Vitamin D deficiency or lower vitamin D levels are associated with a significant increase in the severity of psoriasis, according to US research on nearly 500 cases, one of the largest studies yet.

Brown University dermatologist Eunyoung Cho and colleagues say their findings suggest people with the irritating skin condition that affects over 8 million people in the US may benefit from vitamin D-rich foods or supplements.

Psoriasis is an immune-mediated disease characterized by an abnormally fast turnover of skin cells, and its exact cause is unclear. It's thought to arise from a genetic predisposition triggered by environmental factors.

Anyone who experiences its chronic buildup of dead cells, which causes itchy, scaly patches, knows psoriasis can be painful and involve more than just physical health. Some people even wrongly assume it's contagious.

"Topical synthetic vitamin D creams are emerging as new therapies for psoriasis, but these usually require a doctor's prescription," says medical student Rachel Lim from Brown University, who presented the research at NUTRITION 2023 in Boston on July 25.

Scientists think vitamin D plays a role in preventing the progression of skin diseases by modulating the immune response and acting directly on the skin's repair cells.

"With growing public interest in vitamin supplementation, we wanted to further examine the connection between vitamin D levels and psoriasis severity," Cho says.

Cho, Lim, and other researchers at Brown University and Massachusetts College examined data from more than 40,000 people gathered by the National Health and Nutrition Examination Survey (NHANES) between 2003 and 2014.

Individuals' vitamin D levels and the severity of their psoriasis were evaluated in a group representing the US population.

"Few studies have looked for this association in groups of people, especially in large US populations, or examined this relationship through a clinical nutrition lens," explains Cho.

Data from the NHANES revealed 491 cases of psoriasis, with 162 reported between 2003 and 2006 and 329 reported between 2011 and 2014.

The researchers used self-reported psoriasis-affected body surface area to measure the severity of the disease in each individual. They also collected data on vitamin D levels from blood samples.

"Only one previous study, published in 2013, has used NHANES data to analyze the relationship between vitamin D and psoriasis," says Lim.

"We were able to add more recent data, which more than tripled the number of psoriasis cases analyzed, making our results more up-to-date and statistically powerful than previously available data."

After adjusting the data to account for lifestyle factors like age, gender, race, BMI, and smoking habits, the analysis found that people with lower vitamin D levels had significantly more severe psoriasis.

Those with more psoriasis-affected body surface area had lower average vitamin D levels. On the other hand, the less affected someone's skin was by psoriasis, the higher their average vitamin D levels were.

Cho and colleagues separately analyzed the percentage of people with vitamin D deficiency, defined as less than 50 nanomoles per liter of blood. They compared this to the four levels of psoriasis-affected body surface area.

In the group with the highest level of psoriasis-affected body surface area, 39 percent of people were vitamin D deficient, compared to 25 percent in the group with the lowest level of psoriasis-affected body surface area.

This relationship suggests that vitamin D might affect how psoriasis develops and progresses.

Vitamin D deficiency has been previously linked with an increased risk of depression and COVID-19 mortality, while supplementation, when levels are inadequate, may reduce heart attack risk and ease depressive symptoms.

"Our results suggest that a vitamin D-rich diet or oral vitamin D supplementation may also provide some benefit to psoriasis patients," Lim says.

The researchers note that people with psoriasis should talk to their doctors and dermatologists before taking vitamin D supplements. Vitamin D toxicity is uncommon, but supplementation without medical advice can be dangerous and interfere with other medicines.

The preliminary findings are not yet peer-reviewed, although studies are selected for presentation by a committee of experts.

The study has been presented at the annual meeting of the American Society for Nutrition, NUTRITION 2023.