The present application relates to compositions comprising selenium (e.g., selenium enriched yeast) and methods of using the same to treat and inhibit obesity, diabetes and related conditions. In particular, the present application provides compositions comprising selenium enriched yeast (e.g., selenium enriched yeast comprising 2% or less inorganic selenium), and methods of using the same to enhance mitochondrial activity and function (e.g., in skeletal muscle and liver) in a subject (e.g., as a therapeutic and/or prophylactic treatment for diabetes, obesity and related conditions).