Answer
The total energy E of a particle is the sum of its rest energy $E_{0}$ and its kinetic energy $K$. At very high energies, the kinetic energy is so much greater than the rest energy, that the particle may behave as if it has no rest energy. So, $E\approx K$. This is just like a photon, which has no rest energy because it is massless.
Work Step by Step
The total energy E of a particle is the sum of its rest energy $E_{0}$ and its kinetic energy $K$. At very high energies, the kinetic energy is so much greater than the rest energy, that the particle may behave as if it has no rest energy. So, $E\approx K$. This is just like a photon, which has no rest energy because it is massless.
