Leptin is a hormone that helps to regulate appetite, body weight, and food intake.
During extended dieting and weight loss, leptin levels fall, stimulating an increase in hunger. Simultaneously, your metabolic rate also drops as sympathetic nervous system tone is reduced, thyroid hormone decreases, and your muscle begins to burn fewer calories during activity. This can make losing weight and keeping it off extremely difficult.
Interestingly, in studies where normal leptin levels are restored through injections, many of these adaptations in response to calorie restriction and weight loss are largely reversed. Keeping leptin levels as close to normal as possible while dieting may help prevent the drop in metabolism and increasingly ravenous hunger that usually goes hand-in-hand with diet failure.
For this reason, researchers were excited to discover that leptin levels in dieters are temporarily boosted following a day of higher calorie and carb intake (overfeeding). Theoretically, this might mean that including one or two high-calorie refeed days per week could lessen the harmful impact of a calorie restriction on metabolism, maximizing weight loss success.
However, while this might be possible in theory, we don’t actually know whether having a refeed day or two would keep metabolism from dropping. It’s entirely possible that the transient rise in leptin wouldn’t have much of a meaningful impact at all on metabolism—and based on the few studies that have been done, this appears to be the case.
More research is needed to figure out if refeed days can halt metabolic slowing and improve weight loss, so until we have a clearer picture, we suggest incorporating refeed days based solely on personal preference.