Feeling rough in your first week of keto?

Keto Flu and Electrolytes: What Helps, in Safe Amounts

Reviewed 2026-10-16 · 5 sources · By Anthony Falcone

The short answer: Keto flu is the headache, tiredness, nausea and dizziness many people describe in their first week of keto; in forum reports it peaked in week one and faded after about four weeks. The most popular fix is more salt, and researchers who ran supervised keto studies did give extra sodium and potassium for that reason. We found no trial that tested electrolytes against keto flu, and magnesium has little evidence behind it for cramps.^1,2,5

Before you start

Talk to your doctor before adding salt if you have high blood pressure, heart failure or kidney disease, or take blood-pressure medicine or water tablets (diuretics). Salt raises blood pressure in a dose-dependent way.³
Don't use potassium supplements or potassium-based salt substitutes if you have kidney disease or take medicines that raise potassium (such as ACE inhibitors, ARBs or spironolactone) without asking your doctor first: blood potassium can rise to dangerous levels.
If you take medicine for diabetes, are pregnant or breastfeeding, or have a history of eating disorders, see your doctor before starting keto at all. Heart palpitations or fainting are reasons to stop and get checked, not symptoms to treat with electrolytes.

1 in 3 forum users described keto flu themselves (101 of 300)¹
Week 1 when symptom reports peaked; they dwindled after 4 weeks¹
58 forum tips to add sodium, the most common remedy¹
2 g of sodium a day: the EU's safe and adequate intake for adults (5 g of salt)³
3–5 g of sodium a day in Phinney's supervised keto studies²
11 trials of magnesium for cramps: no clear benefit in older adults⁵

What keto flu is

Keto flu isn't a medical diagnosis. It's the name people use for the cluster of short-lived symptoms that can appear in the first weeks of a ketogenic diet. The most detailed description comes from researchers who read 448 posts by 300 people on 43 online forums about it.¹

About a third of those users (101 of 300) described symptoms they had had themselves. The most frequent were a general flu-like feeling, headache, fatigue, nausea, dizziness, brain fog, stomach upset, low energy, feeling faint and changes in heartbeat. Cramping was mentioned, but rarely (3 users).¹

Of the 60 people who said how bad it was, 15 called it mild, 23 moderate and 22 severe. This is self-reported forum data, so it describes what people go through; it can't tell you how often keto flu happens or what causes it.¹

How long keto flu lasts

In the forum posts, symptoms usually started within days of starting the diet, peaked in the first week and dwindled after four weeks. Only eight users said when their symptoms ended: anywhere from day 3 to day 30, with a median of 4.5 days.¹

Supervised studies show a similar pattern for exercise. In two metabolic-ward studies reviewed by Phinney, endurance or perceived energy dipped in the first week or two of carbohydrate restriction and then recovered. Phinney concludes that adapting to keto takes clearly longer than a week and is probably well advanced by 3–4 weeks, and that dipping in and out of carbohydrates seems to make it harder.²

Why electrolytes come up

When carbohydrate drops very low, as in fasting, the kidneys excrete more sodium. Phinney explains that this can shrink circulating blood volume and lead the kidneys to lose potassium as well, which is why his studies gave participants extra sodium and potassium every day from the start.²

Forum users came to a similar idea from experience. Of 18 remedies suggested, the most common were more sodium (58 mentions), electrolyte supplements (38), broth (27), more magnesium (25) and more potassium (24), with more water mentioned 14 times. The researchers note that these reflect popular beliefs about what causes keto flu; none was tested.¹

So the honest summary: there's a sound physiological reason to pay attention to sodium and potassium on keto, and supervised studies did so, but none of these sources, and no trial we found, compared electrolytes with nothing for keto-flu symptoms.^2,1

Sodium: how much is safe

The European Food Safety Authority considers 2.0 g of sodium a day a safe and adequate intake for adults: enough for most people to stay in balance, and low enough for confidence in a lower risk of heart disease. Labels often list salt rather than sodium. One gram of salt contains 0.4 g of sodium, so 2 g of sodium is 2 ÷ 0.4 = 5 g of salt.³

Most Europeans already eat more than that. Estimates cited by EFSA put average sodium intake in European countries between 3.27 and 4.42 g a day. EFSA also found strong evidence that more sodium means higher blood pressure: in its analysis of trials, systolic blood pressure rose by an average of 5.3 mmHg for every extra 2.3 g of sodium a day.³

In Phinney's supervised keto studies, total sodium intake was kept at 3–5 g a day, including 3 g given as bouillon. That was a research setting with medical oversight, not a target for everyone. If you don't have a reason to limit salt, salting food to taste and having broth in the first weeks is a mild, short-term step; if you do have such a reason, ask your doctor first.^2,3

Potassium: from food first

Phinney's studies kept total potassium at 2–3 g a day, with 1 g of it given as a potassium bicarbonate supplement. In the study of trained cyclists, total body potassium fell by 2% in the first two weeks, which matched the glycogen they lost, and then stayed stable.²

How you cook matters. Phinney points out that roasting or grilling meat, or boiling it and throwing the water away, loses potassium. Traditional Inuit diets used broths and soups that kept both the sodium and the potassium.²

