What the science says about the 5:2 intermittent fasting diet

The 5:2 diet became popular in late 2012 following a documentary by BBC journalist Michael Mosley.

It advocates cutting weekly total kilojoule intake by using intermittent fasting for two non-consecutive days and then eating a usual diet the other five days.

On the fasting days, women limit their total intake to 2,100 kilojoules and men to 2,500 kilojoules, which is about 25 per cent of the recommended intake for normal-weight adults.

On the non-fasting days, you can eat whatever you want, although Mosley's book recommends making healthy choices.

Advocates of intermittent fasting say it helps in weight loss, and some claim it may reduce the risk of cognitive decline, such as Alzheimer's disease, while also promoting longevity.

They also claim it could be protective against diseases such as diabetes.

Sample menu for a fasting day

Breakfast: Two poached eggs with baby spinach or 40g raw oats (cook into a porridge if desired) served with berries.

Dinner: Waldorf salad (apple, celery, endive leaves, walnuts, yoghurt, and mustard) or a few strips of chicken stir-fried with ginger, garlic, coriander and lots of vegetables.

What do experts say about intermittent fasting?

The jury is still out on the benefits of intermittent fasting.

Studies that initially reported superior results for weight loss were short-term and had low numbers of subjects.

Recent research compared larger numbers of people on intermittent fasting with others simply reducing kilojoules.

Over 12 months, there were no significant differences in weight loss for either group.

Nor did the researchers find any difference between the groups in blood pressure, heart rate, or in the levels of glucose, insulin, and triglycerides.

However, LDL or 'bad' cholesterol levels did increase significantly in those on the intermittent fasting diet.

Many of the other claimed health benefits have only been shown in rodents.

Claims of reduced risk of cognitive decline and Alzheimer's disease have not been shown in any studies on humans.

The strengths of the 5:2 diet

If you don't overeat on non-fasting days, your overall kilojoule intake should decrease, so it is likely to be less harmful than many fad diets.

It's also possible that reducing your food intake twice a week could be a new and useful experience, especially if you are in the habit of eating so often that you never experience an empty stomach.

The modified fasting with some food permitted and the inclusion of plenty of vegetables may help avoid some common side-effects to total fasting.

These include irritability, lack of energy, inability to concentrate, dehydration, tiredness, dizziness, and constipation.

Such effects can interfere with work and leisure for most people.

The weaknesses of the 5:2 diet?

The 5:2 diet is unsuitable for many people, including pregnant women, many people with diabetes, those taking certain medications, children and teens.

Given some versions of intermittent fasting fail to promote healthy food choices, there is a possibility you may gorge on your non-fasting days with little regard for the quality of the food you choose.

The 5:2 diet may help some people lose weight. If done well, it may be a valid, but not superior, option to following a traditional reduced-kilojoule diet.

This is general information only. For detailed personal advice, you should see a qualified medical practitioner who knows your medical history.

This story, which was originally written by Pamela Wilson and published by ABC Health and Wellbeing, has been reviewed by Dr Rosemary Stanton OAM, nutritionist and visiting fellow, School of Medical Sciences, University of NSW, and was updated in 2019.