Intermittent fasting (IF) has quickly become one of the most popular approaches to weight loss, overall health and simplifying life. Studies have shown it can have powerful effects on your body and brain, and it may even help you live longer.1,2 Some people find an IF regimen easier than a traditional calorie-restriction approach.3 Because IF divides opinion, it is important to look closely at the research and define the good, the bad and the unknown – including whether it should be recommended after weight loss surgery.
Key takeaways
- IF cycles between periods of fasting and eating; the most popular method is 16/8.
- IF is not typically recommended after weight loss surgery: patients are usually told to eat regularly timed meals with an emphasis on protein.
- Prolonged fasting can cause dehydration, poor calorie intake, nausea and vomiting in bariatric patients.
- Always check with your doctor before starting IF.
What is intermittent fasting (IF)?
IF is an eating pattern that cycles between periods of fasting and eating. Typically, it specifies when you eat rather than which foods. Two common methods are daily 16-hour fasts and fasting for 24 hours twice a week.3 Fasting for a certain number of hours each day (or eating just one meal a couple of days a week) can help your body burn fat, and some scientific evidence points to health benefits as well.
How does intermittent fasting work?
There are several ways to do IF, all of which split the day or week into eating and fasting periods.4 IF works by prolonging the period during which your body has burned through the calories from your last meal and switched to burning fat. Once the body exhausts its sugar stores for fuel, it starts burning fat, which is referred to as metabolic switching.
What are the intermittent fasting methods/plans?
During fasting periods, you eat either very little or nothing at all. The 3 most popular approaches:
- The 16/8 method (also called time-restricted feeding, TRF): skip breakfast and restrict your daily eating period to 8 hours, such as 12:00–20:00, fasting for 16 hours in between.3
- Eat-Stop-Eat: fast for 24 hours once or twice a week, for example from dinner one day until dinner the next.5
- The 5:2 diet: consume only 500–600 calories on two non-consecutive days of the week and eat normally on the other 5 days.5
By reducing calorie intake, all these methods should cause weight loss, as long as you do not compensate by eating more during the eating periods. Many people find the 16/8 method the simplest, most sustainable and easiest to stick with long term – it is by far the most popular.3 Longer periods without food, such as 24, 48 or 72-hour fasts, are not necessarily better and may be dangerous, as going too long without eating might encourage the body to store more fat in response to starvation.2 It can take up to four weeks for the body to get used to IF. Hunger and crankiness are common while the body adjusts, but people who get through this period often stick with the plan long term. It is highly recommended to check with your doctor or healthcare professional before starting IF.
Besides weight loss, what are the benefits of intermittent fasting?
Research shows that intermittent fasting can do more than burn fat.5 The metabolic switch affects the body and brain, and many things happen during IF that can protect organs against chronic diseases such as heart disease, type 2 diabetes, gut health issues, even inflammatory bowel disease and many cancers.2,5 In addition to reducing body weight, IF can lower cholesterol, improve glucose control, reduce liver fat and improve blood pressure. Studies have shown that IF, which leads to calorie restriction, increases the lifespan of even healthy people.5,6
Other benefits revealed by research include:
- A boost in working memory and verbal memory in adults
- Improved blood pressure and resting heart rate
- Reduced tissue damage in surgery and improved results
Is intermittent fasting safe after weight loss surgery?
Weight loss surgery helps patients lose excess body weight. It has the potential to eliminate sleep apnoea, high blood pressure and type 2 diabetes, and reduces the risk of developing heart disease. Practising IF could compromise these positive results. IF is not typically recommended for weight loss surgery patients, who are usually instructed to eat regularly timed meals with an emphasis on increased protein intake. Prolonged fasts can cause weight loss surgery patients to suffer dehydration, poor calorie intake, nausea and vomiting.
One of the main goals after bariatric surgery is to create long-lasting behaviour changes to maintain weight loss and all related health benefits. Food intake is already limited after weight loss surgery, and for this reason alone it is not a good idea to start any type of diet, including IF. Eating healthy, portion-controlled meals on a regular schedule throughout the day is of the utmost importance for weight loss surgery patients.
IF is not for everyone, but for those who have struggled to lose weight, it can be another tool in the kit. It is all about a person's lifestyle and choices. Weighing the options and deciding what will work best for you is the key to success – for example, perhaps fasting for 12 hours a day is enough to achieve the results you want. IF may have different effects on different people.
Talk to your doctor or bariatric team before starting IF, and contact them if you experience unusual anxiety, headaches, nausea or other symptoms after starting.
Sources
- Vidmar AP, Goran MI, Raymond JK. Time-limited eating in pediatric patients with obesity: a case series. J Food Sci Nutr Res. 2019;2(3):236-244.
- Anton SD, Moehl K, Donahoo WT, et al. Flipping the metabolic switch: understanding and applying the health benefits of fasting. Obesity (Silver Spring). 2018;26(2):254-268.
- Moro T, Tinsley G, Bianco A, et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J Transl Med. 2016;14:290.
- Jane L, Atkinson G, Jaime V, Hamilton S, Waller G, Harrison S. Intermittent fasting interventions for the treatment of overweight and obesity in adults aged 18 years and over: a systematic review protocol. JBI Database System Rev Implement Rep. 2015;13(10):60-68.
- Johnstone A. Fasting for weight loss: an effective strategy or latest dieting trend? Int J Obes (Lond). 2015;39(5):727-733.
- Varady KA. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss? Obes Rev. 2011;12(7):e593-e601.
This article is for general information only and does not replace personal medical advice. Always discuss your situation, blood test results and supplementation with your bariatric team or healthcare provider.