What Is Water Fasting and How Do You Do It Properly

During water fasting, you do not eat any food or consume calories. You drink only water (typically 2–3 litres/day).
In the short term, water fasting may reduce weight, blood pressure and some metabolic markers, such as glucose and insulin resistance.
However, many of these effects may not be due to fasting itself. In many cases, they can largely be explained by a calorie deficit. When calories are matched, the supposed “magic” of fasting often becomes much less convincing.

“Fasting” is now a popular umbrella term for different approaches that restrict food intake in some way. For example, fasting on certain days of the week (e.g., the 5:2 diet), eating only within a specific daily window (time‑restricted eating) or alternating fasting and eating days (alternate‑day fasting).

Water fasting, or water‑only fasting, is one of the most extreme forms of fasting because no food or calories are consumed at all. This is why it is surrounded by so many claims, from “resetting the metabolism” and “detoxing” to triggering autophagy and producing rapid weight loss. However, it also carries potential risks.

What is water fasting, and how does it differ from other “fasts”

Water‑only fastingmeans consuming no food or calories and drinking only water. It is often described as a regime where a person drinks around 2–3 litres of water per day and eats no food. It may last 24 hours, several days or even weeks.

In clinical programmes and studies, definitions can vary. For example, in one study, it very strictly defined it as complete abstinence from all substances, including food, tea, juice and even non‑caloric drinks, with distilled water allowed as needed (a minimum of about 1.2 l/day).

Water fasting vs. Buchinger fasting

Studies most often examine water‑only fasting and Buchinger fasting.

Water fasting means no food and only water, typically around 2–3 litres per day.

Buchinger fasting is traditionally medically supervised and, unlike water fasting, usually includes a small amount of energy. This often comes from juice at lunch and vegetable soup in the evening, totalling around 200–300 kcal per day (about 25–25g of carbohydrates a day). Buchinger fasting is generally considered more suitable and safer for longer fasting periods.

Water fasting vs. dry fasting: the main difference is fluids

At first glance, it sounds similar: “fasting” is fasting. In reality, there is one fundamental difference between water and dry fasting – in dry fasting, you do not drink at all.

Why dry fasting is generally riskier

The body can temporarily cope with a lack of food by switching to fat and ketone bodies for energy. A lack of fluids, however, is more dangerous. Dehydration can affect blood pressure, heart rate, temperature regulation and kidney function. For this reason, medical sources typically do not recommend dry fasting and warn about the risk of dehydration and related complications (particularly related to the kidneys).

What happens in the body during water fasting?

When you stop eating, the body has to change where it gets energy from. Scientific reviews often describe this as a metabolic switch: the body shifts from using glucose to using fats and ketone bodies.

The first 1–2 days: glucose and glycogen are depleted

After around 12–36 hours without food, the body gradually shifts away from using glucose, mainly from glycogen stores, and begins relying more on fatty acids and ketone bodies.

In practice, this often means fluctuations in energy and mood, stronger waves of hunger and, in some people, headaches or worse sleep.

Many people find the first 1–2 days the hardest. This is not because the body cannot burn fat, but because it is adjusting to a new state where quick energy from glucose is less available.

Days 3–10: ketone bodies rise and then stabilise

If fasting continues, ketone bodies increase. In people without diabetes, they typically plateau after around 5–10 days.

One review states that beta‑hydroxybutyrate (BHB), one of the main ketone bodies, can reach approximately 4–5 mmol/l after around 8 days and then remain relatively stable.

Similarly, urinary ketones appear to peak around day 5 and then change little with longer fasting.

In a 7‑day fasting study in healthy adults, BHB was almost zero after a normal overnight fast but rose to around 4 mmol/l after prolonged fasting.

Nutritional ketosis therefore does not fully develop on day one. During longer water fasts, ketone bodies rise gradually and stabilise over several days.

What Is Water Fasting and How Do You Do It Properly — Image by freepik.com

The body starts burning more fat and saving carbohydrates

With longer fasting, the body’s main fuel source changes. In a 21‑day study, carbohydrate oxidation dropped rapidly. By around day 3, it was about a third of pre‑fasting levels and by around day 9, it was about 10% of the original level. Fat oxidation, on the other hand, rose quickly (around day 3 ~1.8×) and remained high.

Metabolic adaptation: resting energy expenditure may decrease over time

During short fasts, resting energy expenditure can vary. With longer fasts, however, the body often starts to conserve energy. In the 21‑day study, resting energy expenditure decreased continuously by an average of around 20%.

Why weight drops faster at the beginning

In the same 21‑day study, weight loss was fastest in the first few days: around 0.9kg per dayduring the first 5 days. However, this should not be mistaken for pure fat loss. Much of the early drop is due to water loss, glycogen depletion and some loss of lean mass.

