Tip · Fasting

Electrolytes during fasting periods

Q: Why do I lose weight so quickly when fasting?

A large part of it is water, and the reason is a loss of sodium via the kidney. During fasting, insulin falls, and the keto acids formed must be excreted as anions together with a cation – in the first days, that is sodium. As soon as the kidney provides enough ammonium, ammonium takes over this role, and the sodium loss declines.

Q: Does my fasting headache come from electrolyte deficiency?

That is plausible but not established. In the largest fasting cohort with 1,422 participants, 5.39 percent had headaches and 0.35 percent muscle cramps, with electrolytes in the normal range throughout. A study that attributes headache during fasting to electrolyte deficiency or prevents it through electrolyte supplementation does not exist. Anyone who normally drinks coffee should think of caffeine withdrawal first.

Q: How much salt do I need when fasting?

There is no answer backed by studies. What is established is that the sodium loss occurs, that it is greatest in the first days, and that in the best-documented implementation the sodium came from broths and juices rather than from supplements. The official reference value for general sodium intake is 2.0 grams per day – the 3 to 5 grams cited in the scene are well above that and are backed by nothing.

Q: Do I need magnesium when fasting?

There is no measured deficiency to justify it – in the only large series of measurements, the magnesium level in the blood remained stable over up to 21 days of fasting. Anyone who supplements anyway should observe the official upper limit of 250 milligrams per day from food supplements and divide the amount into at least two doses. The limiting side effect is diarrhea, and that is particularly unfavorable during fasting.

Q: Do electrolytes break the fast?

No. Sodium, potassium and magnesium provide no calories and do not trigger an insulin response. This has never actually been studied, however – there is simply no physiological reason for the concern, and in the large fasting programs, broths with salt are standard.

Q: For whom is extra salt during fasting not a good idea?

For people with high blood pressure, heart failure or kidney disease – there, any increase in sodium intake belongs in a physician’s hands. Potassium supplements are risky with impaired kidney function and when taking ACE inhibitors, sartans or potassium-sparing diuretics. Anyone taking diuretics already has a shifted electrolyte balance.

Headaches and weakness almost always come from a deficiency, not from fasting.

That the body loses sodium during fasting is physiologically well understood. This tip is here because a sensible practice can be derived from it – and because the amounts circulating in the scene have to be measured against the official values. What is meant are fasting periods of about 24 hours or more; with a 16-hour window, this plays no role.

In short

The sodium loss is real and mechanistically explained: the keto acids formed during fasting must be excreted as anions together with a cation, and until the kidney provides enough ammonium, that cation is sodium. Sodium intake during fasting periods lasting several days is therefore physiologically justified – a specific amount, by contrast, is not backed by any study. The official values are clear: EFSA considers 2.0 g of sodium per day a safe and adequate intake for adults, and the BfR recommends not exceeding 250 mg of magnesium per day from food supplements, divided into at least two doses. The 3 to 5 g of sodium and 300 to 500 mg of magnesium cited in the scene are above that. And magnesium in the blood does not fall at all over up to 21 days of fasting.

What is behind it

During fasting, insulin falls. Insulin normally inhibits sodium excretion in the kidney – if it drops away, excretion rises. The quantitatively decisive pathway, however, is ketosis: the keto acids formed appear in the urine as organic anions and need a cation. In the first days of fasting, that is sodium, because the kidney does not yet provide enough ammonium. As soon as ammonium production ramps up, ammonium takes over this role, and sodium excretion declines.

This explains two observations. First, natriuresis is strongest in the first days and subsides afterwards. Second, it stops immediately after glucose intake – the need for electrolytes is coupled to ketosis and ends with it. Water is lost along with the sodium; this is the reason for the rapid weight loss in the first days of fasting and for orthostatic complaints. Potassium is also excreted in greater amounts, partly as an accompanying cation of the keto acids, partly from cell breakdown.

