(Bloomberg) -- The biggest buyers in the $8 trillion market for U.S. corporate bonds, including mutual funds, pension funds and life insurers, should be stress-tested to help prevent instability during a market downturn, according to a research unit of the Treasury Department.

Nonbanks are playing a growing role in the market for corporate debt at a time when the ratio of company debt to gross domestic product has surpassed 2007 levels and is approaching an all-time high, the Office of Financial Research said in a report Tuesday. Entities including mutual funds and insurers now own a quarter of all company obligations.

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