The 5:2 diet

By vegan naturopath Robyn Chuter.

My first Ask Robyn session, for members of EmpowerEd, was held last Tuesday. It was a lively and informative session that covered a range of topics including whether we need to supplement with minerals; how much sleep do people actually require; and is cacao good for you. I wanted to give you a taste of what you’re missing out on if you’re not already an EmpowerEd member, so here’s a summary of the answer I gave to the question

“What is your opinion of the 5:2 diet by Dr Michael Mosley, whereby women consume 500 calories & men 600, two days per week?”

In case you’re not familiar with this diet, and the book written about it (The Fast Diet), it’s a form of intermittent fasting. The idea is to eat a ‘normal’ diet 5 days per week, and then decrease energy intake dramatically on two days per week.

Mosley recommends that women consume 500 calories/2090 kilojoules per day on these ‘fast days’, and men 600 calories/2500 kilojoules. He provides no evidence to support these energy intake recommendations, which appear to be quite arbitrary numbers. He recommends avoiding potatoes (which he incomprehensibly lumps in with ‘refined carbs’) on fast days, and instead eating high protein, low carbohydrate foods on these days such as fish and meat, claiming that these are more satiating (appetite-satisfying); yet the humble high-fibre, high-carbohydrate, low-fat potato has been found to be by far the most satiating food, dramatically outperforming beef, eggs, cheese and yoghurt.

There have been no systematic studies of this diet but anecdotal reports of side effects include:

  • Difficulty sleeping
  • Bad breath (a known problem with low carbohydrate diets)
  • Irritability
  • Anxiety
  • Dehydration
  • Daytime sleepiness
  • Hunger and
  • Low energy

The 5:2 diet is definitely not recommended for people with a history of eating disorders due to the restrictive behaviour required to stick to it.

Like any popular diet, of course, it’s promoted as an effective way to lose weight, particularly for people who struggle with ‘regular’ diets. But research on its effects is singularly unimpressive. For example, overweight women randomly allocated to the 5:2 diet for 6 months only lost 6.5 kg on average, while those randomised to a standard calorie-restricted diet based on the Mediterranean diet (which contained a whopping 30% of calories as fat, 45% low glycaemic load carbohydrate, and 25% protein… in other words, a highly ineffective weight loss plan) lost 5.7 kg in 6 months. An 800 g greater weight loss over 6 months isn’t something I would be getting wildly excited about, considering what the women on the 5:2 diet had to go through to achieve it.

The researchers also found that

“Both groups experienced comparable reductions in body fat, FFM [fat free mass], hip, bust and thigh circumference and composition of weight loss.”

The only difference was a slightly greater (although still very modest) effect on insulin resistance in the 5:2 diet group.

But if you’re concerned about insulin resistance, rather than slashing your calorie intake, why not just add dried beans, peas and lentils to your diet? A trial of legume consumption vs a low calorie diet found that adding 5 cups of beans per week to the diet for 8 weeks was as effective as eating 500 less calories per day in reducing prediabetes risk factors including waist circumference and blood sugar level, and superior to caloric restriction for improving levels of HDL cholesterol and C-peptide.