Is Water Fasting Bad for You?

You know what they say: evil does not die — it reinvents itself. Most of us may have lived through the juice cleanse and detox craze that promised weight loss, gut reset, blood glucose balance, lower cholesterol, cure chronic disease, and a myriad of other benefits, but here comes the water fast. We all know that all health advice starts with drinking more water, but how healthy is consuming only water for days?

In this article, I explore the safety, potential benefits, and science behind water fasting. This method is mostly used in a clinical setting and is not recommended for home use. I also sought expert opinions for deeper insights.

What is water fasting?

Various forms of fasting have been gaining popularity, and one of the more intense ones is water fasting. As the name suggests, this type of fasting only allows the consumption of mineral or distilled water without food for a prolonged period of time. The main difference from other fasting protocols, like intermittent fasting, is that food is not restricted to certain hours but is not allowed at all during a water fast.

The main claimed reasons why some individuals promote or choose to do water fasting are:

• Weight loss
• Blood pressure reduction
• Cholesterol reduction
• Glycemic control

That being said, the research on these potential benefits is still inconclusive and controversial. Furthermore, it has been observed that all metabolic markers tend to return to their pre-fasting values after 3–4 months after completing the fast.

Stages of fasting

There are no set guidelines on how to and for how long to water fast, but most studies opt for 5–20 days. The approach is pretty straightforward: you consume nothing but water, about 2–3 liters, for several days. Some studies suggest using mineral water, while others opt for distilled.

However, drinking distilled water may not be the best option in general, as it is missing its minerals. During a water fast, you are already not consuming any minerals and vitamins, and further removing them by drinking distilled water can be dangerous.

There are several stages of fasting.

After 12–36 hours of starting the fasting period, the body starts using fatty acids and ketones derived from fatty acids instead of glucose for energy. Ketone levels plateau around 5–10 days of prolonged fasting and can serve the body as a primary energy source for up to 90 days. You may also have heard about ketones in conversations about the keto diet. It has been suggested that using ketones as fuel may help with blood pressure regulation, vascular function, inflammation reduction, appetite control, and insulin sensitivity. However, it comes with a myriad of risks, and the research on these claimed benefits is neither conclusive nor substantial to support the efficacy.

Refeeding

The fast is followed by a refeeding phase, where you start eating again. There is a lack of scientific guidance on the best way to do so, but you may want to start with lighter foods like soups and smoothies that are easier to digest. Monitor how you feel and start incorporating balanced meals.

Regardless of your goals, you should follow a healthy and balanced diet after the fast if you want to see sustainable change.

Sandra Vigelienė, who has a degree in Global Nutrition and Health, advises:

Is water fasting safe?

Water fasting is generally proclaimed a moderately safe diet therapy, however, it comes with several side effects, such as fatigue, insomnia, nausea, and more:

Sometimes, prolonged fasting may also lead to more serious health concerns, especially if not done under supervision from your healthcare provider.

• Abnormal liver function. A few studies have documented abnormal liver function, although most studies reported unaffected liver enzymes after 5–10 days of fasting.
• Renal function. Some studies reported a temporary elevation in renal function markers, and one study reported decreased renal function. However, kidney function was generally preserved.
• Electrolyte imbalance. After 8–10 days of fasting, circulating levels of sodium and chloride decreased just below acceptable limits in most studies. To prevent electrolyte loss, it may be useful to supplement with electrolytes during a water fast.
• Ketone levels. Prolonged fasting may increase circulating ketone levels, which can be dangerous for people with type 1 diabetes.
• Severe hunger. Only drinking water without consuming any food can lead to severe hunger that makes adherence difficult or even triggers binges or disordered eating in some.

People with underlying health conditions, pregnant or breastfeeding individuals, and children should not do water fasting unless otherwise advised by their doctor and should do so under supervision.

Expert opinion on water fasting misconceptions

I reached out to Dr. Alan Goldhamer, an expert on clinical water fasting. He has written several books about the benefits of fasting and has a clinic that offers on-site fasting. I asked what are the common misconceptions surrounding fasting.

