(Reuters) - MetLife reported a fall in third-quarter adjusted profit on Wednesday, owing to weak performance in the insurer's group benefits business, sending its shares down 5.5% after the bell.

Adjusted earnings from MetLife's group benefits business — which offers dental, disability, vision, accident, health and life insurance — slid 27% to $373 million from a year earlier.

MetLife said the weakness in the unit was primarily due to the impact of its annual actuarial assumption review coupled with weaker non-medical health underwriting.

Insurers use actuarial assumptions when calculating the life expectancy projection of a person seeking life insurance. Actuarial assumption is an estimate of an uncertain event for the purpose of calculating insurance premiums.

Meanwhile, adjusted net investment income rose 8% to $5.3 billion in the quarter, driven by higher interest rates.

Risk-averse insurers' investment portfolio tends to be heavily weighted toward bonds, which return better yields in a high interest rate environment.

Adjusted earnings available to common shareholders fell to $1.38 billion, or $1.95 per share, in the three months ended Sept. 30, from $1.49 billion, or $1.97 per share, a year earlier.

MetLife shares have risen 26% so far this year, compared with a 22% gain in the benchmark S&P 500 index.

(Reporting by Arasu Kannagi Basil in Bengaluru; editing by Alan Barona)