Nine new diet centers around Philadelphia are promising rapid weight loss with the Medifast weight loss plan. It's not a fast, but it's extreme, allowing only 800 to one thousand calories a day. Stephanie Vander Veur, a weight loss researcher at Temple's Center for Obesity Research, says the rapid weight loss, over several weeks, can be dangerous. "I think that is concerning. They're probably making some pretty drastic changes to their lifestyle that aren’t going to be maintained over a long period of time," she said.