(Bloomberg) -- Investors have a fickle relationship with credit mutual funds lately, pouring cash in one year and yanking it out the next.

As a result, the world’s biggest mutual-fund firms are preparing for when sentiment sours for a prolonged period. They’re increasing the amount of cash in their portfolio and boosting their holdings of corporate-bond exchange-traded fund shares -- which trade like stocks instead of the antiquated, telephone-based system of buying and selling debt.

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