Intermittent fasting (IF) is one of the latest diet trends. It sounds intriguing; finally, we have found an effective diet that does not restrict what or how much we eat. IF allows you to eat the same types of foods and the same number of calories, just within a shorter time period. The idea behind IF is that having a longer time between eating will lower insulin levels and allow us to use our stored energy in the form of fat. Insulin is a hormone that is released to bring glucose (from carbohydrates) from the bloodstream into our cells to be used as energy. Excess glucose that the body does not need for energy immediately is signaled by insulin to be stored away as fat. When we are fasting, our body does not have its favorite form of energy--glucose--so it turns to our fat stores instead.
In theory, IF sounds great. Several human studies have shown that IF is associated with decreased body weight, body fat, and waist circumference (1). While these results are intriguing, a 2017 systematic review and meta-analysis found that IF was just as effective as traditional caloric restriction for weight loss. However, many of these studies in this review were short-term, so we do not know if participants were able to maintain the weight they lost. About 20 of the reviewed studies compared intermittent fasting to continuous calorie restriction (traditional dieting), finding similar drop-out rates between both, suggesting that IF is not easier to follow than any other diet (2).
IF has, however, been found in rats to improve glucose homeostasis and insulin sensitivity, which is especially important for people at risk for diabetes (3). A 2017 review article found that in some studies IF reduced body fat percentage and HbA1c, a measure of blood glucose, more than continuous energy restriction. Further, the review found that IF decreased total cholesterol, LDL cholesterol (the bad cholesterol), blood pressure, and triglyceride levels (4).
IF is appealing because it allows you to consume more food at once, leaving you fuller and more satisfied after meals. Despite it being an alternative to continuous calorie restriction that yields similar weight loss according to many studies, there is no data that suggests it is more sustainable than traditional dieting. In a 2016 systematic review, participants following IF experienced constipation, headaches, bad temper, and lack of energy and concentration (5). Therefore, in the case of IF, it really comes down to personal preference. For some people, eating larger meals could reduce the urge to binge later on in the day. For other people, extreme hunger during times of fasting could make the diet hard to stick to long-term.
Overall, IF does not seem to produce any overt harmful effects. With the research we have right now, we do see some benefits, but few that are unique to IF. Moving forward, we need more clinical human research to understand the long-term effects of IF. Regardless of the supposed pros or cons, be careful when trying this new diet--because that is what it is, a diet. Dieting can lead to an endless cycle of losing weight and gaining it back, which may be even worse for your health. The best diet is one you can stick to long-term, and current research suggests that IF is no better than other diets in that respect.
Written and researched by Sarah Kozlowski Nutrition and Dietetics student UMASS Amherst
Reviewed by Nicole Maslar RDN, LDN