Fasting to lose weight, how safe is it?

WASHINGTON (WJLA) - Crystal Cook fasts 14 hours a day. She's using intermittent fasting to slim down with some friends at "The Body You Want" gym in Georgetown. Each person sets a window of 8 to 10 hours a day to eat and 14 to 16 hours to fast.

"It took a couple days to get used to," explained Alyssa Campbell, intermittent faster. "But after that it was kind of routine."

Most of them skip breakfast and eat two meals and a snack between noon and 8 p.m.

"There is some research and definitely some case studies that show if you play around with that time and make the time you're fasting longer it can produce better fat loss results," explained Josef Bradenburg, owner of The Body You Want Fitness.

A British study in 2013 compared intermittent fasting to daily calorie restriction. It found 25-percent more of the people on the intermittent fasting diet lost weight and they lost twice the amount of weight at that.

Intermittent fasting can also be done as a "five and two" plan. That's five days of eating anything, two days of eating less than 500 calories. Or the "every other day" plan which involves alternating a day of eating anything with a day of fasting.

Dietitian Rebecca Scritchfield warns that fasting to lose weight is never a good idea.

"It's just so dangerous you literally are depriving your brain and heart of essential nutrients they need to function," explained Scritchfield. "And within hours of fasting you can feel irritable, cranky and dizzy."

She also warns that people are more likely to overeat on their non-fasting days.

But these dieters say by filling their plates with protein, veggies and healthy fats, they feel satiated, avoid overeating and maintain a balanced diet.

"I feel great," said Laura Rodriguez. "I feel so much better than how I used to."