I didn't know I was lacking in vitamin D until I collapsed at work

Twenty-three-year-old Saskia Reeves-Priestley was at work when she suddenly collapsed.

"I just felt really dizzy, really nauseous, was having heart palpitations and was really hot," she says.

There had been no prior warning. Saskia, a special educational needs teacher, was a keen goalkeeper who played football four times a week and felt fit and healthy.