For years, weight loss advice has been built around a simple formula: eat fewer calories and burn more through exercise. But according to obesity specialists, the human body is far more complicated than a basic math equation, which may explain why long-term weight loss remains difficult for many people.

Experts say genetics, metabolism, hormones, lifestyle and even social environments all influence body weight, making the process highly individual rather than one-size-fits-all.

Kim Pfotenhauer, a diabetologist and obesity medicine specialist at Michigan State University, says understanding how the body regulates weight could help people better manage their health goals.

Why the body resists weight loss

One major theory is known as “set point weight,” a concept developed in the 1950s. It suggests the body tries to maintain a preferred level of body fat by adjusting hunger signals and energy use.

Research has shown that after weight loss, hormones linked to hunger increase while hormones that promote fullness decrease, sometimes for more than a year. This biological response may push the body to regain lost weight.

Another related concept, called metabolic adaptation, refers to the body burning fewer calories after weight loss than expected for someone of the same size who has not recently lost weight.

This means people often need to exercise more and eat even more carefully as they lose weight, because the body gradually becomes more energy-efficient.

Studies involving participants from the TV show The Biggest Loser found that these metabolic changes could persist for years after significant weight loss.

Doctors say treatments such as bariatric surgery and newer medications like GLP-1 drugs may help reduce hunger signals and support long-term weight management.

The ‘settling point’ theory

Another model, known as the settling point theory, argues that weight is shaped less by biology and more by habits, surroundings and lifestyle.

Under this theory, body weight stabilises when calorie intake and calorie expenditure balance out naturally over time.

For example, someone who switches from an active job to a desk job while consuming larger portions and highly processed foods may gradually gain weight until the body settles into a new equilibrium.

Unlike the set point model, this theory focuses heavily on environmental and behavioural influences rather than biological control.

A middle ground: The dual intervention model

Some researchers believe both theories may work together.

The dual intervention point model proposes that every person has an upper and lower range where the body is comfortable maintaining weight. Within this “zone of indifference,” lifestyle and environmental factors largely determine body weight.

But when the weight drops too low, the body activates survival mechanisms by increasing hunger and reducing energy expenditure to prevent starvation.

Scientists say evidence for the body strongly defending against weight gain is weaker in humans than in animals, but genetics and evolution may still influence how individuals respond to food and fat storage.

The “drifty gene” theory further suggests that modern humans may no longer face the same evolutionary pressures to remain lean for survival, unlike animals trying to escape predators.

Weight loss is not the same for everyone

Experts say no single theory fully explains how body weight works in real life. However, there appears to be a clear difference between how the body reacts during active weight loss and how it behaves while maintaining weight afterwards.

Research suggests reducing calorie intake may be more effective for losing weight initially, while exercise plays a more important role in preventing regained weight over time.

Doctors emphasise that obesity care should go beyond diet alone and include sleep, stress, exercise, mental health and medical support when necessary.