The term "preclinical data" refers to information obtained from animal experiments, for the most part. And " observational studies" are just that - conclusions based on observations without any strong supporting evidence.

So it's not totally surprising when opinions about some treatments have to be reversed in the light of good clinical studies.

Preclinical data suggests that low-dose selenium supplements have a favorable effect on glucose metabolism. And observational studies have indicated that dietary antioxidants may protect against diabetes - and selenium is a mineral component of antioxidant enzymes. Putting two and two together produced a theory that selenium might actually prevent diabetes. This has now proved to be false.

A report in the Annals of Internal Medicine finds that, in a cancer prevention study, 1200 patients taking 200 micrograms of selenium supplements had a 55% greater risk of developing type 2 diabetes than those taking a placebo. During a follow-up of about 8 years, those with the highest levels of plasma selenium had 2.4 times the diabetes risk as placebo recipients.

Selenium is acknowledged to be excellent as a medicated shampoo for dandruff, but it clearly should not be taken internally, unless your doctor has determined that you have selenium deficiency - an extremely rare condition in the Western world.

Source
HealthandAge Blog