As healthcare in many countries faces an aging population and rising costs, mobile Health (mHealth) sensing technologies promise a new opportunity. However, the privacy concerns associated with mHealth sensing are a limiting factor for their widespread adoption. The use of wireless body area networks pose a particular challenge. Although there exist protocols that provide a secure and private communication channel between two devices, the large transmission overhead associated with these protocols limit their application to low-power mHealth sensing devices. We propose an adaptive security model that enables use of privacy-preserving protocols in low-power mHealth sensing by reducing the network overhead in the transmissions, while maintaining the security and privacy properties provided by the protocols.