A system for measuring and tracking metabolic rate, physical activity and calorie intake. The metabolic rate is measured with a design that features an adaptive sampling mechanism for accurate breath sample collection, optimized flow rate measurement for minimizing backpressure while maximizing accuracy, humidity regulation and water condensation reduction mechanism for reliable performance, as well as breath temperature measurement for volume and humidity corrections. The system further comprises an improved algorithm for determining physical activity such as related energy expenditure, and a mechanism for tracking changes in food intake over time.