How to properly water fast

Who should avoid it (or only do it after consulting a doctor)

Avoid fasting (or consult a doctor) if you have a chronic illness, diabetes/metabolic disorder, very low blood pressure, anaemia, stomach/duodenal ulcers, more serious cardiovascular problems, are taking medication (especially for blood pressure/diuretics), are pregnant/breastfeeding or have a history of eating disorders.

Step‑by‑step

1) Preparation (1–2 days before)

Simplify your diet before starting. Choose lighter meals, reduce ultra‑processed foods and avoid beginning a fast immediately after intense training or a demanding period. The aim is to make the transition less abrupt.

2) During the fast

Drink water as needed. Mineral water may be useful.
Avoid alcohol and caloric drinks, otherwise it is no longer water fasting.
Reduce physical demands. Avoid hard training and opt for light walking and rest instead.
Pay attention to your body. If dizziness, weakness or nausea worsen, stop fasting.

3) Ending the fast: refeeding

Do not underestimate the return to eating.

Start gradually with small portions of light food, such as:

soup, yoghurt/kefir, fruit, cooked vegetables, rice/potatoes)
for the first day or two, keep meals simple and avoid overeating
gradually add protein and introduce regular portions

The longer the fast, the more careful you need to be with refeeding.

Start small and choose gentler approaches

Literature shows a big difference between a short fast lasting hours to one day and a prolonged fast lasting several days or weeks.

For beginners, it is more sensible to start cautiously. Shorter periods without food are usually much more manageable than multi‑day water fasts, which are often carried out under supervision in studies.

Reduce training and performance demands

During longer water fasts, activity is often limited in studies. Even moderate activity can significantly increase energy expenditure and may raise the risk of dizziness, low blood pressure, dehydration, electrolyte imbalance or heart rhythm issues.

Consider electrolytes during longer fasts

During water fasting, you do not get electrolytes from food. With longer fasts, sodium and chloride may gradually decrease, and potassium may fall later.

This can contribute to dizziness, weakness, low blood pressure, headaches or muscle cramps – sodium helps maintain fluid volume and blood pressure.

When choosing an isotonic drink/electrolytes, it is important to ensure that they contain zero calories.

Potential benefits of water fasting

Most data on water fasting is short‑term, usually lasting days to weeks, and often comes from supervised programmes.A review of human studies found that fasting protocols lasting around 5–20 daystypically led to a significant increase in ketone bodies and weight loss of around 2–10%, but also a fairly large loss of lean mass.

Weight loss is fast, but not ideal

Fasting leads to a caloric deficit, so weight inevitably drops.

In the short term, weight loss is the most obvious effect. In general, longer fasts lead to greater weight loss, although results vary.

Indicatively, 5 days of water/Buchinger fasting led to a weight loss of about 4–6%, 7–10 days typically brought a higher loss, about 2–10%, 15–20 days were among the largest interventions, and losses were about 7–10%.

However, there is a catch. On average, about two‑thirds of the lost weight was lean mass (so not just fat).

What happens after eating resumes?

After the fast ends and normal eating resumes, weight often partially returns. This can happen quickly, especially as glycogen and water stores are replenished.In one study, a rapid weight gain of around 1.28kg per day was observed during the transition from refeeding to recovery, followed by around 0.6kg per day.

In another study, people lost around 6% after 5 days of water‑only fasting, but regained all of it after 3 months of normal eating.

Other studiesshowed only a small regain of around 1–2% after 2–4 months, but those studies included calorie restriction during refeeding, which may explain the smaller regain.

Blood pressure

Blood pressure often falls with weight loss. In people with obesity, a reduction in blood pressure is commonly seen after losing around 5% of body weight.

With longer water fasts this matches what we see in studies. When participants lost more than around 4% of body weight, systolic blood pressure usually dropped by about 9–14 mmHg, and diastolic pressure by about 6–13 mmHg. Studies where people lost only around 2% of body weight often showed little or no change in blood pressure.

The general rule applies. The longer the fast, the greater the chance of a significant drop in blood pressure. The higher the starting blood pressure, the greater the improvement tends to be.

Why does blood pressure drop during fasting?

It is probably a combination of several factors: Less sodium (when you don’t eat, you usually consume much less salt), higher production of natriuretic peptides, and, after a few days of fasting, the regulation of the nervous system may also shift towards a “rest mode” (parasympathetic).

However, the effect is temporary

Several studies suggest that blood pressure improvements often return to baseline around 3–4 months after fasting ends, even when some weight loss is maintained.

Does water fasting sharpen the mind?

Online, fasting is often said to improve focus and “mental clarity”. However, the evidence is much less clear. Results depend on the length of the fast, time of day, type of task (what exactly is being tested) and whether you are used to fasting. One review found that fasting was more often associated with worse cognitive performance, with only a few studies suggesting benefits after a single fast.

