A fitness sensing system that is configured to be supported by a subject, the system comprising: a processor; an intelligent activity monitor (100, 103) comprising an accelerometer; and a non-invasive blood glucose monitor comprising at least one light source and at least one photodetector, the fitness sensing system being configured to interrogate, by at least one of the light sources, a region of the subject, and receive, by at least one of the photodetectors, radiation from the at least one light source which is reflected from or transmitted through the interrogated region of the subject, and to analyse signals from the photodetectors to provide plethysmography data, blood oxygenation data, and/or heart-rate data; wherein the processor is configured to: process first data received from the accelerometer to identify, from among a plurality of different activities, an activity performed by the subject; compute at least one metric representative of the identified activity; process second data received from the blood glucose monitor of the fitness sensing system to provide at least a blood glucose level reading for a subject; determine an operational mode, from among a plurality of operational modes, for the blood glucose monitor based upon the results of the at least one metric combined with the processed second data; place the blood glucose monitor in the determined operational mode; detect a rate of change of a subject's blood glucose level; compare the detected rate of change to a stored threshold value; issue a control signal to the intelligent activity monitor (100, 103) to collect acceleration data at an increased frequency if the detected rate of change exceeds the stored threshold value; detect, after the control signal is issued, an amount of activity performed by the subject; determine if the rate of change of the subject's blood glucose level is attributed to the activity level of the subject; and revise a time for which a subsequent glucose measurement is to be made. a processor; an intelligent activity monitor (100, 103) comprising an accelerometer; and a non-invasive blood glucose monitor comprising at least one light source and at least one photodetector, the fitness sensing system being configured to interrogate, by at least one of the light sources, a region of the subject, and receive, by at least one of the photodetectors, radiation from the at least one light source which is reflected from or transmitted through the interrogated region of the subject, and to analyse signals from the photodetectors to provide plethysmography data, blood oxygenation data, and/or heart-rate data; wherein the processor is configured to: process first data received from the accelerometer to identify, from among a plurality of different activities, an activity performed by the subject; compute at least one metric representative of the identified activity; process second data received from the blood glucose monitor of the fitness sensing system to provide at least a blood glucose level reading for a subject; determine an operational mode, from among a plurality of operational modes, for the blood glucose monitor based upon the results of the at least one metric combined with the processed second data; place the blood glucose monitor in the determined operational mode; detect a rate of change of a subject's blood glucose level; compare the detected rate of change to a stored threshold value; issue a control signal to the intelligent activity monitor (100, 103) to collect acceleration data at an increased frequency if the detected rate of change exceeds the stored threshold value; detect, after the control signal is issued, an amount of activity performed by the subject; determine if the rate of change of the subject's blood glucose level is attributed to the activity level of the subject; and revise a time for which a subsequent glucose measurement is to be made. process first data received from the accelerometer to identify, from among a plurality of different activities, an activity performed by the subject; compute at least one metric representative of the identified activity; process second data received from the blood glucose monitor of the fitness sensing system to provide at least a blood glucose level reading for a subject; determine an operational mode, from among a plurality of operational modes, for the blood glucose monitor based upon the results of the at least one metric combined with the processed second data; place the blood glucose monitor in the determined operational mode; detect a rate of change of a subject's blood glucose level; compare the detected rate of change to a stored threshold value; issue a control signal to the intelligent activity monitor (100, 103) to collect acceleration data at an increased frequency if the detected rate of change exceeds the stored threshold value; detect, after the control signal is issued, an amount of activity performed by the subject; determine if the rate of change of the subject's blood glucose level is attributed to the activity level of the subject; and revise a time for which a subsequent glucose measurement is to be made.