The Promise

"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 shouldnt fast if youre 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 website, 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. Thats far less than whats usually recommended. Depending on age, gender, and how active you are, you could need three or four times as many calories.

The Fast Diet encourages you to eat lean protein, vegetables, and fruit on fasting days, usually as two small meals plus a few snacks.

A typical 500-calorie fasting day might include oatmeal with fresh blueberries for breakfast, a tangerine for a snack, and a chicken and vegetable stir-fry for dinner. You will drink lots of water and may also have calorie-free beverages such as tea, coffee (no milk or sugar), and club soda.

On your 5 "off days," you can eat anything. Surprisingly, the research team that studied the diet found that people didnt gorge themselves on off days.

The Fast Diet strongly discourages drinking alcohol on fasting days and suggests that if you drink on your "off days," you drink only in moderation. And once you reach your weight loss goal, 1 day of fasting per week is recommended for maintenance.

Level of Effort: Medium

Cooking and shopping: Youll cook or eat as usual on off days, and with a very restricted menu on fasting days. The book provides a months worth of fasting day menus and recipes.

Packaged foods or meals: No.

In-person meetings: No.

Exercise:The Fast Diet recommends exercise, but the diet's website cautions that you shouldn't try to do a lot of endurance training on fasting days, and to stop if you feel uncomfortable.

Does It Allow for Dietary Restrictions or Preferences?

You can adjust the plan for vegetarian, vegan, and gluten-free needs on fasting days. But the book doesn't give you specific plans for those diets.

What Else You Should Know

Cost: Just your groceries.

Support: You do this diet on your own, but you will find lots of tips and discussion forums online.

Does It Work?

Even though many health experts dont recommend fasting, if youre going to do it, the authors of this plan have it right. Eating 500-600 calories a couple of days a week will be tough, but its doable.

One reason experts tell people not to cut way back on calories is that it can slow your metabolism, making it even harder to lose weight. But studies show that fasting from time to time can be an effective strategy for weight loss. In addition, cutting calories on a couple of days instead of every day may help preserve muscle, so you lose mostly fat.

Mosleys team found that people dont overindulge on the non-fasting days. Other research suggests they may be right. But its ultimately up to you to make sure you dont sabotage your weight loss efforts on the off days.

Is It Good for Certain Conditions?

If you have diabetes, this is absolutely not the weight loss plan for you. It could be dangerous, especially if you take medicine, and lower your blood sugar into unsafe -- even deadly -- territory.

For others, there are some early studies that show it may help improve some risk factors for heart disease, including blood pressure and cholesterol. Though any weight loss plan that drops the pounds is likely to also have this effect.

Talk to your doctor first before trying a fasting diet. This approach could put extra stress on your body.

The Final Word

If fasting interests you, The Fast Diet is a decent approach. Its going to take some serious willpower, but if youre up for the challenge, it can work. Its definitely not meant for everyone, so dont ignore the warnings.

It also may be tough to stick with in the long term. But trying it out to see if it suits you is a worthy plan. Have another plan in your back pocket if you or your body tires of this approach.