Research from the University of Illinois at Chicago shows that while "water fasting"—drinking only water for several days—can lead to weight loss, it's uncertain how long the weight loss lasts. Additionally, any metabolic benefits, such as lower blood pressure and improved cholesterol levels, appear to disappear quickly after the fast ends.
Still, Krista Varady, a professor of kinesiology and nutrition who led the study, said there are no obvious negative effects of water fasting or similar regimens that consume very few daily calories. The study was recently published in the journal Nutrition Reviews.
"My overall conclusion is, I guess you could give it a try, but it seems like it would be a lot of work and all these metabolic benefits would be lost," Varady said. However, she stressed that no one should fast for more than five days without a doctor's guidance.
Varady, an expert on intermittent fasting, said she wanted to study water fasting because reporters suddenly started contacting her last fall to hear her thoughts on water fasting. If she were to comment, she thought, she should investigate existing research.
The new paper is a literature review of eight studies on fasting. Buginger fasting is a medically supervised fast popular in Europe in which people drink only minimal amounts of juice and soup each day. Varady's team looked at the results from each of these papers to see what impact they had on the effects of fasting on weight loss, as well as some other metabolic factors.
Researchers found that fasting did appear to promote significant short-term weight loss. People who fasted for five days lost about 4% to 6% of their weight; people who fasted for seven to 10 days lost about 2% to 10%; people who fasted for 15 to 20 days lost about 7% to 10%.
Only a few studies have tracked whether participants gained weight again after fasting ended. In one study, people who fasted from water for five days regained all their weight within three months. In two other studies, only a small amount of the weight lost was regained, but those studies encouraged participants to limit caloric intake after the fast ended.
In contrast, the metabolic benefits of fasting apparently disappeared shortly after the fast ended. Improvements in blood pressure, cholesterol and blood sugar levels were short-lived and returned to baseline levels soon after participants started eating again.
Some study participants had type 1 and type 2 diabetes, and although they were closely monitored and their insulin doses adjusted during the fasting period, they did not experience adverse effects from fasting.
Varady said the most common side effects of long-term fasting are similar to those of intermittent fasting, such as headaches, insomnia and hunger. No serious adverse effects, such as metabolic acidosis or death, occurred in the study.
She did note that participants who fasted for long periods of time lost about two-thirds of their lean body mass and one-third of their fat body mass. This is contrary to most situations during weight loss, where more fat is lost than muscle is lost. The results of such extreme fasting are justified, she says, because "your body needs a constant intake of protein. If you don't have protein, it's going to get it from your muscles."
Varady's research on intermittent fasting includes its effects on weight loss, as well as specific questions like whether intermittent fasting affects fertility -- she found it doesn't.
Varady said she encourages people who want to lose weight to try intermittent fasting rather than water fasting "because there is more data showing that intermittent fasting helps with weight control," she said.