In the same way that being susceptible to obesity is, at least in part, a biological genetic trait, so, scientists found a few years ago, is thinness. Researchers found several genes, over and above those known to be connected to obesity, that were associated with being thin.
As one of the researchers said at the time, with reference to one of society’s last acceptable prejudices:
This research shows for the first time that healthy thin people are generally thin because they have a lower burden of genes that increase a person’s chances of being overweight and not because they are morally superior, as some people like to suggest
Or, again from The BBC’s reporting:
…this supports the idea that, for some people, being thin has more to do with inheriting a “lucky” set of genes than having a perfect diet or lifestyle.