Water fasts may help people lose weight temporarily but there is little evidence to show whether the weight stays off long term, researchers have said.

Water fasting is where people consume nothing but water for a number of days.

The new study also showed that the metabolic benefits associated with water fasting, including lower blood pressure and improved cholesterol, disappear once fasting ends.

Krista Varady, professor of kinesiology and nutrition at the University of Illinois Chicago, said: “My overall conclusion is that I guess you could try it, but it just seems like a lot of work, and all those metabolic benefits disappear.”

No one should continue with this sort of fasting for longer than five days without consulting a medical expert, she said.

Professor Varady, who is a specialist in intermittent fasting, decided to investigate water fasting after being contacted by a number of journalists about the subject. Wanting to know more about water fasting if she was being asked to comment on it, Professor Varady reviewed the findings from existing research.

She and her team looked at eight studies involving water fasting or another type of fasting, Buchinger, which is when someone consumes only a very small amount of juice or soup each day, under the supervision of a medical professional.

The team found that these types of fasts were linked to weight-loss in the short term:

  • Five days’ fasting resulted in around 4% to 6% weight loss;
  • Seven to 10 days’ fasting was linked to weight loss of between 2% and 10%;
  • Fasting for 15 to 20 days led to 7% to 10% weight loss.

However, only a small number of these studies monitored the participants’ weight loss longer term; in one, participants re-gained all the weight they had lost following a five-day fast within three months.

Two other studies found that participants only re-gained a small amount of weight, but these people had been urged to reduce their calorie consumption once the fast ended.

What was more clear was how the initial metabolic benefits from fasting – improvements in blood pressure, cholesterol and blood sugar levels – soon disappeared once fasting ended.

While there were no serious side effects noted in the studies, reports of headaches, insomnia and hunger were common.

The review also found that intermittent fasting does not affect fertility.

Professor Varady said she would recommend intermittent fasting over water fasting “because there’s a lot more data to show it can help with weight management”.

Read the full study in Nutrition Reviews.

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