Fast Diet Overview


Weight Loss Short-term
Weight Loss Long-term
Easy to Follow
For Diabetes
For Heart Health

Scores are based on experts' reviews

Pros & Cons

  • Few rules to keep track of
  • You’re only dieting two days of the week
  • Unsafe for some people
  • You may get hungry

Do's & Don'ts

Do: Stay hydrated. See more Do's & Don'ts




Resembles these U.S. News-rated diets:

Glycemic-Index DietRaw Food DietMedifastSlim-Fast

The aim:

May include weight loss, disease prevention and optimal health. 

The claim:

You’ll lose weight – specifically fat – and reduce your risk of a host of chronic diseases.

The theory:

Michael Mosley, a journalist trained as a doctor at the Royal Free Hospital in London, and Mimi Spencer, a journalist and author, tout the scientific evidence behind significantly cutting calories two days a week as well as their own experiences with this method in their book, “The FastDiet.” They say that when we intermittently fast, we’re fooling our bodies into thinking we may be experiencing a famine, in which case the body switches into maintenance mode and burns energy from fat stores. As our bodies respond to this stressor, they toughen up – essentially following the logic of, “what doesn’t kill you makes you stronger.” 

How does the Fast Diet work?

This pattern of eating is often referred to as the 5:2 diet – you eat normally for five days of the week and cut your calories to about 25 percent of normal intake on two nonconsecutive days of the week. Men consume just 600 calories on their two weekly fast days, while women are limited to 500 calories. Not surprisingly, those calories must be spent wisely on fast days, or you could blow half of them on a flavored latte. The diet authors suggest choosing high-protein foods, such as skinless chicken, nuts and legumes to curb hunger. You’ll also eat strawberries, carrots and other foods with low glycemic loads, meaning they’re less likely than others to affect blood sugar and insulin levels.

On fast days, there are no specific rules for when you eat or meal frequency. Theoretically, the longer amount of time without eating the better, the diet authors say, so dieters would ideally have just one 500 to 600 calorie meal that day or two meals several hours apart. However, Mosley and Spencer also suggest the best approach is one that the dieter will actually stick to, so if it’s easier to spread those 500 to 600 calories on several low-calorie snacks throughout the day, go for it.

On fast days, shoot to meet your calorie count with about 50 grams of “good protein,” such as steamed white fish, skinless chicken, plant-based protein like tofu, nuts, seeds, legumes and eggs. Fruits and vegetables will play a big role on fast days given that many of them have low glycemic loads, few calories and plenty of fiber and nutrients.

On the other five days of the week, there’s no calorie cap, and no food is off limits. This freedom isn’t permission to binge and make up for your two fast days, but it does mean you shouldn’t feel guilty about eating a slice of cake.

Will you lose weight?

Probably. You’ll consume significantly fewer calories than you normally would on two days of the week, so there’s a good chance that if you actually stick with the plan, you’ll steadily drop weight. And because the two fast days are nonconsecutive and allow for at least some eating, the diet authors have found that people don’t typically binge and overeat on the non-fasting days.

  • In a 2011 study published by the American Association for Cancer Research, researchers at Genesis Prevention Center at University Hospital in South Manchester, England tested the effects of three kinds of diets on 115 women. One diet looked like the Fast Diet (five days of normal eating and two days following a calorie-restricted, low-carb diet each week), another restricted carbs two days a week but had no calorie restrictions and a final group followed a calorie-restricted Mediterranean diet for all seven days of the week. After four months, participants following the intermittent low-carbohydate diets lost an average of 9 pounds, while those on the Mediterranean diet lost an average of 5 pounds.
  • In a randomized trial of 107 overweight or obese premenopausal women, researchers found that participants who followed an intermittent food energy restriction plan (25 percent restriction two days a week) lost a comparable amount of weight to the participants who followed a continuous energy restriction plan. After six months, participants following the intermittent calorie restriction plan lost an average of 14 pounds each. Results were published in 2011 in the International Journal of Obesity.
  • A study published in the July 2013 issue of Physiology & Behavior doesn’t discuss intermittent-day fasting, but it addresses the concern of overeating after fasting. Researchers at Cornell University either fed breakfast to or withheld breakfast from a group of student volunteers. They found that those who skipped breakfast reported being hungrier than those who ate breakfast. They also ate more at lunch. Still, the amount they ate didn’t fully compensate for the missed meal. Volunteers who skipped breakfast consumed 408 fewer calories over the course of the day than those who ate breakfast.
  • Studies on every-other-day fasting show mixed results. One published in 2010 in the Nutrition Journal suggested that the technique was effective among a group of obese patients. A group of 16 participants ate only one meal — lunch — every other day, and they were limited to about 500 calories. That’s the same amount of calories women consume on the Fast Diet’s fasting days. On the days when the study participants were not fasting, they were not constrained to any rules. Over the course of eight weeks, the participants lost an average of 12.3 pounds.

Does it have cardiovascular benefits?

It’s likely. If you’re using those 500 to 600 calories wisely on fast days, you’ll likely consume very little (if any) saturated fat and opt for foods high in fiber, such as on fruits and veggies, which are in line with the medical community’s accepted definition of a heart-healthy diet that keeps cholesterol and blood pressure in check and heart disease at bay.

Can it prevent or control diabetes?

The diet appears to be a viable option for prevention of Type 2 diabetes

Prevention: Being overweight is one of the biggest risk factors for Type 2 diabetes. If you need to lose weight and keep it off, and the Fast Diet helps you do it, you’ll almost certainly tilt the odds in your favor. Diet authors also suggest intermittent fasting will increase insulin sensitivity, which reduces the risk of diabetes.

