This must be the most common question I get asked daily….so let’s review the most updated evidence!
First, what is intermittent fasting (IF)? Intermittent fasting is an eating pattern that cycles between periods of fasting and eating. It includes a variety of meal timing approaches that alternate between periods of extended fasting (no intake, or less than 25% of needs) and periods of unrestricted intake. To induce weight-loss, the idea is that we induce ketosis during the period of fasting–because all weight loss happens through ketosis, or a low insulin environment.
Need a refresher on three main types of intermittent fasting (although there are many other variations!)? The most popular methods for intermittent fasting include alternate-day fasting (ADF), 5:2 diet, or daily time-restricted eating (TRE). The most common is TRE where we restrict our day into fasting windows and eating windows. For example, someone who fasts 16 hours, will eat between the remaining 8 hours of the 24-hour day. This has gained traction within weight loss discourse, but how well does it work and what fasting: eating windows work for weight loss?
To discuss the effectiveness of weight loss from intermittent fasting, it is also important to review the definition of significant weight loss. In clinical research, interventions must produce ≥ 5% body weight loss from baseline weight to be considered effective for weight loss. Achieving this minimum amount of weight loss is associated with health benefits such as cholesterol, blood pressure and diabetes control.
With respect to trials evaluating weight loss with intermittent fasting, only TRE has undergone the research standards. Although currently available research is still in its infancy and requires larger trials, it has found on average to produce 3-4% body weight loss from baseline and this is regardless of whether one followed 20:4, 18:6, or 16:8.
To date, no clinical trial has demonstrated significant weight loss (>5% loss from baseline) with intermittent fasting.
Now we are updated!
Source: Cienfuegos, S. et al. Effects of 4- and 6-h time-restricted feeding on weight and cardiometabolic health: a randomized controlled trial in adults with obesity. Cell Metab. 32, 366–378 (2020).
Source: Chow, L. S. et al. Time-restricted eating effects on body composition and metabolic measures in humans who are overweight: a feasibility study. Obesity 28, 860–869 (2020).