From various sites:
A new analysis of data from a large national study found that people who took a 200 microgram selenium supplement each day for almost eight years had an increased risk of developing type 2 diabetes than those who took a placebo or dummy pill.
The data came from the Nutritional Prevention of Cancer Trial (NPC), a large randomized, multi-center, clinical trial. It was designed to evaluate whether selenium supplements prevent skin cancer. In the study being published, researchers selected 1,202 participants who did not have diabetes when they were enrolled in the NPC Trial. Half received a 200 microgram selenium supplement and half received a placebo pill for an average of 7.7 years.
Link to at least one site here.