A cardiorespiratory fitness optimizer apparatus includes a mouthpiece and a valve assembly. The mouthpiece includes lower and upper tooth beds and an anterior manifold. The manifold terminates anteriorly at a centralized valve assembly interface and includes at least one air-letting aperture. The valve assembly is matable with the centralized valve assembly interface and includes an adjustable aperture. The user is able to adjust the aperture for increasing and decreasing airflow resistance therethrough for optimizing cardiorespiratory fitness. The cardiorespiratory fitness optimizer apparatus may include at least one sensor for sensing airflow activity within the valve assembly and communicating data relating to the airflow activity to an external device for displaying human readable output relating to the airflow activity upon the external device. The valve assembly is removable from the mouthpiece and replaceable with a plug element for maintaining anterior formations of the mouthpiece when the valve assembly is removed.