is vs are

"Is' because the subject 'One' is singular.

Thanks.

This could explain why some adults are fat.

Yasser Arafat.

Well, he was.

https://blog.oxforddictionaries.com/2012/11/21/subject-and-verb-agreement/

"more than one in five /ten etc. can cause problems because people may think the subject is singular. In fact, the subject is plural (Britons, not one Briton) so you should use the plural verb are:
✓ More than one in five Britons are now classed as obese."