Reducing energy consumption is a key requirement for Massive Machine-Type Communication (mMTC) devices operating on battery power. Device-to-Device (D2D) communication is a promising technology that can be used in 5G networks for this purpose. In this paper, we identify the interest of using a D2D relay mechanism compared with a direct link with an unfavorable link budget. We first look for an optimal Relay-MTC device configuration that minimizes the energy consumption. We then compare the energy consumption in direct and relaying mode expressing it in terms of the distance between the base station and the MTC device and between the base station and the relay. We show that when the MTC device is close to the base station, D2D relay mechanism can reduce the energy consumed by the MTC device at the expense of an increase in the global energy consumption (i.e. the energy consumed by the MTC device plus the energy consumed by the relay). On the other hand, when the MTC device is far from the base station, D2D relay mechanism allows reducing both the energy consumption by the MTC device and the global energy consumption.