Overweight people who undergo gastric bypass surgery do much better with higher vitamin D levels. In a study at Massachusetts General Hospital, vitamin D levels were measured before surgery. Patients with the lowest levels, below 30 ng/ml, were three times as likely to develop an infection after surgery. This is a particular concern because so many obese patients are deficient in vitamin D. No one knows, however, whether pre-surgical vitamin D supplementation will prevent such complications.