Intermittent Fasting: Acceptability, adherence and participant retention

Lead Research Organisation: Queen Mary University of London
Department Name: Wolfson Institute


Over a quarter of adults in England are obese, with obesity more common in disadvantaged socioeconomic groups. This is important because obesity is considered to be responsible for about 10% of illness and death in the UK. Weight loss improves health, but the menu of effective options for people who want to lose weight is limited.
A few years ago a new approach to weight management appeared called 'the 5:2 diet'. It is unusually straightforward. Dieters simply eat less (about 25% of their normal food intake) on two non-consecutive days per week, i.e. 500kcal for women and 600kcal for men. The approach became popular, with many 5:2 users claiming on social media that the diet helped them lose weight and maintain the weight loss. No studies, however, have evaluated this approach and so it is not known whether it is in fact safe and effective. Before NHS staff can recommend such an approach, we need to know more about its effects.

The 5:2 diet may be a useful strategy to add to existing advice to dieters, but it could also be just another fad diet with limited efficacy and poor participant retention, and it may even pose problems. While the simplicity of 5:2 diet is attractive, it also presents possible disadvantages if healthy eating and exercise are neglected. If on the other hand the 5:2 diet generates sustained and safe weight loss with no adverse effects on other health behaviours, its popular appeal means that it would be easy to disseminate. In view of the need for increasing the repertoire of options for tackling obesity, the 5:2 holds sufficient promise to warrant its evaluation.

As there is almost a total lack of any preliminary data, before launching a large definitive trial, there is a need to see how easy or difficult it is to recruit people to such treatment, how many persevere with the plan over the initial three months, whether the intervention can work in a self-help format or whether it needs face-to-face support from health professionals, whether it has any negative effects, and what results it is likely to generate. This information is important in deciding how large the definitive trial should be.

To provide such information, we propose to recruit 300 obese people from an area of high socio-economic deprivation and allocate them randomly to receive either standard advice on diet and physical activity delivered in a self-help format, 5:2 advice delivered in a self-help format, or 5:2 advice provided with six weekly support session. We would evaluate two different recruitment strategies - referrals from local GPs and adverts in local newspapers. Outcomes would include assessment of the two recruitment strategies in terms of recruitment rates, client characteristics and outcome; ratings of desirability, helpfulness and practicability of the interventions; number of weeks clients in the two 5:2 arms adhere to the 5:2 diet; weight and waist circumference reduction at 4 and 12 weeks; proportion of clients who express preference for switching to one of the alternative treatments at 3 months; change in exercise levels and in blood pressure at 6 and 12 weeks compared to baseline; changes in the content of typical diet at 12 weeks compared to baseline; decision on whether a main trial is warranted and if so, how it should be structured and implemented; and assessment of intervention protocols generating a report on any proposed changes and their rationale.

The proposal was discussed with 27 clients from our Weight Clinics and with a Panel of Service Users comprised of clients who attended or are currently attending our weight management service. Most were aware of 5:2 and several tried it, and these consultations contributed key ideas which informed this proposal.
In terms of potential impact, the project promises to contribute to the much needed broadening of the repertoire of options for tackling obesity.

Technical Summary

Intermittent fasting (IF) is a weight management approach with a strong theoretical rationale and some evidence of positive outcomes in dieters who adhere to it. A version of IF called 5:2 (participants limit their calorie intake on two days per week) has become popular in the UK, with a number of people claiming that it helped them lose weight and maintain weight loss. The approach seems to have a substantial popular appeal, which means that it holds a promise for extensive dissemination. However, its safety and efficacy are not known. The simplicity of 5:2 is attractive, but it also presents possible downsides if healthy eating and exercise are neglected.

We previously proposed a pivotal 5:2 trial to the NIHR Public Health Research Programme. The proposal received good referee feedback and was shortlisted, but the board ultimately decided that more primary research is needed. We agree that before launching a main study, there is a need to establish recruitment and retention parameters; optimal protocols; suitable comparators; acceptability of the approach and participants' adherence to it; and basic outcome data to inform sample size and power calculations.

We propose an early phase study to provide this information. We would randomise 300 participants into 5:2 delivered in a self-help format; 5:2 delivered with group support; and advice on healthy eating and exercise accompanied by standard self-help materials.

To inform the main study, two recruitment strategies would be used, GP referrals and local newspaper adverts. These would be evaluated for recruitment rates and profiles of clients recruited. Treatment attractiveness and acceptability would be evaluated via participants' ratings of the interventions; treatment adherence and participant retention would be monitored throughout the follow-up period; and weight loss, waist circumference, health behaviours and blood pressure would be measured at 1 and 3 months.


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