"Fast," in this case, is not about speed. It's about fasting.
This diet, which started in the U.K., slashes your calories so drastically 2 days a week that you're basically fasting. That's not safe for everyone, so you should check with your doctor if you're considering trying it.
The Fast Diet says that you shouldn’t fast if you’re pregnant or underweight, or if you have a history of eating disorders or diabetes, and that you should check with your doctor first if you take medication. The diet also isn't recommended for kids, teens, frail seniors, or anyone who isn't feeling well or has a fever.
The basic concept behind The Fast Diet by Michael Mosley, MD, and Mimi Spencer is to eat normally for 5 days per week and eat very restricted calories on the other 2 days.
Mosley tried this "intermittent fasting" diet when his doctor showed him that though he was only a few pounds overweight, his cholesterol was high and his blood sugar was headed in the wrong direction. He writes that he knew fasting would be difficult, but his hunger pangs passed quicker than he expected. He also felt that fasting sharpened his senses and his brain. Plus, the diet delivered all the results he hoped for.
On its web site, the Academy of Nutrition and Dietetics says that although there is evidence that intermittent fasting diets may help prevent chronic disease, more research is needed. It doesn't recommend the diet because "it is not a realistic long-term solution.” The academy also notes that "any variation of fasting may make a person irritable, cause daytime sleepiness/sleeplessness at night, and can even lead to dehydration."
What You Can Eat and What You Can't
You eat normally 5 days a week, and fast for the other 2 days. Your fasting days should not be back-to-back; you should have at least 1 normal eating day in between them.
During your fast days, you can eat, but not very much. Women get 500 calories per day; men get 600. That’s far less than what’s usually recommended. Depending on age, gender, and how active you are, you could need three or four times as many calories.