5:2 Diet Review

The 5:2 Diet has quickly become a popular dietary strategy for losing weight and improving wellbeing.   With a plethora of weight loss strategies reported in the media (and endorsed by Hollywood celebrities!), I reviewed the science behind the 5:2 diet to help clarify what we really know about this diet, to help you make sense of this latest diet strategy.

intermittent fastingWhat is the 5:2 Diet?

The 5:2 Diet is an intermittent fasting strategy which involves consuming “normal” caloric levels for five days a week, and then significantly restricting your caloric consumption for two days of the week. On the two days of calorie restriction, you drop your intake to 25% of your normal recommended daily calorie intake. On these fasting days, your calorie allowance can be consumed in one, two or three meals.  My advice is that you should consume plenty of water throughout these fasting days. According to the 5:2 Diet, you may consume other drinks such as black coffee and black or herbal teas.  As always, I strongly advise you avoid diet drinks: these artificially-sweetened drinks can influence your blood glucose and insulin response, which is not ideal for restricted calorie days.

The theory behind the 5:2 Diet is that intermittent fasting activates the SIRT1 gene (sometimes referred to as the “skinny gene”), which is involved in the repair and maintenance of cells; from an evolutionary perspective, this gene is thought to have been important for survival during periods of limited food availability. The SIRT1 gene also inhibits fat storage, so activating this gene should theoretically aid in weight loss. There is currently debate over the effectiveness of the diet, and how sustainable the diet is to maintain for the long term.

What are the positives of 5:2 Diet?

It provides a new approach for people trying to lose weight

Intermittent fasting provides a different approach to weight loss for individuals caught in the long-term continuous calorie restriction diet trap.  Fasting for short periods (two days) is intended to give the digestive system a rest, which is thought to aid in weight loss.

Reduces the risk of some chronic diseases

This 5:2 dietary strategy has shown to reduce the risk of type 2 diabetes, dementia and cancer in individuals. In a recent study involving overweight women, it was found that those who restricted their calories for two days per week lost more fat and had a greater improvement in biomarkers that relate to breast cancer risk, compared with women who participated in common daily dieting. Another study involving fit young men observed improved insulin sensitivity, which is a marker for reduced diabetes risk.

Enhanced physiological functioning

Intermittent fasting has been associated with improved cognitive function, which may decrease the risk of conditions such as Alzeimer’s disease and dementia.  It may also improve blood pressure, metabolic rate, cholesterol level, and insulin sensitivity.

May delay aging processes

Fasting is believed to reduce the levels of IGF-1 in the blood, a hormone which has been associated with accelerated aging processes, and can cause cell divisions similar to those found in cancer.

May increase life expectancy

Reducing your calories by a third has been linked to an increase in life expectancy by a third, and may help to reverse the damage caused to the body by ingesting sugar and processed foods.


What are the negatives of the 5:2 Diet?


Individuals can experience intense and unrelenting hunger during the two days of fasting.   Without appropriate discipline, this can trigger overeating especially after the fasting days, which defeats the purpose of the diet.

Not sustainable

The 5:2 diet may not be sustainable in the long-term for everyone – although it is doable! It may not be a sustainable solution as some people are unlikely to be able to maintain fasting within their diet for the long term. In addition, the 5:2 Diet may not address the underlying eating habits which caused your initial weight gain.

Side effects from fasting

These can include: having difficulty sleeping, bad breath, dehydration, and experiencing anxiety and mood swings.

Nutritional deficiencies

Restricting food can lead to the body not being able to meet its nutrient requirements if the body doesn’t consume all the essential vitamins and minerals it needs.

Reduced energy levels

Fasting can leave you feeling lethargic and having less energy, which will affect your ability to function in daily tasks.  Tiredness can last beyond the day of fasting, impacting not only your mood but also your effectiveness and level of productivity.  Endurance training during fasting days is not necessarily safe and is not advised, which also restricts the exercise routines that are safe for individuals to engage in if adhering to a 5:2 Diet.

Lack of guidelines

There are no guidelines for the non-fasting days, which technically means that junk food could be consumed during these days in place of healthy, whole, nutritious foods.  In this scenario, it would not make the diet sustainable nor healthy!

Not everyone is advised or able to do the 5:2 Diet

Intermittent fasting is not suitable for everyone. People who are not advised to engage in this style of diet include: underweight people, children, people with Type 1 diabetes, pregnant women or breast-feeding mothers, people recovering from surgery, and people who feel unwell.  People who take Warfarin or prescribed medicines need to consult with their GP before embarking on the diet.


More scientific research stills needs to be conducted to determine the long-term effectiveness, sustainability, and suitability of the 5:2 Diet.  Whilst this research is ongoing, at Inspire Fitness, we have had a number of clients who have embarked on the 5:2 Diet with success.  Whatever the strategy you use for changing your nutritional intake and reducing your body weight, ultimately you need the discipline and determination to make long-term changes “stick”.  This diet, along with many before it, still requires you to reduce your calorie intake and change your lifestyle.  These two basic strategies are a good starting point, but you must address your existing eating behaviours and beliefs around food in order to create long-lasting and healthy change.

Brendan Rigby is an Accredited Exercise Physiologist and Corrective Exercise Practitioner at Inspire Fitness for Wellbeing.