Experts at the University of Sydney are urging people, especially those with existing heart or vascular conditions, to seek medical advice before attempting to lose weight using water-only fasting diets. 

The study, published in Molecular Metabolism, was conducted by a team of researchers in Australia, Italy and the US. It examined the impact of prolonged fasting – a diet in which people abstain from consuming calories and instead drink only water for more than four days at a time – on people’s health. 

Prolonged fasting diets have soared in popularity in recent years and have been proven to lead to rapid weight loss, but little is known about the long-term health implications of such diets. 

Lead researcher, Professor Luigi Fontana from the Charles Perkins Centre, said: “Our hypothesis was that prolonged water-fasting would reduce inflammation in the body. 

“However, we found the opposite was true – that prolonged fasting put stress on the body and increased the number of pro-inflammatory proteins in the blood, potentially increasing the risk of health issues for people with existing heart and vascular conditions.” 

In the study, researchers recruited 20 participants (11 women and nine men) to take part in a trial that comprised a 10-day water-only fast followed by 5 days of refeeding, observing their health before, during and after the trial. 

While the participants saw an average 7.7 percent reduction in body weight and a 6 percent decrease in waist circumference, the participants also reported experiencing symptoms such as headaches, insomnia and low blood pressure.

Professor Fontana said: “People are looking for effective ways to manage their weight and while the effects of water-only fasting can be dramatic, more work needs to be done to understand the impact of these diets on the body over a longer period of time.

“For people with existing health conditions – especially heart and vascular issues – we strongly recommend that they speak to their doctor before trying out fasting diets.” 

The study also found that water-fasting reduced amyloid beta proteins – associated with the development of Alzheimer’s – in the blood, suggesting that prolonged fasting could have neuroprotective benefits, although the current evidence is inconclusive and requires further research to establish any potential long-term effects on brain health. 

Hero image: Adobe Stock Photos

Research

Commissati, S, Fontana, L, et al., ‘Prolonged fasting promotes systemic inflammation and platelet activation in humans: A medically supervised, water-only fasting and refeeding study’ (Molecular Metabolism, 2025)

DOI: https://doi.org/10.1016/j.molmet.2025.102152

Declaration

Professor Luigi Fontana reports no conflicts of interest.