Obesity. It’s a global epidemic that now kills more people than malnutrition. Physiologists work to unravel the causes of obesity and the genetic factors involved.

Produced by Orinoco Communications for The Physiological Society
Animation: Hayley Evenett
Illustration: Alex Scarfe
Sound Design/Music: Alexander Bradley
Narration: Elisa Canas
Director: Peter Barker

Scientific advisor: Jeffrey M Friedman
Producer at The Physiological Society: Rachel Wheeley


Sometimes it seems like everywhere we look we’re exposed to adverts about dieting, urging us to lose weight.

Globally, the diet industry rakes in over £150 billion a year, and that figure is rising fast.

But, at the same time, worldwide obesity levels are also growing, and that matters, because obesity now kills more people than malnutrition.

So, what’s going on? Why are self-control and dieting alone unable to stop the growing obesity epidemic?

Research by physiologists suggests there are important biological factors at play.
Namely, that some of us are simply more genetically predisposed to become obese than others.

The root causes of obesity can be summed up like this: food intake, minus energy burned, equals fat stored.

So, obesity occurs when we consume more than we burn, and fat mass builds up.

But what happens when genetics are thrown into the mix?

Our understanding of this field has been advanced by scientific experiments involving a species of obese mouse.

These mice are always hungry and won’t stop eating. They never feel full.

The mice weigh three times more than normal mice because of a defect in a single gene.

Scientists identified that gene as one that codes for a hormone called leptin.

Leptin is made by fat cells and signals to the brain how much fat mass is in the body.

Without it, the brain mistakenly believes the body is starving, so the mice keep eating, even though they’ve had enough food.

Scientists translated this finding to humans when a four-year-old boy with the same genetic mutation became severely obese.

With regular leptin injections, his over-eating stopped and, by the age of eight, he was no longer overweight.

Studies of twins have helped us to identify the genetic basis of obesity.

Identical twins who come from the same egg and the same DNA have very similar body sizes.

But twins that come from different eggs show much more variability.

These studies show that while single genes are occasionally responsible most obesity cases are caused by a combination of genes working together.

Understanding the causes of obesity is far from simple but genetic factors such as leptin play an important role to regulate food intake and body weight.

So self-control alone is rarely enough to overcome the powerful effect of genes.

That’s why physiologists are working hard to assess how a healthy diet and exercise, combined with a better understanding of how genes influence obesity, can help turn the tide on this global epidemic.