Vitamin D plays a number of important roles in the body; it helps maintain bone health, aids immune system function and supports lung and heart health, among numerous other functions. Exposure to sunlight is a key factor in vitamin D levels; ultraviolet radiation from the sun triggers vitamin D synthesis in the body. As such, people with low sun exposure often have lower vitamin D levels, which has been linked to numerous health problems. Now, in a new study published in the journal Obesity Science & Practice, researchers suggest low vitamin D levels may worsen outcomes for patients undergoing weight-loss surgery, or bariatric surgery. Study leader Leigh Peterson, of the Centers for Bariatric Surgery at Johns Hopkins, and colleagues found individuals in the US who underwent weight-loss surgery in winter - when vitamin D levels are at their lowest - were more likely to experience complications than those who had the surgery in summer. In addition, the team found bariatric surgery patients from the northern US - where there is typically less sunshine - had poorer outcomes than bariatric surgery patients from sunnier southern areas. "Sun exposure is critical in the synthesis of vitamin D, so the notion that people living in less sunny northern states may suffer from vitamin D deficiency is not surprising," notes Peterson. "What is remarkable is how closely sun exposure, vitamin D and surgical outcomes were linked."