When we lose body weight from fat, we lose it primarily by exhaling carbon dioxide. (http://www.bmj.com/content/349/bmj.g7257). The weight exchanger is a device that the user blows into and which collects the carbon dioxide from the exhalation, thereby increasing in weight by the amount that the user decreased. The collection is done via a carbon scrubber. The flow rate could be measured to estimate the weight of carbon dioxide transferred and a cumulative total displayed. Or the weight of the scrubber can be measured to see it increase with each exhalation.
Possible components:
Activated carbon for scrubber (http://www.amazon.com/API-Activated-Filter-Carbon-22-Ounce/dp/B002DW0HLU/)
Milligram scale (http://www.amazon.com/Smart-Weigh-GEM20-Precision-Milligram/dp/B00ESHDGOI/)