Water fasts are when a person consumes nothing but water for several days, and it might help with weight loss, lower blood pressure, and improve cholesterol levels, over the short term. According to this study, water fasts or Buchinger fasting (water, juice, and broth/soup medically supervised fasting) are probably not the most effective way to obtain sustained weight loss, while it may help to drop some unwanted pounds in the short term, it is unclear how long the weight will stay off once you start eating solid foods again.
“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,” said Krista Varady, professor of kinesiology and nutrition, who led the research. She stressed, however, that no one should undertake one of these fasts for more than five days without medical supervision.
This study reviewed 8 studies on water fasting or Buchinger fasting to analyze the results to determine the cumulative impacts on weight loss and several other metabolic factors. The water fasting did appear to “spur noticeable short-term weight loss”, with those fasting for 5 days losing about 4-6% of their weight, those fasting for 7-10 days losing 2-10%, and those fasting for 15-20 days losing about 7-10% of their weight.
Varady notes that about two-thirds of their weight loss was in lean mass and one-third was in fat mass which is the opposite of what would typically happen during weight loss, which would be where more fat is lost than muscle. However, she said that this makes sense that these types of extreme fasts would have these results because “your body needs a constant intake of protein. If it doesn’t have that, then it draws from muscles.”
Only a few studies tracked weight regain after the fast ending. One study reported that participants gained back all the lost weight from a 5-day fast within three months, and two other studies reported that only a portion of the weight returned but these studies encouraged sustained restricted calorie intake after their fasts ended.
As for metabolic benefits, it was clear that these disappeared soon after the fasts concluded, with any improvements to blood pressure, cholesterol levels, and blood sugar levels returning to baseline levels rapidly after the participants started consuming solid foods.
The most common side effects were headaches, insomnia, and hunger, and there did not appear to be very serious adverse side effects for those who did a water fast or similar kind of fast wherein people consumed a small number of calories per day. Despite there being no serious negative side effects across the studies, the researchers encourage those hoping to lose weight to try intermittent fasting over a water fast because “there’s a lot more data to show it can help with weight management.”
