There's no evidence that intermittent fasting reduces your metabolic rate any more than other diets. The argument against intermittent fasting is that it puts your body into "starvation mode", which prevents you from burning fat and shuts down your metabolism. While it’s true that long-term weight loss can cause you to burn fewer calories over time, this happens no matter what method you use.  A short-term fast can actually increase your metabolic rate because norepinephrine levels increase, which stimulates your metabolism and signals your fat cells to break down body fat. Research shows that fasting for up to 48 hours can boost metabolism by 3.6–14%. However, with longer fasts of 3 or more days, your metabolism can reverse, decreasing instead.