Thousands of people are being admitted to hospital with vitamin D deficiency at the East Lancashire Hospitals NHS Trust.
The vitamin helps people keep healthy bones, muscles and teeth, according to the NHS.
Vitamin D deficiency cases at the trust have soared over the last three years, with 3,535 diagnoses in the 12 months to March 2018, data from NHS Digital shows. This was one of the highest numbers seen by any trust in England.
During the same period in 2015-16, there were just 1,530 cases.
Public Health England has encouraged people to consider taking a vitamin D supplement during the winter months, after a surge in diagnoses at hospitals across England.
Over the last three years, cases have increased by 84%, climbing from 54,850 in 2015-16 to 101,140 in 2017-18. The vast majority were secondary diagnoses, meaning vitamin D deficiency was not the main cause for the patient's admission to hospital.
Vitamin D helps the body to regulate calcium and phosphate levels, which are needed to maintain healthy bones, muscles and teeth, according to the NHS. A lack of it can lead to bone deformities in children or painful musculoskeletal conditions such as osteomalacia in adults.
Sunlight is the main source of vitamin D, but a small amount can be obtained through dietary sources.
Dr Umesh Chauhan, GP at Pendle View Medical Centre and East Lancashire Clinical Cardiovascular Lead, said: “The main reason for the increase is because GPs and hospitals are more aware than ever of vitamin D deficiency, they’re doing more tests and identifying more patients at risk.
“Vitamin D plays an important role throughout the body and is important for our muscles and general health. Vitamin D production needs sunlight to fall directly on to bare skin (through a window is not enough). This is not the same as sun tanning; the skin simply needs to be exposed to sunlight.
“During winter, our local climate makes it difficult to create enough vitamin D naturally, but including foods and drinks fortified with vitamin D in your diet will help.
“In relation to these statistics, it’s important to point out that vitamin D deficiency is not the prime reason for these hospital admissions; the statistics are produced from tests conducted after a patient is admitted with other medical problems.”
The British Nutrition Foundation said one in four 11 to 18 year olds and one in six adults in the UK are believed to have low levels of vitamin D.
Professor Louis Levy, head of nutrition science at PHE, said: "Vitamin D is essential for healthy bones and most of us get enough from sunshine and a healthy balanced diet during summer and spring.
"During autumn and winter, those not consuming foods naturally containing or fortified with vitamin D should consider a 10 microgram supplement.
"Those who don’t expose their skin to the sun may not get enough vitamin D from sunlight and should take a supplement all year round."
Black or ethnic minority people and those who spend a lot of time indoors - such as people living in care homes - could benefit from supplements throughout the year, he added.
The NHS says vitamin D is only found naturally in a small number of foods, such as oily fish, egg yolks, and liver.
It is also found in fortified foods such as low-fat spreads, cereals and fortified milk and dairy substitutes.