Why dieting can cause fat regain: what the research found

Many people end a diet weighing less than when they started, only to find themselves heavier a few months later than before they began. A 2018 review article in the European Journal of Clinical Nutrition by Dulloo, Miles-Chan, and Schutz proposed a mechanism for why: collateral fattening. The concept describes what the body does when it tries to restore lean mass, such as muscle and organ tissue, lost during a calorie deficit. Because fat mass recovers faster than lean mass after a diet ends, the body’s compensatory hunger signals stay elevated for longer than fat stores actually need. This drives caloric intake beyond what fat storage requires, and the result is fat overshoot. You end up carrying more fat than you did before losing the weight. How pronounced this rebound becomes depends largely on how much lean mass you lost during the diet, which comes back to protein intake and resistance training.

What the research describes

The paper describes collateral fattening as part of the body’s broader autoregulation of body composition. It is not a passive process. The body actively defends its lean mass reserves, and when those reserves drop below a certain level, appetite increases to support restoration.

The sequence during a diet: both fat mass and lean mass decrease. How much lean mass you lose depends substantially on whether you are eating enough protein and maintaining resistance training through the deficit. Strip both of those away, and lean mass losses are significantly higher.

Fat mass and lean mass do not recover at the same rate after a diet

Dulloo et al. (2018) identified that fat mass returns faster than lean mass after a calorie deficit ends, and that the body’s hunger-driving signals persist until lean tissue is restored, often producing more fat storage than was originally lost.

Why recovery is not equal

The core problem is asymmetry. After a diet ends, fat stores can replenish relatively quickly. Lean mass takes longer because muscle requires protein synthesis and stimulus to rebuild.

The researchers identified two mechanisms that drive fat overshoot:

1. Reduced lean mass triggers increased appetite. The body reads a lean mass deficit as a signal to eat more.
2. Because lean tissue restores slowly, continued appetite-driven eating delivers more calories than lean tissue actually needs, and the surplus goes to fat storage.

The result: appetite stays elevated during a window where fat mass has largely recovered but lean mass has not. Calories that come in during that window are disproportionately stored as fat.

Why lean mass loss matters during a deficit

This is not an argument against calorie restriction. It is an argument for how you carry out the restriction.

Calorie deficits that cause significant lean mass loss, typically those with inadequate protein and no resistance training, create the conditions for more pronounced collateral fattening. A larger lean mass deficit means a longer recovery period, more persistent hunger, and a bigger fat overshoot.

By contrast, preserving lean mass during the deficit reduces the size of the lean mass hole the body needs to climb out of. Less lean mass lost means less compensatory hunger, a shorter recovery window, and a smaller fat overshoot.

This is one of the clearest practical arguments for prioritising protein and resistance training during a fat loss phase, not just for the muscle itself, but for what muscle preservation does to the recovery trajectory after the diet ends. Thinking about sustainable approaches to fat loss means accounting for what happens after the deficit, not just during it.

Reducing the rebound

The practical implications from the research are direct:

• Adequate protein during the deficit. This is the primary tool for sparing lean mass in a calorie restriction phase. Spreading intake across meals supports how protein is used by the body. This matters equally when using intermittent fasting as your calorie deficit approach, since a compressed eating window can limit how often protein feedings occur.
• Resistance training throughout the deficit. Training signals help preserve the muscle you have when calories are restricted.
• Avoid extreme deficits. Severe restriction, the kind of undereating that leaves training underfuelled, accelerates lean mass loss and worsens the autoregulatory rebound. Moderate, sustained deficits preserve more lean tissue, which is how we structure fat loss coaching from the start.
• Return to maintenance gradually. After a diet ends, coming back to maintenance calories slowly reduces the window where compensatory hunger outpaces actual tissue needs.
• Keep daily non-exercise movement up. Total energy expenditure is shaped by far more than structured training sessions. Non-exercise activity thermogenesis (NEAT) can vary by nearly 800 kilocalories per day between people with similar exercise habits, and maintaining it during the recovery period limits the energy surplus available for fat overshoot.

The weight regain that follows many diets is not simply a willpower problem. For some people, it is the body’s lean mass restoration system doing exactly what it is designed to do, at a time when fat storage happens to be faster than muscle recovery. Knowing that changes what you focus on.

Reference: Dulloo AG, Miles-Chan JL, Schutz Y. Collateral fattening in body composition autoregulation: its determinants and significance for obesity predisposition. Eur J Clin Nutr. 2018;72(5):657-664.