Jan 27 (Reuters) - Commercial insurer W R Berkley WRB.N reported a 45% rise in fourth-quarter profit on Monday, helped by strong performance in its investment portfolio and underwriting business.

Equity markets have been rallying due to rate cuts and were further fueled by the U.S. presidential election victory of Donald Trump, who is expected to ease regulation and implement more business-friendly policies.

The company's net investment income rose 1.3% to $317.4 million in the quarter, compared to the same period last year.

"We positioned our investment portfolio well for changes in the environment, which resulted in robust growth in net investment income from our fixed-maturity portfolio and a strong contribution to total return from net unrealized gains on our equity portfolio," the company said in the statement.

Its profit for the quarter ended Dec. 31 was $576.1 million, or 1.44 cents per share, compared with $397.3 million, or 98 cents per share, last year.

Greenwich, Connecticut-based W R Berkley saw its net premiums written increase by nearly 8% to $2.94 billion as businesses spent more on insurance policies amid easing recession concerns.

The company reported a GAAP combined ratio of 90.2%, compared with 88.4% a year earlier. A ratio below 100% shows that an insurer earned more in premiums than it paid out in claims.

(Reporting by Jaiveer Singh Shekhawat in Bengaluru; Editing by Alan Barona)

((JaiveerSingh.Shekhawat@thomsonreuters.com))