Length of fasting and time of day matter

One meta‑analysis found that people tested later in the day performed worse when fasting compared with those who had eaten. In other words, fasting may amplify normal tiredness and dips in performance as the day goes on.

The relationship between fasting length and performance may also be somewhat U‑shaped. Short fasts may have little effect, medium‑length fasts may feel worse as hunger peaks and the body transitions, while longer fasts may sometimes feel more stable once ketosis is established. However, this has limits, and prolonged fasting may still impair performance.

Is it fasting, or just a calorie deficit?

When someone does water fasting, they enter a major calorie deficit because they are not eating.

That deficit alone can explain many of the positive changes often attributed to fasting, including weight loss, improved glycemia and lower blood pressure, etc.

To understand whether fasting has benefits beyond calorie restriction, we need studies that separate:

the effect of a caloric deficit (less energy)
the effect of fasting itself (going longer without food)

What did the research find?

In one study in healthy lean adults, researchers compared three approaches:

Daily energy restriction: participants ate 75% of their usual intake every day.
Alternating‑day fasting with the same overall deficit: participants ate 0% one day and 150% the next day, creating the same average calorie deficit.
Alternating‑day fasting without overall deficit: participants ate 0% one day and 200% the next day, meaning fasting occurred without reducing total calorie intake.

This is exactly the type of design we want:

75:75 vs 0:150 = fasting vs. “isocaloric” (energy‑matched) diet with the same deficit
0:200 = test if fasting has something “extra,” even when total calories remain the same

The result: the calorie deficit mattered

Daily restriction led to greater fat loss than alternate‑day fasting with the same overall calorie deficit.

One possible reason is that people doing alternate‑day fasting spontaneously reduced their movement and natural daily activity.

No unique benefits appeared in the fasting group that could not be explained by calorie deficit.

What to take from this? If your goal is health or weight loss, the main driver is often the calorie deficit. Fasting may be one way to create that deficit, but it does not necessarily have a special advantage when calories are matched.

Potential negatives and side effects of water fasting

With water fasting, it is good to keep a simple rule in mind: the longer the fast, the greater the risk of side effects. Reviews of medically supervised fasting programmes mostly report milder issues, but more serious complications have also been described.

Common side effects

Typical mild adverse effects repeatedly include hunger, headaches, nausea up to vomiting, dry mouth, fatigue and sometimes dizziness and insomnia.

If you experience severe headaches, nausea, dizziness or extreme fatigue, do not interpret this as “detox working”. It may be a sign that your body is under stress and that fasting is not a good fit for you.

Signs to stop water fasting

Stop fasting if you experience:

fainting or near‑fainting or significant “dizziness” when standing up (orthostatic hypotension)
repeated vomiting or inability to keep fluids down
palpitations, irregular heartbeat, chest pressure/pain
rapidly worsening weakness, confusion, significant decline in concentration
any symptom that is worsening instead of staying stable

What to take from this?

Water fasting is an extreme form of calorie restriction. It can temporarily reduce weight and may lower blood pressure or improve some metabolic markers, but it is not a “metabolic reset”. Much of the effect is likely due to being in a deep energy deficit. Similar benefits can often be achieved with a less drastic, more sustainable diet that better preserves muscle mass.

If you still choose to fast, treat it as a period of slowing down. Drink enough fluids, consider electrolytes for longer fasts, and return to eating gradually. If you experience significant dizziness, fainting, repeated vomiting, palpitations or rapid deterioration, it is sensible to end the fast.

FAQs

Is water fasting safe?

It depends on who is doing it and for how long. In healthy people, studies most often describe milder side effects such as hunger, headache, fatigue, dry mouth and insomnia. However, more serious complications have also been reported.

Longer unsupervised fasts are the riskiest, especially if you have a chronic illness, low blood pressure, diabetes, kidney or heart problems, take medication or have a history of eating disorders.

Should I take vitamins during water fasting?

For short fasts, vitamins are usually unnecessary. For longer fasts, it is often more practical to focus on electrolytes rather than vitamins, because sodium, chloride, and later potassium may decrease during prolonged water fasting.

If you do take vitamins, remember that some, especially multivitamins, iron and zinc, can irritate the stomach when taken without food and may worsen nausea.

Can I take medication during water fasting?

Do not stop taking medication simply because you are fasting. However, fasting can affect blood pressure, hydration and electrolytes. This means some medications, particularly blood pressure medication, diuretics and diabetes medication, may become riskier during a fast. Speak to your doctor before fasting if you take regular medication.

Can I drink coffee during water fasting?

It depends on how strictly you define water fasting. In strict water‑only protocols, only water is allowed. That means no tea or coffee.

Black coffee without sugar or milk contains almost no calories, so from a calorie perspective it usually does not “break” a fast. However, it is no longer a true water‑only fast. Coffee may also worsen nausea, reflux, anxiety or poor sleep during fasting. If you do drink it, keep the amount small and monitor how you feel.