Water fasts may help with weight loss, but the benefits are short-lived

CHICAGO — Are water fasts an effective path toward sustained weight loss? Probably not, according to researchers from the University of Illinois Chicago. Study authors find that while a water fast (consuming nothing but water for several days) may help shed pounds in the short term, it’s unclear just how long that weight will stay away once solid foods come back into play.

Moreover, the other observed metabolic benefits of water fasts, such as lower blood pressure and improved cholesterol, appear to “disappear” quite quickly once a water fast ends. Despite all that, study authors also say they did not observe serious adverse effects associated with engaging in a water fast or any similar type of fast in which the dieter consumes a very small number of calories daily.

“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,” says project leader and professor of kinesiology and nutrition at UIC, Krista Varady, in a university release.

Prof. Varady also stresses that no one should try one of these fasts for more than five days without medical supervision. Varady is considered an expert on intermittent fasting, and explains she set out to conduct this specific study focusing on water fasting because she was recently contacted by multiple journalists asking about her thoughts on the subject.

This latest report is a review of eight studies focusing on either water fasting or Buchinger fasting — which is a medically supervised fast quite popular in Europe in which people consume nothing but tiny amounts of juice and soup daily. Prof. Varady and her team analyzed the results from all of those earlier papers in an attempt to uncover just how effective these approaches are when it comes to weight loss, as well as numerous additional metabolic factors.

Fasting did indeed appear to spark noticeable short-term weight loss. Those who fasted for five days lost about four to six percent of their weight. Others who fasted for seven to 10 days lost roughly two to 10 percent of their weight and those who fasted for 15 to 20 days lost seven to 10 percent.

Weight loss: woman measuring her waist size
Photo by Bill Oxford on Unsplash

Just a handful of studies in the review tracked if participants gained that weight back once the fast ended. During one of those studies, people gained back all of the weight they had lost during a five-day water fast within three months. During two other studies, just a small amount of lost weight returned, but those projects in particular had encouraged dieters to continue restricting their calorie intakes after the fasts ended.

Conversely, researchers say it’s also clear that the metabolic benefits of these fasts tend to disappear quickly after people return to their usual diets. Noted improvements in cholesterol, blood sugar, and blood pressure all faded fast, eventually returning to baseline levels after participants started eating again.

Some of the analyzed earlier studies featured patients with Type 1 and Type 2 diabetes. These participants dealt with no negative side-effects tied to the fasting, but they were monitored very closely and had their insulin doses adjusted while fasting.

The most common side-effects associated with these prolonged fasts were similar to those seen from intermittent fasting, including headaches, hunger, and insomnia. However, no serious negative side effects (metabolic acidosis, death) were seen across studies.

All in all, participants in these prolonged fasts lost about two-thirds of their weight in lean mass and one-third in fat mass. Interestingly, that’s just about the opposite of what usually occurs during weight loss, where more fat is lost than muscle. According to study authors, it makes sense such extreme fasts would result in this, because “your body needs a constant intake of protein. If it doesn’t have that, then it draws from muscles.”

Prof. Varady’s research into intermittent fasting has also analyzed how well the regime works for weight loss, as well as more specific questions, such as whether intermittent fasting affects fertility.

In conclusion, Prof. Varady says she would encourage someone 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.”

The study is published in the journal Nutrition Reviews.

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