The performance of private equity portfolio companies backed with high-yield bonds has fallen short of expectations, raising concerns that firms are loading businesses with more debt than they can handle.

Moody's analysed financial data from 144 highly-leveraged companies, which were rated for the first time between January 2010 and July 2013. Eighty-eight of the 144 firms -all of which are based in Europe, the Middle East and Africa-were private equity-backed.