It's not reduced at all. PoE utilizes a common mode signal (in the form of a DC bias) to send around 48 volts across as many twisted pairs as possible (to increase ampacity). This is referred to "Alternative A" POE, in that it does not use dedicated twisted pairs to send a DC signal, like Alternative B does. Alternative B can be used on base-t that does not utilize all four twisted pairs, such as 100 and 10 base-t.
Gigabit Ethernet over base-t MUST utilize alternative A, as it utilizes all conductors for data signals. Alternative A works for base-t fabric, as signals across twisted pairs are processed using differential signaling, meaning signals are processed by measuring the difference in potential across two waveforms, rather than a single waveform with respect to ground. With this, we can bias those data signals up to 48V with absolutely no impact on fidelity.
The DC bias is filtered out when we get to the powered device transformers, and we're left with a differential waveform on the DSP devices and a flat healthy DC current going to the power supply.