A biosensor includes an optical circuit that obtains a plurality of photoplethysmography (PPG) signals from light at different wavelengths that is reflected from or transmitted through tissue of a user. A processing circuit determines a measurement value for a nitric oxide (NO) level in blood flow using first and second PPG signals and determines an insulin response from caloric intake using the measurement value for the NO level in blood flow. The first PPG signal is obtained from light at a wavelength having a high absorption coefficient for NO in blood flow and the second PPG signal is obtained from light at a second wavelength having a low absorption coefficient for NO in blood flow. The processing circuit also determines one or more phases of digestion using at least one or more of the plurality of PPG signals.