Want to lose weight quickly? Water fasts -- where people consume nothing but water for several days -- might help you, but it’s unclear how long you’ll keep it off, according to research.

Researchers from the University of Illinois Chicago showed that the metabolic benefits of water fasts, such as lower blood pressure and improved cholesterol, also seem to disappear soon after the fast ends.

However, there do not appear to be any serious adverse effects for those who do a water fast or a similar kind of fast where people consume a very small number of calories a day, said Krista Varady, professor of kinesiology and nutrition, who led the research, which is published in Nutrition Reviews.

"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 Varady, an expert on intermittent fasting.

She stressed, however, that no one should undertake one of these fasts for more than five days without medical supervision.

The new study is a review of eight studies on water fasting or Buchinger fasting, a medically supervised fast that is popular in Europe where people consume only a tiny amount of juice and soup a day.

Fasting did seem to spur noticeable short-term weight loss, the researchers found. People who fasted for five days lost about 4 per cent to 6 per cent of their weight. Those who fasted for seven to 10 days lost about 2 per cent to 10 per cent, and those who fasted for 15 to 20 days lost 7 per cent to 10 per cent. However, people gained back all they had lost in a five-day water fast within three months.

Some of the studies included participants with Type 1 and Type 2 diabetes, who suffered no ill effects from the fasting, though they were monitored closely and had their insulin doses adjusted while fasting.

The most common side effects of these prolonged fasts were similar to those from intermittent fasting, Varady said, such as headaches, insomnia and hunger. There were no serious negative effects in the studies, such as metabolic acidosis or death.

Varady noted that the participants in these prolonged fasts lost about two-thirds of their weight in lean mass and one-third in fat mass. This is the opposite of what happens most of the time during weight loss, where more fat is lost than muscle.

It makes sense that these extreme fasts would have this result, Varady said, because "your body needs a constant intake of protein. If it doesn't have that, then it draws from muscles."

She said she would encourage someone hoping to lose weight to try intermittent fasting instead of water fasting, "because there's a lot more data to show it can help with weight management".