, Grundler F, Bergouignan A, Drinda S, Michalsen A. Safety, health improvement and well-being during a 4 to 21-day fasting period in an observational study including 1422 subjects. PLOS ONE, 2019. Case series. doi.org/10.1371/journal.pone.0209353
  3. Wong PS, Wan K, Dai Z, Yu APH, Wong SHS, Poon ETC. Effect of 8-Hour Time-Restricted Eating (16/8 TRE) on Glucose Metabolism and Lipid Profile in Adults: A Systematic Review and Meta-Analysis. Nutrition Reviews, 2025. Meta-analysis. doi.org/10.1093/nutrit/nuaf206
  4. Parr EB, Charrouf R, Hutchison AT, Flint SA, et al. Time-restricted eating versus dietetic guidance on glycaemic outcomes in adults at risk of type 2 diabetes: a non-inferiority randomised clinical trial. Diabetologia, 2026. Randomised trial. doi.org/10.1007/s00125-026-06762-x
  5. Phinney SD. Ketogenic diets and physical performance. Nutrition & Metabolism, 2004. Review. doi.org/10.1186/1743-7075-1-2
  6. EFSA Panel on Nutrition, Novel Foods and Food Allergens (NDA). Dietary reference values for sodium. EFSA Journal, 2019. Official reference values. doi.org/10.2903/j.efsa.2019.5778

The Fasting Protocol lays out the eating-window options (16:8, 20:4, OMAD), a guide to 24–48 hour fasts, a refeeding strategy with meal plans and an electrolyte management formula. The Hydration Protocol adds a sodium/potassium ratio guide, exercise fluid timing and DIY performance drink formulas. Fasting Protocol · Hydration & Electrolyte Protocol

Keep reading