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.

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 - 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.

"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 was in the middle of making dinner at her university house in Liverpool when she started shaking.

"I couldn't really see in front of me," she says. Her housemate took her to A&E - later she too would be diagnosed as vitamin D deficient.

Other than feeling tired, she says she had no symptoms: "I'm a 21-year-old student. I've got work and uni - I'm always feeling tired."

She remembers her mother suffering from low vitamin D but says she did not realise it could happen to her.

After taking supplements, Amaka feels a lot better, but she is anxious in case it happens again. "It was scary and I don't want to experience that again."

Uday joins other clinicians and academics who have formed a group - ACT NOW Vitamin D - calling for the threshold at which you are classified as deficient to be raised.

She says that would "catch" more people with low vitamin D and give a more accurate picture of how widespread the issue is. It would also mean they can treat the deficiency earlier, she says.

Uday warns of "a lifelong deficit" in women - in adolescence, then pregnancy, and then the newborn is affected.

"If you don't correct it from the very beginning," she says, "this is going to affect multiple generations."

"It's shocking," says Prof Martin Hewison, from the University of Birmingham.

He tells me that despite being 100 years on from the discovery of vitamin D "rickets hasn't gone away".

"If we have rickets in the community, there are going to be many more people who are vitamin D deficient."

He says the government should look into wider fortification of food in which vitamins are added to foods during production. Countries like Canada and Sweden have mandatory policies.

In the UK, some foods such as breakfast cereals and spreads are already fortified with vitamin D, but it is voluntary.

During the winter months, people start to lose their vitamin D levels.

"We might have to accept that for certain groups vitamin D supplementation is a matter of course," says Hewison.

Saskia is now taking vitamin D supplements regularly - and is back in goal.