Recently, the Orthogonal Frequency Division Multiplexing (OFDM) technique has been extensively studied for fiber-based optical transmissions in the context of access networks. The Adaptive Modulation Optical (AMO) OFDM system has been proved to be one of the cost-effective solutions. Among adaptive modulation techniques, the Levin-Campello (LC) bit/power loading, which is widely implemented for xDSL systems, is shown to bring excellent performances in unamplified optical Intensity Modulated/Direct Detected (IMDD) transmissions. In this paper a novel adaptive Discrete Fourier Transform (DFT) Precoded OFDM (POFDM) is proposed and investigated. By means of numerical simulation, the proposed modulation scheme is shown to reduce the Peak-to-Average Power Ratio (PAPR) of the transmitted OFDM signal up to 2 dB at 10^-3 clipping rate. As a consequence, a smaller input back-off (IBO) can be applied to the laser-driving current before the power amplifier, resulting in a lower power consumption of the POFDM system when compared to the conventional LC AMOOFDM system. According to simulation results, the energy consumption of the power amplifier is reduced by a factor of two in unamplified optical IMDD transmissions. Moreover, in terms of capacity-versus-reach performance, the proposed system achieves the same performance as the conventional adaptive modulation scheme.