Water Fasting For Weight Loss: Does It Work And Is It Safe?

Could water fasting increase weight loss and how many days consuming nothing but water is safe?

Could water fasting increase weight loss and how many days consuming nothing but water is safe?

Water fasting is a type of diet plan undertaken by people who want to lose more weight quickly.

A water fast can last for five days or more and during this period a person will eat nothing but drink water and natural zero calorie beverages.

Scientists have reviewed the evidence for water fasting and Buchinger fasting, a medically supervised diet therapy consisting of taking very little food for several days to almost three weeks.

The research reveals that fasting for five to 20 days increases ketones production due to the body breaking down fat for energy, and promotes weight loss up to ten percent of body weight.

However, four months after the fast ends, the metabolic benefits such as reduced blood pressure, reduced LDL (bad) cholesterol and triglycerides, improved insulin sensitivity and blood sugar levels seem to fade.

The adverse effects were metabolic acidosis (too much acid in the body), insomnia, hunger, and headaches.

Otherwise, a water fast or a similar diet therapy where a person consumes very few calories daily did not cause any severe injury.

Professor Krista Varady, the study’s lead author, 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.”

Five days maximum

However, Professor Varady advised that without medical supervision, people should not use these type of fasts for more than five days.

The review analysed eight studies on prolonged water fasting or Buchinger fasting where fasters under medical supervision consume a tiny amount of vegetable soup, juice, tea, and water.

The findings show that the weight loss effect after a short period of time was visible.

In five days participants lost four to six percent of their weight, those on seven to 10 days fasting lost between two to 10 percent, and those on 15 to 20 days fasting slimmed down by seven to 10 percent.

After the fasting period was over, participants maintained their weight loss if they followed a restrictive calorie intake, otherwise in three months they gained the weight back.

About one-third of the weight loss was fat mass and two-thirds was lean mass suggesting that extreme fasts cause people to lose more muscle than body fat.

Professor Varady said:

“Your body needs a constant intake of protein.

If it doesn’t have that, then it draws from muscles.”

Professor Varady added she prefers those who want to lose weight to consider intermittent fasting instead of water fasting,

“Because there’s a lot more data to show it can help with weight management.”


The study was published in the journal Nutrition Reviews (Ezpeleta et al., 2023).

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