Water fasting for health: Stacking up the claims against the research

Among those who pursue extremes in the name of health and longevity, water fasting has gained traction.

Yet, new research suggests the practice, whereby people consume nothing but water for varying periods of time, may not benefit our health in the way people think – and can come with serious risks.

Athlete Lachlan Clancy says water fasting left him feeling energised.Credit: Josh Hilliam

Many ancient civilisations practised fasting because they believed it enhanced their physical and spiritual health. Forms of fasting include intermittent fasting, in which energy intake is limited on certain days of the week; time-restricted eating, whereby energy is consumed within a specific window of time each day, typically about eight hours; bone broth, juice or smoothie fasts, in which a limited intake of nutrients is consumed in liquid form; and water fasts, in which nothing but three or more litres of water is consumed for the duration of the fast.

Bold claims and wishful thinking

Today, the popularity of water fasting has been revived by podcasters such as Joe Rogan, Andrew Huberman and Gary Brecka (who sells a range of supplements and products he claims help to “ease” you in and out of the fast).

This is despite various risks associated with water fasts, including headaches; gout; metabolic acidosis, in which the body’s pH drops too low; hyponatremia (abnormally low sodium in the body); dehydration; loss of lean muscle mass; risk of exacerbating eating disorders and diabetes; and, in extreme cases, death.

Supporters point to the “magic” that happens when we fast, and mention improved brain and metabolic health, weight loss, reduced inflammation, longevity, and increased autophagy – the body’s process of clearing away damaged or unused cells so it can renew itself and function more efficiently.

Many claims, however, are scientifically unsupported, based on rodent studies or extrapolated from human studies looking at calorie restriction.

In animal studies, calorie restriction does increase autophagy. And in humans, chronic calorie restriction without malnutrition can have a powerful anti-inflammatory effect.

But, just because something works in animals, doesn’t mean it works in humans. And just because there is an effect from calorie restriction, doesn’t mean a more extreme form of fasting will enhance the effect. Calorie restriction and intermittent fasting typically involve reducing energy intake by about 20 per cent, not removing nutrients altogether.

“[Prolonged fasting is] a completely different metabolic and physiological state,” says Professor Luigi Fontana, director of the Healthy Longevity Research Group at the University of Sydney.

Of fasting, he says: “There’s a lot of wishful thinking and inaccurate conclusions. News and socials are loaded with partial truths and misinformation.”

Now, researchers are interested in better understanding the effects.

Surprising findings

For a new study, published in Molecular Metabolism, Fontana and his team sought to test the biological effects of prolonged fasting (defined as four or more days) with the hypothesis that it would reduce markers of inflammation.

Twenty volunteers were examined before, during and after a water-only fast, and contrary to the hypothesis, the researchers found prolonged fasting led to increased inflammation and a prothrombotic environment, which can increase the likelihood of blood clots in at-risk people.

The effects were transient and participants returned to normal when they began eating again.

“I don’t understand why the body is reacting that way,” Fontana says.

He speculates that it may be the result of lipolysis, which is the metabolic process of breaking down triglycerides into free fatty acids and glycerol to fuel the body during fasting or intense exercise. He says that perhaps during this process the liver is being flooded with excess fat or the toxins stored in the fat, leading to inflammation.

It may also be due to a stress response. “Indeed, we see a huge increase in cortisol,” says Fontana. “The body perceives fasting as a stressful condition.”

For people with a high risk of thrombosis or unstable plaque, Fontana says: “Even a bout of inflammation and prothrombosis could potentially lead to a major event.”

Associate Professor Tim Sargeant, a cell biologist at the South Australian Health and Medical Research Institute who was not involved with this study, was surprised by the findings.

Clancy consumed nothing but four litres of water a day when water fasting.

“In popular culture, fasting is seen as a way to reduce something like this,” Sargeant says. “I think what Luigi has shown is that an intervention of this magnitude is more complex than what people realise.”

In January, 34-year-old athlete Lachlan Clancy completed a three-day water fast in which he consumed nothing but four litres of water a day.

Clancy, inspired by Gary Brecka’s podcasts on water fasting, did it as a mental challenge and to activate a gut reset. There is some evidence that fasting triggers changes in the gut microbiome, but those changes don’t last.

He also hoped it would kickstart autophagy: “For me, longevity is quite a big one.”

This was the second time he had done such a fast, and by day three, he says, he felt energised and clear. He would not, however, fast several weeks before a Hyrox fitness competition as he did this time: “I didn’t quite appreciate the stress it puts on your body.”

Water fasting: A complex reality

Royal Australian College of General Practitioners obesity management chair Dr Terri-Lynne South says the biological changes of fasting are intriguing and may hold promising health benefits.

“In practice, fasting interventions are often short-term, and the biological changes they induce typically revert to baseline once regular eating patterns are resumed,” she says.

South advises medical supervision for anyone considering a prolonged fast, reminding that risks increase in likelihood and severity with the duration of a fast.

“Ensuring safety and adherence can also become more challenging as fasting periods lengthen,” she says.

For his part, Fontana says he isn’t against fasting. “I’m agnostic,” he says, adding that if researchers can prove it increases autophagy, then there will be a demonstrable benefit.

“The message is, be careful” he says. “Fasting is not for everyone. It does cause a pro-inflammatory, prothrombotic response.”

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