Magnesium behaves differently

In the largest cohort to date, the serum magnesium level remained stable over 4 to 21 days of fasting, while sodium and potassium fell significantly and calcium rose. This does not mean that the body’s stores remain unchanged – serum magnesium is a poor indicator of stores. But there is no measured drop of magnesium in the blood on which a supplement recommendation could be based.

For magnesium from food supplements, on the other hand, there is a firm official limit. The BfR recommends that persons aged 4 years and older not exceed 250 mg per day from food supplements, based on the Tolerable Upper Intake Level of 250 mg for supplemental magnesium derived in 2001. The limiting side effect is diarrhea, which did not occur at intakes of up to 250 mg per day, and the maximum daily amount should be divided into at least two doses. Diarrhea during a fasting phase is particularly unfavorable because it costs additional fluid and electrolytes.

As for the complaints

The common attribution that headaches and weakness during fasting almost always come from a deficiency is not supported by the data. In the cohort with 1,422 participants, all electrolytes stayed within the normal range, and of 1,311 evaluated questionnaires, 169 people (14.94 %) reported sleep disturbances, 155 (13.70 %) exhaustion, 100 (8.84 %) dry mouth and 61 (5.39 %) headache. Muscle cramps, the classic electrolyte symptom, were reported by 4 people (0.35 %).

The statement that can be supported therefore reads differently: the sodium loss during fasting is physiologically well described – the attribution of individual complaints to a deficiency is not. Anyone who normally drinks coffee should think of caffeine withdrawal first when a fasting headache occurs.

What is well supported

The mechanism is described quantitatively and has not been superseded to this day. Nine women with obesity were stabilized on a constant sodium and calorie intake and then fasted while maintaining their sodium intake; measurements were taken at three-hour intervals. The rise in urinary excretion of sodium, ammonium and potassium correlated with the simultaneous rise in organic acid anions (r = 0.891; p < 0.001). The sodium loss was considerably greater than the chloride loss, and after refeeding with glucose, sodium excretion promptly fell. How large the loss can become is shown by a paper on 40 people with obesity undergoing therapeutic fasting: either a steady loss at a low level or a fluctuating loss that in some cases led to marked sodium depletion.

What the studies show

The mechanism of fasting natriuresis

Nine women with obesity, first stabilized on a constant sodium and calorie intake, then fasting with unchanged sodium intake and an unchanged activity program, measurements at three-hour intervals. The hypothesis that the obligatory cation accompaniment of the metabolically formed anions explains the sodium loss was confirmed. The paper additionally tested three corollaries: the rise in ammonium excretion lagged considerably behind that of the anions, the sodium loss was much greater than the chloride loss, and after glucose intake sodium excretion promptly fell.

Electrolytes during supervised fasting in 1,422 people

Buchinger fasting over 4 to 21 days with 200 to 250 kcal per day from juices and broths and 3 liters of water daily. Serum sodium fell significantly from 140.1 ± 0.1 to 138.7 ± 0.1 mmol/l (p < 0.001), potassium also fell (p = 0.001), magnesium remained stable, calcium rose – all values stayed within the normal range. The authors report six cases of mild hyponatremia with a lowest sodium value of 127 mmol/l, all of them not serious and normalized again; hyponatremia in 3 participants (0.21 %) and hypokalemia in 1 participant (0.07 %) are documented as adverse events.

Natriuresis even at 500 kcal per day

25 people with obesity and essential hypertension as well as 9 overweight normotensive persons on a supervised 500 kcal diet with unrestricted salt intake, 24-hour urinary electrolytes on days 0, 4, 7 and 10. The urinary sodium concentration almost doubled in the hypertensive participants on day 4 and rose 1.4-fold in the normotensive controls. Plasma ANP rose to almost three times the baseline in the hypertensive participants and to about double in the normotensive ones. This explains why natriuresis is more pronounced in high blood pressure.

