5:2 Fasting: What Is the 5:2 Diet and How Does It Work

The 5:2 diet alternates between five days of normal eating and two days of significant calorie restriction (500-600 calories), effectively stimulating fat burning, autophagy, and metabolic improvements. Research demonstrates its effectiveness for weight loss and enhancing metabolic markers, though adherence declines rapidly.

5:2 Fasting

The 5:2 fasting diet is an eating pattern which respects:

• 5 days of normal eating (regular calorie intake)
• 2 days of fasting (<20%, or 500-600 kcal intake)

During normal eating days, one consumes all the usual meals without having a time or calorie restriction, focused on a healthy and balanced diet.

During fasting days, which can be consecutive or non-consecutive, one’s calorie intake is limited to <25%. For most people, this is around 500 (women) and 650 (men) calories daily.

What is the 5:2 Diet

The 5:2 diet is a dietary strategy that emphasizes fasting. It’s a form of fasting scheduled in a simpler, flexible, and straightforward way. Essentially, one can choose 2 fasting days a week in which total calories consumed are <25% of usual requirements.

This diet can offer a practical way to reduce total energy intake to potentiate weight loss and improve metabolic function. Compared to intermittent fasting (IF), which is typically 16/8 hours, it is more intense for the body due to the severe drop in total energy intake on a weekly basis.

How the 5:2 Diet Works

The 5:2 diet leverages the metabolic effects of fasting by creating a significant calorie deficit which stimulates the body to burn stored fat for energy. During such state, insulin levels drop, which can result in further improvements in insulin sensitivity. These hormonal shifts contribute to metabolic efficiency and regulate energy metabolism better, particularly in obese or overweight populations.

During fasting days, the severe calorie restriction can stimulate autophagy, a natural mechanism where the body clears itself of damaged, dysfunctional, or junk cells. Such cellular repair aids in numerous benefits related to lower inflammation, greater energy levels, and fat loss.

5:2 Fasting Benefits

The benefits of fasting or keto-mimicking diets are amplified in people with poor metabolic function, characteristic for obesity and diabetes. This occurs due to the shift in energy metabolism, from glucose to fats.

Generally, the benefits don’t differ much from regular fasting. The same mechanisms take place; the only difference is the schedule. 5:2 can be more intense, as during fasting days calories are severely reduced.

How to do 5:2 Fasting?

The 5:2 diet starts by choosing two non-consecutive days weekly to limit calorie intake at around 500-600 kcal. As the diet restricts calories, it can be hard to get all the needed nutrients. This amplifies the importance of choosing nutrient-dense foods like vegetables, eggs, meat, nuts, fruits, and starches.

Staying hydrated is also essential to prevent hunger misinterpreting thirst. Herbal tea, black coffee, or water are not breaking the fast, considering no sugars or creamers are added. On regular eating days, it’s advisable to avoid overeating to compensate for the reduced calories, as in most cases this is where the calorie deficit that brings those benefits takes place.

Exercise needs to be adjusted around eating days, so glycogen reserves are fuller and the muscle can take advantage of it. Intense exercise during fasting is not recommended as blood sugar levels drop fast and energy crashes will occur.

By maintaining consistency and listening to one’s body, the 5:2 diet can be a simple and sustainable method for managing weight and enhancing metabolic function.

Adverse Effects of The 5:2 Diet

One of the key adverse effects present in any restrictive diet is nutrient deficiency. Research shows the intake of key minerals such as calcium, zinc, magnesium, and potassium were lower than recommended, while sodium exceeded suggested levels, during a 5:2 diet experiment. On fasting days, the total amount of protein increased from 21-25%. (8)

During the adjustment period to CR or 5:2, side effects like hunger, irritability, fatigue, and brain fog on fasting days may occur. The body needs time to adapt to such a strong shift in metabolism, burning fat for fuel.

Some people may also experience headaches or dizziness, often linked to low blood sugar and dehydration. For people with metabolic conditions like diabetes, the 5:2 diet may lead to hypoglycemia, so monitoring blood glucose level is strongly advisable.

In some, it may cause sleep disruptions, particularly as hunger stimulates cortisol and sympathetic activity.

To minimize these adverse effects, it’s essential to pay attention to the cues of one’s body, avoid extreme calorie restriction on fasting days, and have professional guidance that monitors the process. Consulting with a healthcare professional before starting the diet is particularly important for individuals with medical conditions, pregnant or breastfeeding women, or those who are underweight.

The 16/8 fasting provides most of the benefits of the 5:2 diet, while being more convenient, less intense, and more practical. It tackles the main problem of 5:2, which is low adherence. For a diet to work, sustainability is key. Research shows a strong decline in adherence to the 5:2 diet from 74% at 6 weeks, to 31% at 6 months, to 22% at one year. (8)