Methods and systems for determining a proportion of a metabolic fuel being metabolized are disclosed. A computer can receive concentration measurements of substances exhaled by a user during a time period. The computer can determine a time of interest that occurs during the time period when a change occurs in the proportion of a first metabolic fuel type being metabolized by the user. The change in the proportion can be determined based on the concentration measurements of the substances, such as a respiratory quotient which corresponds to the levels of the first metabolic fuel type and a second metabolic fuel type being metabolized. These proportions can be indicated to the user as well as the time of interest when the change in the proportion occurs in the first metabolic fuel type being metabolized.