Control: People with Type 1 diabetes are not advised to try this diet. Those with Type 2 diabetes should consult with their doctors before trying this diet. If they choose to follow this meal plan, it’s extra important to choose low glycemic index foods on fast days to manage blood glucose levels.

Are there health risks?

There are several groups of people who should not fast, including women who are trying to get pregnant, women who are already pregnant, children and adults with Type 1 diabetes, eating disorders or who are already lean. Also, those who have medical conditions or are on medication should talk to their doctors before trying this diet.

How well does it conform to accepted dietary guidelines?

Fat. The government recommends that 20 to 35 percent of daily calories come from fat. A sample daily menu on the Fast Diet provides 40 percent. The sample menu was based on a fasting day.

Protein. In a sample fasting day menu, 17 percent of daily calories are from protein, compared with the 10 to 35 percent the government recommends.

Carbohydrates. The government advises that between 45 and 65 percent of daily calories come from carbs. The Fast Diet provides about 45 percent on a fasting day.

Salt. The majority of Americans consume too much salt. The recommended daily maximum is 2,300 milligrams, but if you’re 51 or older, African-American, or have hypertension, diabetes or chronic kidney disease, the limit is 1,500 mg. A sample fasting day menu adds up to only 262 mg.

Other key nutrients. The 2010 Dietary Guidelines call these “nutrients of concern” because many Americans get too little of one or more of them:

  • Fiber. Getting the recommended daily amount of 22 to 34 grams for adults helps you feel full and promotes good digestion. Veggies, fruits, beans and whole grains – all major sources – are encouraged on this diet. A fasting day on this diet provides 14 grams.
  • Potassium. A sufficient amount of this important nutrient, according to the 2010 Dietary Guidelines, counters salt’s ability to raise blood pressure, decreases bone loss and reduces the risk of developing kidney stones. It’s not that easy to get the recommended daily 4,700 mg. from food. (Bananas are high in potassium, yet you’d have to eat 11 a day.) The majority of Americans take in far too little potassium. A sample menu came up short at 950 mg.
  • Calcium. It’s essential not only to build and maintain bones but to make blood vessels and muscles function properly. Many Americans don’t get enough. Women and anyone older than 50 should try especially hard to meet the government’s recommendation of 1,000 to 1,300 mg. A sample daily menu provides 529 mg.
  • Vitamin B-12. Adults should shoot for 2.4 micrograms of this nutrient, which is critical for proper cell metabolism. Fish like salmon and trout, along with eggs and yogurt, are good sources. A sample daily menu provides 2.1 mcg.
  • Vitamin D. Adults who don’t get enough sunlight need to meet the government’s 15 microgram recommendation with food or a supplement to lower the risk of bone fractures. A sample daily menu provides 3.3 mcg.

Supplement recommended? N/A

How easy is it to follow?

That depends. Drastically cutting your calories on fast days will likely be a challenge. But for five days of the week there are no rules to keep track of. 


Convenience: The Fast Diet’s minimalist set-up makes it fairly convenient, given you can count calories and restrain yourself. The biggest inconvenience is likely the very premise of the diet – ignoring hunger and consuming few calories on two days of the week. 

Recipes. Both “The Fast Diet” and “The Fast Diet Cookbook” include numerous recipes and meal plans for 500 and 600 calorie fast days.

Eating out. Dining out should be no problem five days of the week. On fast days, however, the inflated portions at many restaurants may be a challenge, given that one serving of fries could possibly add up to your whole day’s worth of calories. Choosing restaurants with disclosed calorie counts may take some guesswork out of ordering. Be prepared to stick to the diet plan while other friends are ordering freely.

Alcohol. Again, five days of the week, do what you like with your calories. On fast days, however, your calories are so precious that it’s better to spend them on foods with nutritional value. So steer clear of alcohol on these days.

Timesavers. There’s a reasonable chance you’ll save time preparing meals on fasting days, especially if you choose simple recipes. Other than that, there areno timesavers, unless you hire somebody to plan your meals, shop for them and prepare them.

Extras. None.


Fullness: Nutrition experts emphasize the importance of satiety, the satisfied feeling that you’ve had enough. However, there’s a good chance that, at least toward the beginning of dieting, you won’t feel particularly full on fast days.


Taste: You’re making everything, so if something doesn’t taste good, you know who to blame.

How much does it cost?

Cost isn’t really a factor for this diet, given that you’re eating normally most days. There’s a good chance you’ll wind up buying fewer foods on fast days, and thus spend less on your grocery bills. But keep in mind that if you follow the diet authors’ advice and get your calories from high quality foods, they can be a bit pricier than cheap, high-calorie fast food cheeseburgers.

Does the diet allow for restrictions and preferences?

Most people can customize the Fast Diet to fit their needs – pick a preference for more information.

Yes, with a few minor tweaks you can easily replace any animal products with vegetarian- or  vegan-friendly options.

People who can’t tolerate gluten, a protein found in wheat, barley and rye, can easily followthis diet. The key is selecting gluten-free ingredients when possible.

Doable, but it’s up to you to check the nutrition information on recipes and keep track of your sodium intake.

Yes, you can make sure your diet is kosher.

Yes, but it’s up to you to ensure your food conforms.

What is the role of exercise?

The Fast Diet is only an eating pattern, but that doesn’t mean you shouldn’t exercise. Being physically active lowers your risk of heart disease and diabetes, helps keep weight off and increases your energy level. Most experts suggest getting at least 30 minutes of moderate-intensity exercise – like brisk walking – most or all days of the week.

Last updated by Laura McMullen | January 03, 2014

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