A Beginner Guide To 5:2 Fasting + How To Do It Safely

5:2 fasting1, also known as The Fast Diet, is a type of intermittent fasting that involves eating normally for five days of the week and restricting your calorie intake to approximately 500 to 600 calories (or roughly 25 to 30% of your normal calorie intake) for the remaining two days of the week. 

This type of fasting was popularized by Michael Mosley, a British journalist who published a book titled The FastDiet in 2013. In his book, Mosley describes the 5:2 eating plan, which was developed in response to his own health issues, and explains how it helped him improve his cardiometabolic health.

People often try 5:2 fasting because it can be easier to follow2 than diet regimens that require you to count macros every day, like the high-protein, low-carb diet.

"For some, this approach may be more appealing than traditional dieting, as there are only two days of the week where there's a focus on reducing caloric intake," says Laura DeCesaris, IFMCP, M.S., D.C., a functional medicine practitioner. 

You have the freedom to choose which two days of the week you want to fast, and they don't have to be consecutive. Another major plus of the 5:2 fast is that there is no restriction on which foods you can eat on fasting or non-fasting days since no food groups are off limits.

A randomized trial that included 137 adult participants with Type 2 diabetes found that 5:2 fasting helped lower participants' HbA1c levels3. HbA1c, or hemoglobin A1c, is an indicator of your average blood sugar levels over the last two to three months. High HbA1c levels are associated with a greater risk of long-term diabetes-related complications4.

Another smaller randomized trial with 16 young, healthy-weight participants found that 5:2 fasting helped reduce participants' fasting blood glucose levels5. Fasting blood glucose is an indicator of your blood sugar levels several hours after eating. High fasting blood glucose levels can be a sign of insulin resistance, which means your body is not effectively able to use insulin to metabolize the glucose in your blood.

5:2 fasting can support cardiometabolic health6 by lowering insulin resistance, oxidative stress, blood pressure, cholesterol, and triglyceride levels, according to a 2022 review published in the journal Nature Reviews Endocrinology.

A randomized trial conducted in China with 61 participants who had both Type 2 diabetes and nonalcoholic fatty liver disease (NFLD) found that following the 5:2 eating plan for 24 weeks helped participants reduce the amount of fat stored in their liver and improve liver function7 considerably.

A 2021 review published in the journal Nutrients notes that intermittent fasting, including 5:2 fasting, may help boost brain health8 and potentially reduce the risk of conditions such as epilepsy, ischemic stroke, Alzheimer's disease, and more in animal models.

Yes, 5:2 fasting can help you lose weight. "It has been shown to produce between 3 and 7% weight loss6 over three to six months in people with obesity, which roughly translates to 5 to 20 pounds. Most of the weight loss is achieved in the first three months," says Varady. 

A 2022 review published in the journal Diabetes, Metabolic Syndrome and Obesity notes that 5:2 fasting may be as effective in reducing weight9 as other, more traditional diets that involve restricting your calorie intake every day.

The review notes that in addition to helping you lose weight, 5:2 fasting can also help improve your waist-to-hip ratio (WHR), which is considered a more accurate anthropometric metric10 than BMI.

However, it's important to note that the amount of weight you can lose on a 5:2 fast varies from person to person. "With any weight loss journey, the amount of weight lost will depend on factors like initial body weight, extent of calorie restriction, exercise, and stress levels, among others," says DeCesaris.

For instance, what you eat on days when you're not fasting can play a major role in determining how much weight you lose. Eating a healthy, balanced, and nutritious diet on non-fasting days can help you achieve your weight loss goals.

On the contrary, eating a lot of calorie-dense junk food on non-fasting days will make it hard for you to lose weight or even cause you to gain weight.

If you've tried 5:2 fasting and feel like you're not losing much weight, it may be helpful to try other, more consistent types of fasting such as time-restricted eating.

On days when you're fasting, since you have limited calories to consume, you should spend them wisely on nutrient-dense foods that will help you meet your body's needs. Nutrient-dense foods are high in nutrition but relatively low in calories, essentially giving you the highest bang for your buck if you're restricting calories.

Not getting adequate nutrition on fasting days can raise your risk of developing nutritional deficiencies11. 

It may not be easy for you to suddenly reduce your calorie intake to just a quarter of what you normally eat. It can help to start reducing your intake gradually, so your body has a chance to get used to it. 

While 5:2 fasting doesn't restrict any foods, it does require you to be mindful of the number of calories you're consuming on fasting days. 

When you're shopping at the grocery store, be sure to check product labels for calorie and nutrition information. For foods that don't have labels, you can look up the information you need online12.

Most studies investigating the benefits and effects of the 5:2 fasting plan do not go more than 24 weeks, so the long-term consequences of this eating plan are still unknown, says Keatley.

However, since 5:2 fasting has been shown to reduce muscle mass and slow metabolism9, you may be more prone to regain the weight once you go back to your normal eating patterns, Keatley explains.

5:2 fasting may be helpful to some people, but it's not for everyone. You should avoid this diet if you are pregnant or planning on becoming pregnant, breastfeeding, under 20 years old, underweight, or have a history of eating disorders.

To ensure your safety while you're on this eating plan, Keatley recommends consulting a health care provider such as a doctor or a dietitian before you start and in case you experience any symptoms or side effects.

Some of the side effects of fasting13 include hunger, fatigue, irritability, difficulty concentrating, low mood, food-related thoughts, and fear of losing control and overeating on non-fasting days.

Another risk of 5:2 fasting is nutrition deficiencies14, which can lead to long-term health problems if left untreated. It may be helpful to consult a health care provider and regularly check your nutrition levels, to ensure you're not at risk for a deficiency.

5:2 fasting is an eating plan that calls for significantly restricting your calorie intake two days a week and eating normally on the other five days. It's been shown to lead to weight loss and metabolic health improvements, but it's not for everyone. Consult a health care provider to determine whether it's safe and appropriate for you.