# How can one define a covariant rate of change of rest mass for an extended body?

For a point mass I can define a covariant rate of change of its rest mass as $$\frac{d}{d\tau}m_0$$ where $$\tau$$ is the proper time. How can I also define a covariant rate of change of rest mass for an extended body, bearing in mind that different parts will have different world lines and hence proper times?