Some think fasting and starving are synonymous. Fasting is the period of time when labile tissues still exist and can last 40–70 days in a 70 kg male. Once available reserves have been depleted, you enter starvation, and death will then ensue. We only fast people who have that reserve and monitor patients with laboratory testing and twice daily examinations with doctors experienced in fasting supervision. We have done this with 25k patients in the past 40 years.

Dr. Alan Goldhamer

Potential benefits of water fasting

Water fasting is promoted and studied for several health benefits, so I decided to investigate the science behind its preached benefits.

Weight loss

A meta-analysis of the efficacy and safety of prolonged water fasting found that a 5-day fast led to 4–6% weight loss, a 7–10-day fast to 2–10%, and a longer fast of 15–20 days led to 7–10% weight reduction. The results were seen in various groups: individuals with normal weight, people who are overweight or obese, and those with type 1 or type 2 diabetes. All studies that looked at waist circumference found a significant decrease of 3–9 cm. Reduction in waist circumference correlated with the amount of weight lost.

Two studies from the above-mentioned meta-analysis compared fat vs. lean mass loss and found that water fasting resulted in about two-thirds more lean mass loss than fat mass reduction. Losing lean mass indicates that fasting causes muscle breakdown, which is never a desired body composition change. Also, losing more muscle mass in comparison to fat increases your body fat percentage, which is associated with worse health.

Long-term weight loss requires sustainable approaches. While water fasting can help you lose weight in a short period of time, most studies report partial or full regain in about three months after completing the fast.

Furthermore, completely restricting your food intake for several days may lead to binges post-fasting, severe digestive issues, and cravings, as your body will try to compensate for the ‘starvation’ it just went through. That is not to mention the risks associated with certain health conditions and water fasting.

I would not suggest opting for water fasting in order to lose weight. Lifestyle changes, a moderate calorie deficit, and movement will give you much more sustainable results and improve your health.

Lower blood pressure

Water fasting has been studied for its potential ability to improve blood pressure (BP). This was confirmed by a meta-analysis that has found consistent reductions in systolic (9–14 mm Hg) and diastolic (6–13 mm Hg) BP if at least 4% weight reduction was achieved.

A study of 1,422 subjects used Buchinger's periodic fasting, which is similar to water fasting but allows 200–250 calories of food per day. The evidence suggests that people with higher BP at baseline experienced greater improvements. The same correlation was observed in all fasting groups: five, 10, 15, and 20 days.

It is not entirely clear why fasting causes changes in BP, but it may be due to a reduction in sodium intake and a shift in circulating peptides and ketones resulting from the lack of food intake.

The available studies do point to positive changes in blood pressure with prolonged water fasting. However, improvements in systolic and diastolic BP are associated with moderate (5–10 %) weight reduction, regardless of the method. Thus, it is hard to discern whether BP changes are due directly to water fasting, no sodium intake, weight loss, or a combination of all three.

If you have concerns about high blood pressure, I would advise you to consult with a doctor and follow their advice regarding diet and lifestyle changes and prescribed medications. You may want to bring up water or Buchinger's fasting during the consultation, but you should only do it with the permission of your healthcare provider.

Cholesterol control

While most studies show that fasting reduces weight and blood pressure temporarily, the changes in cholesterol are highly diverse.

A meta-review summarized seven studies that looked at the effects of water fasting on low-density lipoprotein (LDL) cholesterol and found that three studies showed a decrease, two studies an increase, and two reported no change. On the other hand, high-density lipoprotein (HDL) cholesterol decreased by 7–18% in four studies and remained unchanged in three studies. In both cases, the level of decrease correlated with the length of the fast.

Furthermore, studies from the same meta-analysis that looked at refeeding found that cholesterol levels return to baseline after the fast is over, even if weight loss is maintained.

Water fasting does not seem like an effective strategy for managing cholesterol levels, with the results varying greatly between studies and improvements being very temporary. If you are concerned about your cholesterol level, I would suggest looking at your lifestyle, especially diet and exercise, and consulting your doctor to see if you need to take medication.

