Losing weight can be a frustrating game: after months of successful slimming, the kilos can soon pile back on, leaving people right back where they started.
No factor drives the yo-yo effectbut new research points to fatty tissue as a leading culprit. Fat “remembers” previous obesity and resists attempts to lose weight, scientists have found.
Researchers identified the biological memory after examining adipose tissue from people with obesity before and after losing weight after bariatric surgery. The tissues were further compared with fat from healthy individuals who had never been obese.
The analysis showed that fat cells were affected by obesity in a way that changed how they responded to food, possibly for years. In tests, the cells grew faster than others by absorbing nutrients faster.
Prof Ferdinand von Meyenn, a senior author of the study at the Federal Institute of Technology in Zurich, said: “Our study indicates that one reason why it is difficult to maintain body weight after initial weight loss is that the fat cells retain their previous obese state and probably intend to return to this state.
“It seems that the memory cells are primed to respond more quickly, and perhaps in unhealthy ways, to sugars or fatty acids.”
Further work on mouse cells traced the biological memory to chemical modifications on DNA or the proteins that DNA is wrapped around. These epigenetic changes alter gene activity and metabolism.
Write in Naturethe scientists describe how previously obese mice gained weight faster than others when put on a high-fat diet, suggesting a shift in metabolism that made it easier for them to gain weight. However, the memory of obesity in fat cells was not solely to blame. The scientists suspect that a similar memory exists in brain cells that influences how much food animals take in and how much energy they use.
Dr Laura Hinte, first author of the study, said: “From an evolutionary perspective this makes sense. Humans and other animals have adapted to defend their body weight rather than lose it, as food scarcity has historically been a common challenge.”
Almost two thirds of adults in England are overweight or living with obesity and worldwide the condition affects more than a billion people. Obesity is costing the NHS £6.9 billion a year and is the second largest preventable cause of cancer.
Another researcher, Dr Daniel Castellano-Castillo, said: “At a societal level, this may offer some comfort to individuals struggling with obesity.” Struggling to keep the kilos off may be driven by a cellular memory that is “actively resistant to change,” he said.
The work could pave the way for better weight management programs, although the cellular memory of obesity may also fade over time. “It’s possible that maintaining a reduced or healthy body weight for long enough is enough to erase the memory,” Hinte said.
Prof Henriette Kirchner at the University of Lübeck called the finding “very plausible”. “I’m convinced that it plays an important role in the yo-yo effect after dieting,” she said. “The researchers convincingly show that the longer you’ve been obese, the harder it is to erase the memory.”
People who lose weight through a diet or after weight loss dolls like Wegovy typically regain weight when they stop.
David Benton, an emeritus professor at Swansea University and author of the 2024 book Tackling the Obesity Crisis: Beyond Failed Approaches to Lasting Solutions, said more than 100 factors have influenced obesity.
“Obesity reflects consuming more calories than you burn. When a diet removes energy, you lose weight,” he said. “However, the mantra is that diets fail. They fail because to prevent you from regaining lost weight , you have to change your diet permanently. Often after we finish the diet, we go back to the lifestyle that caused the problem in the first place. The result is yo-yo dieting.”
