Intermittent fasting (IF) has quickly become one of the world’s most welcomed 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 find following an IF regimen easier than a traditional calorie-restriction approach (3). IF's concept seems to be divisive, so it's important for us to take a closer look at the research and define the good, bad, and unknown, including whether it's a practice that should be recommended to weight loss surgery patients.

What is Intermittent Fasting (IF)?

IF is defined as an eating pattern that cycles between periods of fasting and eating. Typically, the types of food eaten are not specified; rather, the eating time is specified. Two common intermittent fasting methods are daily 16-hour fasts and fasting for 24 hours, twice per week (3). Fasting for a certain number of hours each day (or eating just one meal a couple 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 different ways to implement IF, all of which involve splitting up 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 consumed during your last meal and has 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 either eat very little or nothing at all. Here are the 3 most popular approaches to IF:

1. The 16/8 method (also referred to as time-restricted feeding or TRF): This involves skipping breakfast and restricting your daily eating period to 8 hours, such as 12pm-8pm. You fast for 16 hours in between (3).
2. Eat-Stop-Eat: This involves fasting for 24 hours, once or twice a week, for example by not eating from dinner one day until dinner the next day (5).
3. The 5:2 diet: With this method, you consume only 500-600 calories on two non-consecutive days of the week, eating normally the other 5 days (5).

By reducing your calorie intake, all these methods should cause weight loss, as long as you don't compensate by eating more during the eating periods. Many find the 16/8 method the simplest, most sustainable, and easiest to stick with long-term (3). It is by far the most popular. Longer periods without food, such as 24, 48, or 72-hour fasting periods 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 become accustomed to IF. Feelings of hunger and crankiness are common as the body adjust to the new routine; however, patients who make it through this adjustment period often stick with the plan for the long-term. Please remember, it's highly important and recommended to check with your doctor or health care professionals before starting IF.

Besides weight loss, what are the benefits of intermittent fasting?

Research shows that intermittent fasting periods can do more than burn fat, thus resulting in other benefits (5). A 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 reduced body weight, IF can lower cholesterol, improve glucose control, reduce liver fat, and improve blood pressure. Studies have shown that IF, which leads to caloric restriction, increases the lifespan of even healthy people (5,6).

Other benefits research has revealed include:

• 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 following weight loss surgery?

Weight loss surgery helps patients lose excess body weight. It has the potential to eliminate sleep apnea, high blood pressure, type 2 diabetes, and reduces the risk of developing heart disease. Practicing IF could compromise the positive results of bariatric surgery. IF is not typically a recommended practice for weight loss surgery patients. Patients who have bariatric surgery 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 caloric intake, nausea, and vomiting.

One of the main goals after bariatric surgery is to create long-lasting behavioral changes to maintain weight loss and all other related health outcomes and benefits. Food intake is already limited post-weight loss surgery, and for this reason alone, it is not a good idea to begin any type of diet, including IF. Eating healthy, quantity-controlled meals on a regular schedule throughout the day is of the utmost importance for weight loss surgery patients. 

IF isn't for everyone, but for those who've struggled to lose weight, it can be another tool in the kit. It's all about a person's lifestyle and the choices they make. Weighing the options and deciding what’s going to work best for you is the key to success. For example, perhaps fasting for 12 hours a day is enough to obtain the results desired. Keep in mind that IF may have different effects on different people. Please talk to your doctor or health care professional if you start experiencing unusual anxiety, headaches, nausea, or other symptoms after beginning IF.

References:

1. 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.
2. 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.
3. 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 14, 290 (2016).
4. 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. 
5. Johnstone A. Fasting for weight loss: an effective strategy or latest dieting trend?. Int J Obes (Lond). 2015;39(5):727-733.
6. Varady KA. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?. Obes Rev. 2011;12(7):e593-e601.