Weight-loss surgery is associated with low vitamin D levels, but new research suggests seasonal changes in sun exposure may play a role in this complication.

A Johns Hopkins study involving more than 930,000 patients found that people in the northern United States who have weight-loss surgery during winter face more complications than patients in the South.

"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," study leader Leigh Peterson, a nutritionist and postdoctoral research fellow at the Johns Hopkins Center for Bariatric Surgery, said in a hospital news release.

"What is remarkable is how closely sun exposure, vitamin D and surgical outcomes were linked," Peterson said.

A review of medical records of patients who had the surgery between 2001 and 2010 found that 71 percent of the 300,000 operations that resulted in an extended hospital stay involved patients living north of South Carolina.

In addition, more than twice as many patients experienced delayed healing in winter than in summer, according to the study published online Dec. 14 in Obesity Science and Practice.

Nutritional deficiencies can trigger inflammation, increase infection risk and slow wound healing, but the researchers said more research is needed to determine if routine use of vitamin D supplements could help prevent these complications after weight-loss surgery.

Excessive vitamin D intake, while rare, can cause nausea, constipation, confusion and an abnormal heart beat. Most people can get enough vitamin D from routine sun exposure. The researchers added, however, that obesity is a known risk factor for low levels of vitamin D and said people planning to have weight-loss surgery should be screened for this deficiency.