Magnesium: what it can and can't do

Magnesium is the third most popular electrolyte tip on keto forums. For a reference point, EFSA could not set an average requirement for magnesium and instead based its adequate intake for adults on what healthy people in the EU typically eat.^1,4

If you're reaching for it to stop cramps, the evidence is weak. A Cochrane review found 11 trials with 735 people. In older adults with ordinary night-time leg cramps, magnesium didn't clearly reduce how often cramps happened compared with placebo (3 trials, 177 people, moderate-certainty evidence). The review found no trials at all in people who cramp during exercise.⁵

Minor side effects, mostly diarrhoea and nausea, were reported more often on magnesium than on placebo (low-certainty evidence): in the trials, roughly 11% to 37% of people on magnesium had them, against 10% to 14% on placebo. Stomach upset is already one of the keto-flu complaints people report, so that's worth knowing.^5,1

Sweat, heat and training

Sweat is the part that varies most. 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 moderate temperatures and at moderate activity levels, sweat losses are small.³

So someone training hard in summer loses far more sodium than someone at a desk, which is one reason a single salt number can't fit everyone. Expect hard, fast efforts to feel worse on keto in any case: in Phinney's cyclist study, endurance came back after the first week but sprint ability stayed limited.^3,2

What to do with this

Expect the first week to be the hardest. In forum reports, keto flu peaked in week one and faded after about four weeks.¹
Unless your doctor has told you to limit salt, don't cut it on keto: salt food to taste and consider broth early on. Keep in mind that the EU's safe and adequate sodium intake is 2 g a day (5 g of salt), and most people already eat more.^2,3
Get potassium from food, and keep the cooking liquid from meat and vegetables in soups and broths rather than pouring it away.²
Don't count on magnesium to stop cramps; the trials so far don't show a clear benefit, and it can upset your stomach.⁵
Keep training easy in the first week or two and stick with the diet consistently; adaptation seems to take 3–4 weeks.²

How sure is this?

What we know about keto flu itself comes from self-reported forum posts. There was no comparison group, users' health and reasons for the diet weren't verified, and only eight people said when their symptoms ended.¹
We found no trial that tested sodium, potassium or magnesium as a keto-flu remedy. The electrolyte amounts here come from two small metabolic-ward studies run by one researcher, described in his own review.^2,1
The magnesium evidence is about older adults with night-time leg cramps; there are no trials in people on keto or in people who cramp during exercise.⁵
We could read only the summary of EFSA's magnesium opinion and of the Cochrane review, so we use only their main conclusions and the numbers stated there.^4,5

Common questions

What are the symptoms of keto flu?

In the largest collection of reports, the most common were a flu-like feeling, headache, fatigue, nausea, dizziness, brain fog, stomach upset, low energy, feeling faint and heartbeat changes. Heartbeat changes or fainting should be checked by a doctor.¹

How long does keto flu last?

Reports peaked in the first week and dwindled after four weeks. The eight forum users who said when it ended gave anywhere from 3 to 30 days.¹

Do you need electrolytes on keto?

The kidneys lose more sodium when carbohydrate is very low, and supervised keto studies added sodium and potassium for that reason. We found no trial testing whether that eases keto flu.^2,1

Does magnesium help keto flu or cramps?

There are no trials on magnesium for keto flu. For ordinary night-time leg cramps in older adults, a Cochrane review found it unlikely to give a meaningful benefit, and minor side effects such as diarrhoea were reported more often than on placebo.⁵

Is there a good electrolyte drink recipe for keto?

We don't give a recipe here. Forum users suggested broth, sugar-free sports drinks and pickle juice, all untested. Whatever you use, check the label's sodium (salt × 0.4) against the 2 g a day reference, and ask your doctor first if you have been told to limit salt.^1,3

Sources

Bostock ECS, Kirkby KC, Taylor BV, Hawrelak JA. Consumer Reports of “Keto Flu” Associated With the Ketogenic Diet. Frontiers in Nutrition, 2020. Survey / forum study. doi.org/10.3389/fnut.2020.00020
Phinney SD. Ketogenic diets and physical performance. Nutrition & Metabolism, 2004. Review. doi.org/10.1186/1743-7075-1-2
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
EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific Opinion on Dietary Reference Values for magnesium. EFSA Journal, 2015. Official reference values. doi.org/10.2903/j.efsa.2015.4186
Garrison SR, Korownyk CS, Kolber MR, Allan GM, Musini VM, Sekhon RK, Dugré N. Magnesium for skeletal muscle cramps. Cochrane Database of Systematic Reviews, 2020. Meta-analysis. doi.org/10.1002/14651858.CD009402.pub3

The Keto Blueprint is the step-by-step version: a 14-day plan to start keto safely, with your macros, what to eat, how to handle the keto flu, and how to train, measure and decide what comes next. For hydration beyond keto, the Hydration Protocol covers sodium/potassium ratios, exercise fluid timing and DIY performance drink formulas. The Keto Blueprint · Hydration & Electrolyte Protocol