Are you overweight? Don't just blame fast food, it's in your DNA!

It’s not just what you eat that makes those pants tighter — it’s also genetics, researchers have claimed.

In a new study, scientists discovered that body-fat responses to a typical fast-food diet are determined in large part by genetic factors, and they have identified several genes they say may control those responses.

The study is the first of its kind to detail metabolic responses to a high-fat, high-sugar diet in a large and diverse mouse population under defined environmental conditions, modeling closely what is likely to occur in human populations.

The researchers found that the amount of food consumed contributed only modestly to the degree of obesity.

“Our research demonstrates that body-fat responses to high-fat, high-sugar diets have a very strong genetic component, and we have identified several genetic factors potentially regulating these responses,” first author Dr Brian Parks, a postdoctoral researcher at the David Geffen School of Medicine at UCLA said.

“We found that obesity has similar genetic signatures in mice and humans, indicating the mice are a highly relevant model system to study obesity. Overall, our work has broad implications concerning the genetic nature of obesity and weight gain,” he said.

The dramatic increase in obesity over the past few decades has been tightly associated with an increase in obesity-related conditions such as type 2 diabetes, heart disease and cancer.

And while high-calorie diets containing high levels of fat and sugar, along with sedentary lifestyles, have been considered the most significant environmental factors contributing to this epidemic, the new UCLA research demonstrates that body-fat responses to food are strongly inherited and linked to our DNA.

The researchers note that overconsumption of high-calorie, high-sugar food is an important factor contributing to the obesity epidemic, but stress that food consumption is only one of many environmental factors that affect obesity.

The findings are published in the online edition of the journal Cell Metabolism.