About 40% of American adults live with obesity — and for many, it can feel a bit like a roller-coaster ride as their weight fluctuates up and down.

The cycle of losing and regaining weight, commonly known as the yo-yo effect, could be due to a type of "metabolic memory," in which the body remembers and strives to return to its former state of obesity, according to a new study.

"Obesity is a chronic condition with significant metabolic consequences, strongly linked to various metabolic and cardiovascular diseases," study author Dr. Ferdinand von Meyenn, assistant professor for nutrition and metabolic epigenetics at ETH Zurich in Switzerland, told Fox News Digital.

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"A well-documented observation is that the body tends to defend increased body weight, making weight loss and maintenance notoriously challenging."

Obesity is a common chronic disease in the U.S., with one in five children and two out of five adults meeting the criteria, according to the Centers for Disease Control and Prevention (CDC).

The new research, published in November in the journal Nature, points to epigenetics (genetic activity) that may play a role in regaining weight after weight loss. 

What is epigenetics?

"Epigenetics, which involves chemical markers that influence gene activity without altering the DNA sequence, is crucial in how cells function and respond to environmental factors," Dr. Fatima Cody Stanford, obesity medicine physician at Massachusetts General Hospital and Harvard Medical School, told Fox News Digital.

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"These markers can be altered by lifestyle factors, such as diet, and can remain stable for years, effectively creating a cellular ‘memory’ of past states, such as obesity," added Stanford, who was not part of the study.

It is well-known that cells keep their genetic identity as they divide. The researchers were curious about what happens to fat cells — which live on average for 10 years before the body replaces them, von Meyenn said.

This differs from other cells in tissues, which divide much faster – intestinal cells, for example, generally divide every other week, he added. 

Fat cells still must adapt to external stimuli and undergo epigenetic adaptations, von Meyenn noted.

The researchers set out to determine whether these changes could be reversed.

Fighting ‘memory’ in fat cells 

In mouse studies, the researchers found that even after significant weight loss, their cells have a "memory" of obesity encoded in the epigenome, which controls the activity of genes, said von Meyenn.

"Our study indicates that one reason maintaining body weight after initial weight loss is difficult is that the fat cells remember their prior obese state and likely aim to return to this state," he told Fox News Digital.

"This means one would have to ‘fight’ this obesogenic memory to maintain body weight."

Based on this research, a failure to maintain weight loss after dieting is not necessarily due to lack of effort or willpower, but could also be driven by an underlying biological phenomenon, von Meyenn added.

Potential limitations 

The research looked only at fatty tissue, and the genetic mechanisms were studied only in mice. Even so, the researchers said they believe that similar mechanisms also apply to humans. 

Other experts cautioned, however, that the study shows only association and does not prove that epigenetic changes cause the yo-yo effect.

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"The authors pointed out that they cannot say for certain that the epigenetic modifications directly cause people to regain their lost weight, nor pinpoint which specific epigenetic markers may be driving this effect," Petronella Ravenshear, board-certified nutritionist and author of "The Human Being Diet," told Fox News Digital.

Florida-based Ravenshear, who was not part of the study, noted that the results should not lead people to claim, "It’s not my fault, it’s my genes!"

Plans for future research

"We will need to now expand, see how the memory can be erased and whether other cells or tissues are also affected, [such as] the brain or the liver," von Meyenn said.

It’s possible that if people maintain a healthy weight for a year or longer after dieting, that may be enough time to erase the memory in fat cells, according to Ravenshear.

"This discovery underscores the importance of preventing obesity, particularly in children and adolescents, to avoid establishing this epigenetic memory that complicates long-term weight management," Stanford noted.

"Understanding these mechanisms further could lead to more effective treatments and prevention strategies, emphasizing the need for a proactive approach to weight management from an early age," she added. 

‘Way of living’

The term "diet," in its original connotation, means "way of living" — but it is now synonymous with short-term dietary changes that are unsustainable, Ravenshear noted.

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People often regain the weight they lose when they return to the eating habits that led to weight gain in the first place, many experts agree.

Ravenshear cited Professor David Benton at Swansea University in the U.K., author of "Tackling the Obesity Crisis," who recently shared in a Guardian interview, "The mantra is that diets fail."

"They fail because to avoid regaining lost weight, you need to permanently change your diet."

After finishing a diet, many people return to the previous lifestyle that caused the problem in the first place, he noted.

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"Frequently snacking, and eating calorie-dense, nutrient-poor foods, disrupts our blood sugar, raises insulin levels and adds to inflammation – and the inflammation itself makes it harder to lose weight," Ravenshear told Fox News Digital.

She recommends focusing on breaking addictions to sugar and refined carbohydrates and embracing a new way of eating.

The expert suggests choosing nutrient-dense whole foods, eating three meals a day, and fasting with nothing but water for five hours between meals.

The weight loss occurs as a side effect of rebalanced hormones and lowered inflammation, she noted.