Researchers at Cambridge University says some people have a genetic advantage over others when it comes to being thin.
The study claims being slim is sometimes down to DNA and not only diet and exercise.
Those involved in the research found obese people had a higher genetic risk score than those with a "normal weight", which contributes to their chances of being overweight.
Professor Sadaf Farooqi says people can be quick to judge others.
"The people who can lose weight very easily or can even stay thin, they're not necessarily morally superior; they're just lucky.
"They inherited the right genes that allow them to keep weight off.
"What we want to do in the future is now to pinpoint some of those genes so we can find out how people can stay thin".
