(Bloomberg) -- Hedge funds have amassed the greatest share of the $1.2 trillion U.S. junk-bond market since the credit crisis, raising concern bets with borrowed cash will accelerate losses when the Federal Reserve stops printing record amounts of money.

The funds, which typically use leverage to bolster returns, hold as much as 23 percent of outstanding dollar-denominated high-yield bonds, from as much as 18 percent last year and the highest since 2008, according to Barclays Plc. Credit hedge funds have boosted assets by 89 percent since 2008, outpacing the 66 percent growth of the junk market, data from Hedge Fund Research Inc. and Bank of America Merrill Lynch indexes show.

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