The design of advanced closed-loop life support systems requires that the CO2 removed from the cabin atmosphere be reduced to recover its oxygen content. An optimum physical/chemical technology choice for reduction will balance reaction products with the demands and products of the human metabolic process. Although water electrolysis is the technology of choice for oxygen generation, many different processing strategies for CO2 reduction have been proposed. The Sabatier and the Bosch carbon formation reactions and Solid Oxide electrochemical reduction along with Steam Reforming and Fischer-Tropsch synthesis, are among the candidates. Each strategy provides a different level of oxygen recovery. The consequence of each choice directly impacts the water required for electrochemical oxygen generation.This paper reports on a theoretical study of CO2 processing options, both individually and in combinations that result in improved loop closure. The measure of closure that is presented is the overall number of moles of water that must be provided from outside the loop in order to obtain one mole of metabolic oxygen. Descriptions of the technology are included along with some qualitative estimates of technology development status. No quantitative measures of system weight, volume or power are provided.