What Is the Fast Diet?

fast diet

Verywell / Debbie Burkhoff

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The fast diet, a form of what's called the 5:2 diet, uses modified intermittent fasting to help people lose weight. Specifically, people following the fast diet eat very few calories (approximately 500 calories for women and 600 calories for men) on two non-consecutive days per week and then eat what they want on the other five days.

Proponents of the diet say it works short-term to help them lose weight, and long-term to keep the weight off. The diet is simple to follow: It involves calorie-counting only on two days per week and doesn't ban any particular types of foods.

However, this can be a difficult diet to stick to, especially at first, since the substantial variation in day-to-day caloric intake may lead to extreme fluctuations in blood sugar (which can affect energy and mood), significant hunger, and possibly other side effects, including headaches. This type of eating plan doesn't teach followers how to eat a healthful diet on the days that they can eat regularly and it's not a healthy diet choice for people with certain health conditions.

What Experts Say

The fast diet endorses a 5:2 style of intermittent fasting. Some research suggests this aids in weight loss and disease prevention, but the evidence is limited and long-term compliance can be tough. Certain people should skip this diet, such as pregnant women or people with diabetes.

Chrissy Carroll, RD, MPH


People fast for a variety of reasons. Fasting is commonplace in religious customs, such as Lent, Yom Kippur, or Ramadan. Emerging research suggests fasting may also have health benefits.

The practice of intermittent fasting—avoiding eating for some period of time—has been studied for its potential effects on longevity and various health conditions, including cancer and diabetes, however, most of the research is limited to animal studies. One small-scale human study in obese subjects found fasting may improve insulin sensitivity and reduce inflammatory markers, but long-term efficacy, safety, and adherence still need to be evaluated.

Still, fasting is most often used for weight loss, and proponents of the fast diet and other intermittent fasting diets maintain that the technique works nearly effortlessly to help people shed unwanted pounds.

The fast diet is considered a modified intermittent fasting program because you don't actually fast during your two fasting days per week; instead, you eat a minimal amount of calories. Diet creator Michael Mosley, M.D., a medical journalist for the BBC and a New York Times bestselling author, says he developed the diet following his own personal experiences of being slightly overweight with high blood sugar.

The diet's philosophy—which Dr. Mosley says is sustainable long-term—is that your body is designed to fast and that intermittent fasting encourages weight loss even when you eat what you want on your non-fasting days.

Still, he notes that he experimented with fasting for several days running (he did a four-day fast), and rejected that approach because it's not sustainable. In addition, he says, people who go on prolonged fasts generally lose muscle in addition to fat, and they run the risk of their metabolisms slowing to the point where the weight will return as soon as they stop fasting. Dr. Mosley also investigated alternate-day fasting and found it to be too difficult.

The fast diet program is designed to be easier to follow. In addition, Dr. Mosley claims that intermittent 5:2 fasting (two fast days per week) will not cause your metabolism to slow down. In fact, he says, it may even improve your mood. However, there is no research to support this claim.

How It Works

When following the fast diet, you pick two days per week to eat only 500 calories (for women) or 600 calories (for men). It's generally recommended to split these calories into two meals: breakfast and dinner.

On the other five days of the program, you eat whatever you want. In fact, Dr. Mosley encourages those following the program to use thoughts about the foods they'll eat on non-fasting days to get them through fasting days: "Unlike deprivation diets that have failed you before, on this plan tomorrow will always be different. Easier. There may be pancakes for breakfast, or lunch with friends, wine with supper, apple pie with ice cream."

Nonetheless, the authors report that people following the program don't necessarily binge-eat on their non-fasting days. They urge people to hide snacks such as cookies and candy on their fasting days and eat them in relative moderation on their non-fasting days. Eventually, they say, your tastes will change and you'll be less inclined to reach for sweets.

What to Eat

Compliant Foods
  • Fresh fruit

  • Low-fat cottage cheese or yogurt

  • Lean chicken

  • Salmon, tuna, and other fish

  • Steamed vegetables

  • Boiled or poached eggs

  • Oatmeal

Non-Compliant Foods
  • Fried foods

  • Red meat

  • Cookies, candy, and other sweets

  • Sugary drinks (including sweetened coffee and soda)

  • Ice cream

  • Large portions of any food

When deciding what foods to eat during fasting days, the program recommends consuming foods with a low glycemic index, along with some protein.

