No evidence the 5:2 diet prevents heart disease

Monday March 19 2018

"Eat your heart out" and "5:2 dieters lose weight 'quicker' and may have healthier hearts," are the misleading headlines from The Sun and The Daily Telegraph this morning. These reports are based on a very small trial comparing the 5:2 diet with a standard low-calorie diet.

The researchers also took blood samples to measure levels of fat and sugar in the blood immediately after eating. High levels of both fats and sugar are a risk factor for heart disease.

The 5:2 diet is based on the principle of intermittent energy restriction (fasting). People are encouraged to "eat normally" for 5 days a week and then restrict the calorie intake for the remaining 2 days (typically to around 500-600 calories per day).

The trial included only 41 people. There was no significant difference between the two groups in the time it took them to reach their weight loss targets, or the number of people achieving this.

The study only assessed blood samples at the beginning and the end of the study. There was no difference in blood sugar levels between the two groups. Blood fats were slightly lower in the 5:2 group, but this is not enough evidence to say that the 5:2 diet will definitely reduce blood fat levels and reduce risk of heart disease in the future.

If you are overweight or obese, losing weight (and keeping the weight off) will help improve your heart health. The NHS weight loss plan encourages safe and sustainable weight loss through a combination of healthy eating and regular exercise.

Where did the story come from?

The study was carried out by researchers from the University of Surrey and King's College London and was published in the peer-reviewed British Journal of Nutrition. It was funded by Lighterlife, who provided the food packs used for the 5:2 intervention, presenting a potential conflict of interest.

The UK media grossly overestimates the results of this study, for example the Mail Online confidently states the 5:2 diet "reduces the risk of heart disease". This research did not test the participants for the risk of heart disease, and a study population of this small size would never provide strong enough evidence to support this claim.

What kind of research was this?

This was a randomised controlled trial (RCT) that aimed to compare the effects of the intermittent energy restriction (IER) diet plan, more commonly known as the 5:2 diet, with continuous energy restriction (CER) – a "traditional" low-calorie diet.

RCTs are the most reliable way of assessing the effect of an intervention. The study design takes into account the effects of potential confounders. However, to improve the reliability of this study's findings you would need a larger sample size and longer-term follow-up of weight and health outcomes.

What did the research involve?

This study recruited participants from Surrey between May 2015 and August 2016. Eligible participants had to:

  • be overweight (body mass index of 25 or above)
  • be aged between 18-65 years
  • have a waist circumference of >94cm for men and >80cm for women
  • have no changes in weight in the 3 months prior to recruitment date
  • have no significant medical history

There were 41 participants, 24 following the 5:2 diet, and 17 following the traditional diet.

Each participant was provided with healthy eating advice and individualised food portion lists by the researchers. Participants on the 5:2 diet were given 4 commercially available Lighterlife food packs, which provided approximately 25% of their estimated calorie needs. They ate these for 2 consecutive days and on the remaining 5 days they were advised to choose their own foods, but asked to keep this healthy.

Participants on the traditional diet were advised to consume 600 fewer calories per day. All foods consumed in the traditional diet group were self-selected.

The main outcome measured was changes to level of fats in the blood after eating. At the start of the study participants had blood samples taken following a 12-hour fast and following a liquid meal. They were then asked to follow their diets and return to the lab for the same test when they had achieved a 5% weight loss target.

To ensure compliance with the diets, participants were contacted fortnightly by the study investigators via phone, email and/or text message, and had monthly face-to-face clinic appointments where their weight was recorded. Every 2 weeks participants were asked to complete questionnaires about their morning weight.

What were the basic results?

Out of the 41 participants, 27 achieved their 5% weight loss target – 15 in the 5:2 diet group and 12 in the traditional diet group. Average weight loss was 5.4% in the 5:2 group and 5% in the traditional diet group.

It took 5:2 diet participants an average of 59 days to achieve their 5% weight-loss target and those following the traditional diet 73 days. However, the difference did not reach statistical significance.

Neither were there any differences between groups in blood sugar control after eating.

The 5:2 group did, however, have reduced triglyceride blood fats immediately after eating, a difference that just reached statistical significance.

How did the researchers interpret the results?

The researchers state: "Our preliminary data suggest that the mode of energy restriction may have different cardio-metabolic effects, which in turn could be important to long-term disease risk."

They suggest the superiority of the 5:2 diet on blood fat levels after eating "now warrants targeted mechanistic evaluation within larger study cohorts".

Conclusion

This study does not provide strong evidence that there is a difference between the diets. While more people on the 5:2 diet achieved their 5% weight loss target, this was only a difference of 3 people. The 5:2 dieters also reached the target 2 weeks quicker than those following a traditional diet, but again this wasn't a significant difference.

The study did find some evidence that certain blood fats were lower immediately after eating for those following the 5:2 diet. However, it's a big jump to say that this will have long-lasting effects and lower risk of heart disease.

The study has several limitations:

  • the sample size was very small and there may not have been enough people to reliably detect differences for many of the outcomes – making it difficult to know whether the results are valid or due to chance
  • weight and dietary compliance were self-reported, which may lead to inaccuracies – some participants may have been following their diet more closely than others
  • almost all the participants were white, which is an unrepresentative study population, especially as cardiovascular risk differs depending on ethnicity

Overall this study doesn't tell us much about weight loss and the most appropriate diet, apart from very weak evidence in favour of the 5:2 diet for helping you lose weight quicker.

It does reinforce the importance of achieving and maintaining a healthy weight, however you choose to do so, as this will prevent obesity-related disease.

For anyone who is looking for a free weight loss plan, advice can be found here.

Analysis by Bazian
Edited by NHS Website