Selenium offers important health benefits, including the prevention of some types of cancer. The traditional selenium indexes, such as selenium concentration, do not account for the metabolic status of this element regarding its chemoprotective effect. Then, the knowledge of a group of proteins that respond to selenium supplementation could be useful in the assessment of the metabolic status of selenium. The effect of dietary supplementation of rats with sodium-selenate on the blood plasma proteome is investigated. A group composed of six rats is fed a basic diet supplemented with sodium-selenate at 1.9 μg of Selenium per g of food, and a control group is fed a diet that covers the minimum selenium requirements, each for ten weeks. A proteomic approach is used to both quantify the changes in the abundance of some plasmatic proteins and to identify them. Fibrinogen, apolipoproteins, haptoglobin, and transthyretin changed significantly their abundance due to selenium administration. Those proteins are indirectly related to selenium metabolism. Then, the change in the proteomic profile due to selenium supplementation could probably be considered as a new index to assess the metabolic status of selenium. This index might help in the prevention of some diseases by nutritional diagnosis and, consequently, the adequate dietary recommendation.