The Fast Diet provides numerous suggested menus that add up to 500 or 600 calories per day. For example, breakfast could include oatmeal with skim milk and half a medium pear (307 calories in total) or two poached eggs with one sliced tomato (198 calories). Alternatively, you could have one large boiled egg with half a small grapefruit (130 calories), or a scant half-cup of low-fat cottage cheese, half a sliced pear, and one fresh fig (152 calories).

Dinner could feature salmon with tuna sashimi and broccolini topped with olive oil (341 calories), vegetarian chili (383 calories), chicken stir-fry with vegetables (360 calories), no-carb Caesar salad with chicken, Canadian bacon, grated Parmesan cheese, and romaine lettuce (295 calories), or pesto-topped salmon with baked vegetables (400 calories).

Your choices of what to eat on fasting days will be limited to foods that fit into the low-calorie pattern; a giant slice of chocolate cake (650 calories) or large flavored coffee drink (around 500 calories) are unlikely to fit into your caloric allowance. However, on this diet, you can save those for your non-fasting days.

Recommended Timing

On fasting days, Dr. Mosley recommends splitting your allowed calories between breakfast and dinner; on the days when he fasts, he says he has breakfast with his family at around 7:30 a.m., and then eats dinner at around 7:30 p.m., with no food in between. The Fast Diet co-author Mimi Spencer reports that she eats 500 calories on her fasting days, mostly split between breakfast and dinner, but that she uses some of her calorie allotment for small snacks, such as an apple or some carrot sticks.

There's no "right" way to do it, they write: "As far as we are aware, there have been no studies, as yet, that attempt to compare the health benefits to people on a fast day of either eating their calories in one go or splitting them into two meals and including the odd snack."

Resources and Tips

The key to making your fasting days work on the fast diet is to keep your calorie count at the recommended maximum. This may be harder than it sounds—500 or 600 calories is not very many calories.

To help you stay in compliance on fast days, The Fast Diet includes an extensive list of foods, along with their calories per serving. For example, six almonds are 42 calories, 1/2 cup of frozen peas is 62 calories, and five medium asparagus stalks are 22 calories.

You may want to consider investing in a scale to weigh your food portions, since estimating quantities of foods by weight can be challenging. Since 3 ounces of skinless chicken thigh is 170 calories, you easily can go over your daily caloric allotment by more than 100 calories if you miscalculate and serve yourself 5 ounces of chicken instead.

Also, although you're allowed to eat whatever you want on your non-fasting days, you should try for moderation there as well. The people who report the most success on the diet are those who don't overindulge—at least too much—on their five non-fasting days of the week.

Drink plenty of water, especially on your fasting days. This may help to keep headaches at bay when you first start the diet. You can add a spritz of lemon (but no sugar) to give it some zest, but no additional calories.

Finally, don't stress if you consume a few too many calories on a fasting day. The goal is for women to consume 500 calories and men to consume 600 calories, but if you wind up at 525, it's just not going to affect your diet noticeably, so you shouldn't worry about it.


The fast diet works fine in conjunction with vegetarian or vegan diets, or with diets that are intended to treat food allergies or intolerances. You just need to eat the foods you normally would eat, while making sure you follow the caloric intake rules on the fast diet.

For example, if you're vegetarian, you'll stick with vegetarian foods. On your fast days, you'll aim to eat foods that are high in complex carbohydrates (oatmeal or bran flakes, for example), in addition to protein-rich foods (tofu, at 90 calories per 3 oz serving, gets you vegetarian protein with relatively few calories).

It's just as simple to do the fast diet if you're gluten-free or dairy-free; you just need to avoid the foods you normally would avoid while following the diet's simple 5:2 rule. Since no foods are required and none are forbidden, what you eat is up to you.

Pros and Cons

  • Diet is simple to follow

  • Allows plenty of choices of foods

  • Some scientific backing for intermittent fasting

  • Plan doesn't teach healthy food choices

  • Intermittent fasting could encourage binge eating

  • Program may not be safe for everyone

  • Fasting may lead to side effects


Some diets are complicated, with rules about combining foods and timing meals. The fast diet is simple in comparison—you just need to know how to count calories and keep track of which days you're fasting.

