Of the 30 studies, can you present a table of what each one presented? Perhaps if you can't unify how the units are presented, you may have to throw some out or contact the authors. You may also want to think about how you are going to weight each of these measurements, since I am guesing each study did not have the same sample size.

What was predicted in all of the regression models, and did they control for the same covariates. Please provide much more information.