To convert kilocalories per day to millimoles per gram per hour, one must consider the specific metabolic reaction involved, as the conversion depends on the energy yield of the substrate being metabolized. The discussion highlights the importance of knowing the enthalpy or free energy changes associated with the metabolism of the substance in question. It is noted that for Basal Metabolism Rates (BMR), which are measured in kilocalories per day, the conversion requires understanding the molecular weight of the substrate. A suggested approach involves calculating the energy yield per gram based on the reaction's energy output and the molecular weight of the metabolized compound. Ultimately, the conversion is contingent on the specific metabolic context and the properties of the substrate.