_ RCGP Learning

Intermittent fasting (IF) is a pattern of eating whereby the person undertakes a regular (usually daily) fast of at least 16 hours. In light of National Obesity Awareness Week in January or in keeping with New Year’s resolutions, patients may seek advice on weight loss from their GP and opinions on the efficacy of certain diets. This blog will discuss some of the evidence around fasting for weight loss and management of type 2 diabetes (T2D) and other cardiovascular risk factors.  

Ingested food is stored in the body as liver glycogen and as fat, the process of lipogenesis being mediated by insulin. Energy is provided by glycogenolysis, however after around 12 hours of fasting, hepatic glycogen becomes depleted and adipose tissue lipolysis begins to occur. A recent NEJM review article suggested that ‘intermittent  fasting  elicits  evolutionarily  conserved,  adaptive  cellular  responses  that  are  integrated  between  and  within  organs  in  a  manner  that  improves  glucose  regulation,  increases  stress  resistance,  and  sup-presses inflammation”¹

For patients, we can explain that across our evolution we not always had access to three meals a day, and that after after about 8 to 12 hours the the body enters a ‘fat burning’ phase.

There are many ways to fast, with the most basic being a fast of at least 16 continuous hours in every 24. To get results in terms of weight loss or increased health benefits, the fasts should ideally be done on a daily basis. Two popular IF methods are as follows:

• 16:8 – Fast for 16 hours and eat all meals within an 8 hour window. This typically means skipping breakfast and eating two meals a day.

• 20:4 – Fast for 20 hours and eat all meals within a 4 hour window. This would involve eating one meal or having two smaller meals a day.

 A relatively short fast of 16 hours can be incorporated as part of daily life. The ‘intermittent’ nature of IF is that the fasting is flexible and at the convenience of the individual – the number of fasts per day and the exact hours of fasting can be altered if needed e.g. to allow the patient to eat normally at a social event.

Alternative methods include the 5:2 diet, which includes eating a balanced diet for 5 days out of the week and only consuming 500 calories for the remaining 2 days. Whilst this has been shown as an effective method for weight loss, it differs from intermittent fasting as it involves reducing calorie intake instead of not eating at all. 

There is evidence to suggest that IF may reduce the need for prescribed insulin in patients with diabetes².

IF is a relatively new concept, with some studies being small and of short duration; longer and larger studies are needed. Some examples of the evidence around IF are given below:

• A 2014 crossover study of 54 patients with T2D found that the group which ate only two meals per day, had better outcomes than those who ate the same calories spread out over six smaller meals. The group eating only two meals per day had a greater reduction in BMI, waist circumference and fasting plasma glucose³.

• A 2015 literature review of 15 studies found that various types of fasting reduced weight and improved lipid profiles and called for further research, with greater information about food intake so as to differentiate between the benefits of fasting and the benefits of calorie restriction⁴

• A 2019 study randomised 88 obese women into four groups (control, calorie restriction and IF with or without calorie restriction). All food was measured and provided by the study. Those who fasted for 24 hours three times a week lost more weight than those who simply restricted their calories, even though the calorie intake of the two groups was identical. The fasting group also had lower lipids and fat mass than the calorie restriction group⁵.

• A 2019 study of 15 men who limited their food intake to nine hours in each 24 hour period, whilst wearing a continuous glucose monitor, were found to have improved glucose tolerance and fasting triglycerides during the fasting period compared to the control period⁶.
• Another 2019 study followed 19 participants with metabolic syndrome who limited their dietary intake to a 10 hour period for 12 weeks. There were significant reductions in BMI, waist circumference, LDL cholesterol and blood pressure⁷.

It seems that IF therefore has the potential to be beneficial for patients with T2D and metabolic syndrome; it may be the case that educating patients on the benefits of IF can help in the management of T2D and reduce the need for pharmacological interventions⁸.

IF is a relatively new concept in relation to T2D so there are no official guidelines for healthcare professionals on how to manage a diabetic patient who wishes to do try it. As with any kind of fast, it is not recommended for pregnant/breastfeeding women or for children.

RCGP members can find out more about using diet to manage Type 2 diabetes by accessing our eLearning course on ‘Type 2 diabetes and the low GI diet’. Non-members can pay £25 to access this course.

The following resources are free to both members and non-members:

• ‘Five Minutes to Change your Practice’ screencast on Updated management of type 2 diabetes in adults
• EKU Journal watch item on Effects of intermittent fasting on health, aging, and disease

RCGP members can also benefit from free access to the following resources on Type 2 diabetes:

• EKU17: Diabetes in Adults, Children & Young People (Type 1 & 2)
• EKU18: Type 2 Diabetes in Adults: Management
• EKU Screencast: Type 2 diabetes in adults: management

References

1 De Cabo, R., & Mattson, M. P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 381(26), 2541–2551. doi:10.1056/nejmra1905136

² Diabetes UK,  2019. About Type 2 diabetes. [Online] Available at: https://www.diabetes.org.uk/Type-2-diabetes

³ Kahleova H, Belinova L, Malinska H, et al. Eating two larger meals a day (breakfast and lunch) is more effective than six smaller meals in a reduced-energy regimen for patients with type 2 diabetes: a randomised crossover study [published correction appears in Diabetologia. 2015 Jan;58(1):205]. Diabetologia. 2014;57(8):1552–1560. doi:10.1007/s00125-014-3253-5

⁴ Grant M. Tinsley, Paul M. La Bounty, Effects of intermittent fasting on body composition and clinical health markers in humans, Nutrition Reviews, Volume 73, Issue 10, October 2015, Pages 661–674,

⁵Hutchison A.T. et. al., 2019. Effects of Intermittent Versus Continuous Energy Intakes on Insulin Sensitivity and Metabolic Risk in Women with Overweight. Obesity (Silver Spring). 2019 Jan;27(1):50-58

⁶Hutchison A.T., et. al., 2019. Time-Restricted Feeding Improves Glucose Tolerance in Men at Risk for Type 2 Diabetes: A Randomized Crossover Trial. Obesity (Silver Spring). 2019 May;27(5):724-732. Available at https://www.ncbi.nlm.nih.gov/pubmed/31002478

⁷Wilkinson M.J. et. al., 2019. Ten-Hour Time-Restricted Eating Reduces Weight, Blood Pressure, and Atherogenic Lipids in Patients with Metabolic Syndrome. Cell Metab. 2019 Dec 2. pii: S1550-4131(19)30611-4. Available at https://www.sciencedirect.com/science/article/pii/S1550413119306114

⁸ Firmly S., et al., 2018. Therapeutic use of intermittent fasting for people with type 2 diabetes as an alternative to insulin. [Online] Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6194375/

Many thanks to Dr Rachel Hudson and Dr Claudia Camden-Smith for their help with producing this blog.