Glycemic control

Fasting may help reduce blood glucose levels and potentially improve insulin sensitivity in some individuals.

A study of 1,422 participants with obesity who followed Buchinger's periodic fasting for 4–21 days found that the protocol improved their blood glucose levels, bringing them down to normal.

Another clinical trial on people with obesity without diabetes found that fasting for 17 days increased insulin resistance score despite improving some other metrics. This highlights the complexity and the need for more studies on fasting and insulin resistance.

A non-randomized controlled pilot study of 20 participants with type 1 diabetes found no significant improvements in blood glucose after a 7-day water fast. However, on the last day, the daily insulin dose was decreased from 24 units of insulin (IU) to 8 IU.

Studies on water fasting for glycemic control are pretty scarce, with most evaluations coming from what is generally known about fasting. Fasting does not seem to improve blood glucose levels in people with diabetes. One study has observed improvements in blood glucose levels in people without diabetes; however, another has seen an increase in insulin resistance, requiring more studies and caution regarding fasting and blood glucose control.

Writer’s insight

I hold a Master’s in Biomedical Sciences and live and breathe health, wellness, nutrition, and exercise trends. Looking at the science, I do not see water fasting as some miracle cure for all diseases. In fact, as discussed above, most benefits lack uniform supportive evidence and have contradicting studies.

Urtė Gumuliauskaitė, MSci

Furthermore, consuming no food at all for several days can be dangerous, even if you have a lot of fat stores to burn through, and the weight lost is most likely to be water weight. Lastly, it can feel very exclusive and off-balance, as eating is an integral part of our lives. You are likely to feel tired and have low energy and not many of us have the luxury to take time off for 20 days to water fast.

If you are interested in the benefits of fasting for health, weight management, or longevity reasons, I would suggest looking into intermittent fasting (IF). It involves eating during a restricted eating window but does not necessarily involve cutting calories or drastically changing your habits. IF can also be done essentially as long as you want to, thus bringing on more sustainable and long-term benefits, as it encourages habit and lifestyle change. However, despite being a lighter approach, IF still needs to be approached with caution as it can cause some blood sugar fluctuations or nutrient deficiencies if not done properly.

Safer alternatives

Water fasting is quite extreme, and regardless of the benefit you seek, there are several safer and more sustainable alternatives.

• Intermittent fasting. One of the most popular forms of fasting is eating only for a set number of hours and fasting for the rest. The most common method is eating for eight hours and fasting for 16 hours. Intermittent fasting has been suggested to reduce waist circumference, fat mass, insulin levels, LDL, and total cholesterol and increase fat-free mass in healthy individuals. However, the research on the benefits of all fasting forms is still contradictory.
• Calorie deficit. If you want to lose weight, you need to be in a calorie deficit created by diet and exercise. How much weight you lose will depend on the length of the deficit and your adherence. The typical approach is a 500-calorie deficit, as you should never starve yourself with too big a deficit. A gradual change in calories may help with adherence and sustaining the deficit.
• Healthy diet. If you do not have underlying medical conditions, following a healthy diet may help you manage your blood glucose and cholesterol levels. This also ensures you are getting essential nutrients, which is the biggest difference when you compare fasting/water fasting.
• Regular exercise. Staying active and engaging in physical exercise may reduce the risk of several chronic diseases.

No matter your goals, the key pillars of health remain a healthy diet, regular exercise, sleep, and a balanced lifestyle.

Bottom line

Water fasting is one of the most extreme forms of fasting, as it allows for no food. Despite being promoted for several health reasons, it still lacks solid scientific backing.

I would not advise doing a water fast for fun at home and would encourage you to do a lot of research and to carefully consult with your doctor before considering it.

Other forms of fasting, like time-restricted eating, may offer the same potential benefits in a more sustainable way. If your primary goal is to lose weight, you need to be in a caloric deficit for long enough to lose it and develop better lifestyle habits to maintain the new weight. No miracle diet or fast will give you a shortcut.