What is Intermittent Fasting?

Intermittent fasting (IF) is periods of voluntary abstinence from food and drink. It is an ancient practice followed in a variety of different formats by populations globally and can vary in terms of length, frequency and consumption. Intermittent fasting became a hot trend after a popular TV documentary and accompanying book where Michael Mosely proposed the 5:2 diet which touted the benefits of restricting energy intake severely for 2 days a week but eating normally during the rest of the week.

What are the benefits?

A recent review by Patterson and Sears in 20171 concluded that modified fasting regimens appear to promote weight loss and may improve metabolic health. They explain that intermittent fasting regimens are hypothesised to influence metabolic regulation via effects on (a) circadian biology, (b) the gut microbiome, and (c) modifiable lifestyle behaviours such as diet, activity, and sleep. Intermittent fasting is also known to improve cardiometabolic health. Within 5 weeks Sutton et al in 20182, showed IF to improve insulin sensitivity, blood pressure and oxidative stress even without weight loss in men with prediabetes. An additional benefit to the gut microbiota could be to encourage the movement of pathogenic bacteria through the digestive system, preventing a more recently discussed condition known as SIBO (small intestine bacterial overgrowth) where bacterial fermentation could be causing painful gas production after meals. The migrating motor complex (MMC) is a cyclic, recurring motility pattern that occurs in the stomach and small bowel during fasting; it is interrupted by feeding3 but increased by the hunger hormone ghrelin4.

How long should a fast be?

Fasting times can vary. Alternate-day fasting involves alternating fasting days, during which no calories are consumed, and feeding days, during which foods and beverages are consumed as desired. Whereas modified fasting regimens generally specify that energy consumption is limited to 20–25% of energy needs on regularly scheduled fasting days. Time restricted feeding on the other hand involves having a longer overnight fast of 12-16 hours.

It may be wise to start with periods of a restricted diet opposed to a strict water fast and to watch out for any signs of low blood sugar. Until blood sugar levels are balanced or in those with low blood pressure, it maybe that long periods without eating may cause dizziness, fainting, confusion, irritability, weakness and tiredness. We are creatures of habit and it may be that changing our learnt eating patterns may take some time to get used to. Hunger may be experienced in the short term especially in the mornings.

It may be advisable that on the days you choose to fast that you refrain from excessive exercise. It could be considered that this would be pushing the body beyond natural limits. However, this will vary from person to person and individual health, reserves and blood sugar balance would need to be taken into consideration. It may be that gentle walking, swimming, cycling, qigong or yoga may be acceptable for some. With any forms of fasting it is important to listen carefully to the body and what you need.


  1. Patterson RE, Sears DD. Metabolic Effects of Intermittent Fasting. Annu Rev Nutr 2017;37:371-393.
  2. Sutton EF, Beyl R, Early KS, Cefalu WT, Ravussin E, Peterson CM. Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss in Men with Prediabetes. Cell Metab 2018;27(6):1212-1221.e3.
  3. Deloose E, Janssen P, Depoortere I, Tack J. The migrating motor complex: control mechanisms and its role in health and disease. Nat Rev Gastroenterol Hepatol 2012;9(5):271-85.
  4. Fu RG, Wang L, Zhang J, et al. [Effect of ghrelin on duodenal migrating myoelectric complex in rats with chronic renal failure]. [Article in Chinese] Nan Fang Yi Ke Da Xue Xue Bao 2011;31(4):624-7.