In an effort to cut calories and lose weight, some dieters will eat lower-calorie foods or smaller portions of everyday foods. Others go to the extreme – no food at all.

Beyonce Knowles is among the celebrities who admit to fasting, consuming a liquid-only diet. Oprah Winfrey claims to have lost weight through a special cleansing fast.

Going without solid food over several days or even weeks may help you lose weight in the short term, but, Natalie Newell, a registered dietitian with Rex Healthcare said you may end up gaining even more weight back.

“It could … slow down your metabolism,” said Newell. “When you go back to your regular diet, or to even a healthy diet, you’re actually going to be eating the same amount of calories that you were previously eating, and your metabolism is not working as efficiently as it should.”

Newell pointed out that most fasting diets deny people nutrients that their body needs, like proteins. Without the energy provided by solid food, fasters can also see their energy level suffer.

Newell said healthy weight loss is all about controlling calories. On average, women should be able to lose weight on about 1,200 to 1,600 calories a day. For men, the range is 1,800 to 2,200 calories per day.

Newell allowed that certain fasts – for religious reasons or to jump-start a healthy eating plan – can be safe. She warned against going without solid food for any period of time beyond a day or two.

Newell also addressed the claims that fasting cleanses the body of toxins. The body is designed to get rid of its own excess toxins, she said.