Water fasting involves only consuming water. Some other names for it are extended fasting and prolonged fasting.
Water fasts typically last 24–72 hours.
This approach can be traced back thousands of years, and it has recently become fashionable. People often try it because they want to improve their health and lose weight.
But is there any convincing scientific evidence that water fasts work? Not really.
Is it safe? Based on very limited research, water fasting appears to be safe, but only in carefully controlled settings. There are other considerations, too — we’ll explore them in more depth below.
If you’re thinking about water fasting, ask a healthcare professional for guidance first.
The research so far
To date, very few studies have explored the health effects of water fasting.
One small study from 2021 included 12 middle-aged men, and it tested whether water fasting was safe.
After the participants had fasted for 8 days, the scientists found significant changes in their blood and urine markers. They also saw changes in body composition, including reduced weight, and lower perceived levels of stress.
The researchers concluded that while water fasting for 8 days was safe, it could have damaged the participants’ health if they’d continued.
It’s important to note that the participants were all nonsmokers with no history of chronic health conditions. They had healthy blood pressure and weight at the start, were taking no medications, and they’d all tried water fasting before.
Another study, from 2022, recruited 48 men and women with overweight or obesity. The participants each did a water fast for at least 10 days.
The researchers found that fasting was associated with decreases in weight and body mass index scores. There were also decreases in blood pressure.
But, as the authors note, there was no control group for this study. So, we can’t know whether other factors played a part in these changes.
Strikingly, almost 50% of the participants dropped out of the study. This highlights the challenges of only consuming water for extended periods.
Some of the reasons that the participants left the study included headaches, feeling unwell, acid reflux, fatigue, anxiety, cramping, panic attacks, vomiting, and heart palpitations.
As with the previous study, the participants were all healthy at the start and were closely monitored by healthcare professionals throughout their fasts.
Lastly, a review looked at adverse events during medically supervised water fasts that lasted at most 2 days. The participants were all in a hospital setting.
The researchers found that most adverse events were mild to moderate, and these included fatigue, nausea, difficulty sleeping, back pain, indigestion, and headaches.
Is water fasting worth it?
In short, probably not.
There’s very little evidence that water fasting is a healthy approach to managing health or losing weight.
Weight loss is inevitable if you’re eating nothing, but we know that moving toward a healthier lifestyle often involves small, simple changes that are realistic and manageable.
What’s more, the weight loss likely stems from the body releasing glycogen (energy) stores from the liver and muscles. These stores hold a lot of water.
So, much of the weight lost during water fasting is simply water.
Plus, some weight loss is likely from muscle, and we generally want to preserve muscle.
Meanwhile, there’s been little research into how quickly people regain the weight lost during an extreme fast.
Overall, water fasting can be hard to sustain, even in the short term. And understandably so.
It’s also likely that any side effects of water fasting would impact your day-to-day activities, at work or school and in your free time.
Finally, you’re more likely to feel tired, so you might not feel like staying active. And we know that physical activity is key for maintaining your overall health and a moderate weight.
It’s important to note: Health experts don’t recommend trying water fasting at home. Going it alone is very different from fasting under the supervision of a healthcare professional, who regularly makes sure you’re safe.
Risks of water fasting
Water fasting carries risks, including side effects such as mood changes, migraine headaches, and disordered eating. It can make you feel physically and mentally drained, too.
And this type of fasting can be particularly dangerous for certain people. For example, pregnant people, those with type 1 diabetes, and anyone who needs to take medication with food should avoid it.
Unlike intermittent fasting, which we’ll touch on later, water fasting is at the extreme end of the fasting spectrum, in part because people often try it for long periods.
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And even after you’ve finished an extended water fast, there’s another risk — refeeding syndrome.
This is a potentially fatal complication that can develop when people reintroduce food too quickly after a prolonged fast. This can trigger severe shifts in levels of fluids and electrolytes.
In the 2021 and 2022 water fasting studies we mentioned above, the scientists were careful to reintroduce foods in a safe, gradual way.
Intermittent fasting is a safer alternative to water fasting. It involves focusing on when you eat rather than what you eat.
Several studies have shown that intermittent fasting can lead to weight loss.
Although it’s still early days, emerging research suggests that intermittent fasting may positively affect brain health. It may, for example, improve mental health and memory.
There are several types of intermittent fasting, and one is time-restricted eating (TRE).
It involves eating within a set window of time, which may last for 4, 6, or 8 hours, for example. The rest of the day you fast.
The good thing about TRE is that it’s flexible — you pick the time frame that suits you.
And unlike water fasting, TRE allows you to meet your daily nutrient requirements.
But, as with all fasting approaches, there’s not much evidence about the long-term effects. And there are still many questions about how sustainable intermittent fasting is.
ZOE’s latest study — The Big IF Study, which more than 100,000 people joined — aims to answer some of the key questions about TRE.
Early findings from this research show that some common benefits are increased energy, better digestion, less bloating, and weight loss.
Remember, like all fasting approaches, intermittent fasting may be unsuitable for certain people.
If you’re unsure, ask a healthcare professional, like a registered dietitian, whether this approach would be safe to try.
While water fasting may seem attractive, very little evidence suggests that it’s a good way to stay healthy or lose weight.
Achieving a healthier lifestyle often involves making small, realistic changes that are easy to manage.
Water fasting, on the other hand, is an extreme approach at the far end of the fasting spectrum.
A type of intermittent fasting, like TRE, may be a safer alternative since it offers more flexibility. Plus, several studies on intermittent fasting have found some health benefits.
Still, intermittent fasting may not be safe for certain people. It’s a good idea to ask a healthcare professional for guidance if you have any concerns.
The Big IF Study aims to answer some key questions about intermittent fasting. Watch this space.
Effects of intermittent fasting and energy-restricted diets on lipid profile: A systematic review and meta-analysis. Nutrition. (2020). https://www.sciencedirect.com/science/article/abs/pii/S0899900720300848
Effects of time-restricted feeding on body weight and metabolism. A systematic review and meta-analysis. Reviews in Endocrine and Metabolic Disorders. (2020). https://link.springer.com/article/10.1007/s11154-019-09524-w
Is fasting safe? A chart review of adverse events during medically supervised, water-only fasting. BMC Complementary and Alternative Medicine. (2018). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5819235/
Is water-only fasting safe? Global Advances in Health and Medicine. (2021). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8369953/#__ffn_sectitle
Refeeding syndrome: What it is, and how to prevent and treat it. BMJ. (2008). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2440847/
The effectiveness of intermittent fasting to reduce body mass index and glucose metabolism: A systematic review and meta-analysis. The Journal of Clinical Medicine. (2019). https://www.mdpi.com/2077-0383/8/10/1645
The effects of prolonged water-only fasting and refeeding on markers of cardiometabolic risk. Nutrients. (2022). https://www.mdpi.com/2072-6643/14/6/1183