Corporate creditworthiness in the U.S. is deteriorating at the fastest pace since 2009, with earnings growth slowing as yields rise from record lows.

The ratio of upgrades to downgrades fell to 0.89 in the first five months of the year after reaching a post-crisis high of 1.55 in 2010, according to data from Moody's Capital Markets Group. At Standard & Poor's, the proportion has declined to 0.83 as of last week from a year earlier.

The Federal Reserve has pumped more than $2.5 trillion into the financial system since markets froze in 2008, helping companies improve profitability by lowering their borrowing costs. Policy makers are considering curtailing $85 billion in monthly bond buying intended to prop up the economy as analysts surveyed by Bloomberg forecast earnings growth of 2.5 percent in the current quarter, the least in a year.