Where the numbers come from – and where they do not

There is no controlled study that has investigated a specific electrolyte intake during fasting. None of the available papers randomized electrolyte supplementation against no supplementation, and none tested a dose – all the amounts in circulation are empirical values. The size of the loss per day has not been published in a usable form either: the papers report excretion curves and correlations, not daily balances from which an intake in grams could be derived. Added to this is the origin of the data: the studies on natriuresis come from people with obesity undergoing therapeutic fasting – 9 women in the mechanism study, 40 patients in the clinical paper, hypertensive participants in the ANP study. For people of normal weight on a 24- or 48-hour fast, there are no corresponding measurements. And the link between fasting headache and electrolyte deficiency is not established: a study that attributes headache during fasting to electrolyte deficiency or prevents it through electrolyte supplementation does not exist.

How to do it

The best-documented implementation works with food, not with powder: 1,422 participants, 4 to 21 days, 200 to 250 kcal per day from juices and broths – that is, sodium and potassium via food – plus 3 liters of water daily. Under these conditions, all electrolytes stayed within the normal range, with a significant but small drop in sodium and potassium. Two findings put this in context: in the mechanism study, sodium intake was kept at the previous level, and excretion still rose considerably – salt intake does not prevent natriuresis, it only compensates for it. And after refeeding with glucose, excretion promptly fell. On the amounts, in the corrected version: the official reference value for general sodium intake is 2.0 g per day; the common figure of 3 to 5 g per fasting day is 1.5 to 2.5 times that and is not backed by any study. For magnesium from supplements, the BfR upper limit of 250 mg per day applies, divided into at least two doses – the common figure of 300 to 500 mg exceeds it and should be corrected to at most 250 mg. For potassium the direction is reversed: 1 to 2 g per day is well below the EFSA reference value of 3,500 mg per day. And on the question of whether electrolytes break the fast: sodium, potassium and magnesium provide no calories and do not trigger an insulin response.

Safety

The obvious danger during fasting is not too much sodium but too little: in the largest cohort, six cases of mild hyponatremia occurred, the lowest value was 127 mmol/l – and that even though the participants received broths. A great deal of water without any salt over several days of fasting is therefore the riskier route, not the safer one. At the same time, the amount of sodium circulating in the scene is high: EFSA considers 2.0 g of sodium per day a safe and adequate intake for adults, and beyond that there are no safety data for time-limited fasting days in healthy people. There is, however, a clear contraindication: anyone with high blood pressure, heart failure or kidney disease should not increase their sodium intake on their own – and it was precisely in hypertensive participants that natriuresis during fasting was most pronounced, which does not make the question simpler but places it in a physician’s hands. For magnesium, the BfR upper limit of 250 mg per day from food supplements applies, with diarrhea as the limiting side effect. Potassium supplements are not harmless: with impaired kidney function, or when taking ACE inhibitors, sartans, potassium-sparing diuretics or aldosterone antagonists, additional intake can lead to hyperkalemia, and anyone taking diuretics already has an altered electrolyte balance – according to the NICE criteria, diuretics even count among the risk factors for refeeding problems. Fasting for several days, including self-managed electrolytes, is not suitable for people who are underweight, pregnant or breastfeeding women, children and adolescents, or people with a history of eating disorders.

BK-Score Thin human evidence

Human evidence		4
Mechanism		7
Safety data		7
Hype gap		7
Track record of use		8

The loss of sodium and potassium during fasting is physiologically described and explains the typical complaints well. However, there are no controlled studies that support a specific intake – the amounts cited in the scene are empirical values.

The score rates the state of knowledge, not the effect. “Safety data 9” means well studied – not harmless.
Subjective assessment by Biohacking Kompakt based on published scoring rules – not a scientific rating and not a medical recommendation. Rules and all ratings (German)

Frequently asked questions about electrolytes during fasting periods

Why do I lose weight so quickly when fasting?