The nature of wireless channel in a wireless body area network is highly dynamic and hence, any static design is trivially sub-optimal. In this paper we propose a cross-layer channel-adaptive framework to adjust the MAC frame body length of the data packets. We propose two different approaches to incorporate the impact of the time-varying channel gain in the transmission policy aiming to improve the overall energy efficiency of the system. The first approach is based on autoregressive channel prediction and the second scheme is a novel procedure inspired by the well-known slow-start mechanism. Explicitly, in our first scheme the channel gain is predicted and the optimal frame length is selected. In the second method, MAC frame length is sequentially increased after receiving the acknowledgment message or decreased if no acknowledgment is received. Our simulation results show that the proposed schemes can significantly improve energy efficiency in comparison with the case with fixed frame length.