A method for analyzing the mitochondrial function of a subject involves: conducting a first test wherein the average oxygen consumption of the subject during exercise is measured while the subject is breathing air having a first oxygen concentration, preferably room air having a concentration of 20-21% by weight; conducting a second test wherein the average oxygen consumption of the subject during exercise is measured while the subject is breathing air having a second oxygen concentration, preferably air having an oxygen concentration of 30-33% by weight; measuring first and second energy quotients obtained from the first and second tests, respectively; and then using the energy quotients to calculate the subject's Mitochondrial Factor. The Mitochondrial Factor is directly proportional to the subject's mitochondrial efficiency and function. If conducted periodically, the method can be used to determine whether the subject's mitochondrial efficiency is increasing or decreasing over time. The rate of increase or decrease will indicate the subject's risk of developing one or more age-related diseases.