MetLife Inc., the largest U.S. life insurer, was downgraded by Fitch Ratings on the prospect of losses tied to investments including commercial real estate holdings. The issuer default rating was lowered to A from A+ on “concerns regarding the fragile nature of the economic recovery and continued deterioration in the commercial real estate market,” the ratings firm said today in a statement on the New York-based insurer. MetLife will probably report $2.2 billion to $2.6 billion in investment losses in the five quarters ending in December 2010, Fitch said.