Scientists at Cambridge University say drugs to keep people slim are now a possibility after they identified the handful of genetic factors that prevent overeating.
Medics have known for several years that genes can influence a person's weight.
However, the new study is significant because it reveals in granular detail which variants suppress or encourage appetite.
The research team analysed the genetic profiles of more than half a million volunteers from the UK Biobank.
They found that around six per cent of British people with European ancestry have a particular combination which means they are more likely to avoid putting on weight regardless of their lifestyle.
Published in the journal Cell, the study focused on a gene known as MC4R which was previously identified by the same Cambridge scientists to play a role in appetite by controlling a receptor in the brain called melanocortin 4.
People who had certain variants of MC4R which disrupted this receptor tended to gain weight easily, the study found, while those who had a different combination caused the receptor to stay "switched on", enjoying the opposite effect.