Reported by BBC Health
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.
More from the original report
She would later be diagnosed as vitamin D-deficient – blood tests showed her levels were "next to nothing".
In the UK, low vitamin rates are increasing in some age groups , external - particularly in girls and young women.
According to data from the National Diet and Nutrition Survey, last year nearly a quarter of 11- to 18-year-olds were classed as deficient.
Vitamin D can come from our diet, in foods such as oily fish and meat, or supplements.
But most is made by our body when UV light in sunlight hits the skin. Lower levels of sunlight - depending on where in the world you live or the time of year - decrease the body's ability to produce it.
Covering parts of the body and wearing sunscreen also has an impact - despite their protective advantages.
Those with darker skin need exposure to stronger light to produce the same amount of vitamin D as lighter skin.
Doctors don't routinely offer blood tests to check for vitamin D deficiency, but will offer a supplement if a patient is concerned they might be high risk.
Vitamin D deficiency is linked to the bone disease rickets in babies and infants. In adults it causes a similar weakening of the bone called osteomalacia , external .
"They call it the silent disease," says Dr Suma Uday, a paediatric endocrinologist who wants to raise awareness of the problem.
She warns that vitamin D deficiency in young people and adults is difficult to spot and can be missed or misdiagnosed. Symptoms include fatigue and joint and bone pain.
Uday says: "It's basically silently eating away at the bones, leaving them softened and prone to breaks."
Amaka Osu says she doesn't think many of her friends realise what the symptoms are