The fast diet also allows plenty of food choices. Nothing is expressly forbidden, and in fact, some indulgence is encouraged on non-fasting days. This should help you fight any feelings of deprivation, which can derail a diet.

Finally, there's some scientific backing for intermittent fasting, which has been studied for its effects on longevity. Although the research on it isn't definitive, studies suggest that intermittent fasting could be helpful in improving cardiovascular health and markers of inflammation.


The fast diet allows you to eat whatever you want, which is a benefit, but also a detriment since it doesn't teach you how to make healthier food choices on your non-fasting days. The book does include multiple recipes, but they're aimed only at your fasting days and so they're extremely low in calories.

In addition, intermittent fasting isn't safe for those with eating disorders, since it could encourage both binge eating and anorexic behavior. It's also not safe for women who are pregnant. And, anyone who's been diagnosed with diabetes and/or heart disease should talk with their doctor before trying the diet, since it may not be the best choice for people with those conditions.

Finally, fasting can be extremely uncomfortable. People who fast often complain of headaches, fatigue, and general malaise. They also may feel light-headed. The fast diet can be difficult to stick with (and potentially unsafe for some people) because of this.

How It Compares

The Fast diet is similar to other fasting diets, especially those that follow a 5:2 (two non-consecutive fasting days interspersed with five days where you eat normally) pattern of intermittent fasting.

Diets that call for you to drink nothing but smoothies for several days (such as the Body Reset diet or the Smoothie diet) contain around the same number of calories per day, but those diets extend the "fasting" period beyond the fast diet's one day to five days or more. A juice-cleanse diet follows similar patterns.

Another popular intermittent fasting diet, the 16:8 diet, calls for you to eat all your calories for the day during an eight-hour period, and then to fast the other 16 hours. This approach to intermittent fasting is newer and has less scientific backing than the 5:2 intermittent fasting program.

The 2019 U.S. News and World Report Best Diets ranks the fast diet number 33 in Best Diets Overall and gives it an overall score of 2.4/5.

USDA Recommendations

There's no reason you can't incorporate the U.S. Department of Agriculture's MyPlate guidelines for a healthy diet into your eating plan while also following the fast diet. However, the fast diet falls far short of the USDA daily calorie goals, which are 1,600 to 2,400 calories for adult women and 2,000 to 3,000 calories for adult men.

Similar Diets

Water Fasting

Water fasting means consuming nothing but water for an extended period—water fasts can be as short as one day or as long as three days. One-day water fasts are similar in concept to the fast diet, although you may be more comfortable on the fast diet since it allows you to eat some food.

Body Reset Diet

In the body reset diet, you diet for 15 days, starting with five days of only smoothies (one red smoothie, one white smoothie, and one green smoothie per day). This provides around the same number of calories as you would consume in one intermittent fasting day on the fast diet. However, the period of extreme calorie restriction lasts for much longer.

Master Cleanse Diet

This diet, also known as the lemonade diet, requires you to drink the juice from lemons with lots of maple syrup (up to 1 1/2 cups of maple syrup per day) along with saltwater and laxative tea. The diet lasts between 10 and 40 days. This is a more extreme form of fasting and is less healthy than a fast diet.

A Word from Verywell

It's likely that you'll lose weight if you follow the fast diet—studies show that people don't consume enough extra calories on their non-fasting days to make up for their caloric deficits on their fasting days. There also may be some health benefits to intermittent fasting, although more research is needed.

However, fasting can be hard, and you may feel bad (not to mention quite hungry) on your first few fasting days if you follow this program. In addition, the fast diet may not be safe for people with certain health conditions, including eating disorders, diabetes, and heart disease. And, remember that the fast diet doesn't necessarily focus on healthy food choices.

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  1. Harvie MN, Howell T. Could Intermittent Energy Restriction and Intermittent Fasting Reduce Rates of Cancer in Obese, Overweight, and Normal-Weight Subjects? A Summary of Evidence. Adv Nutr. 2016;7(4):690-705. doi: 10.3945/an.115.011767.

Additional Reading
  • Mosley, Michael, and Mimi Spencer. The Fast Diet. Short Books, 2014.