TORONTO, Aug 4 (Reuters) - Canada's two biggest life insurers on Wednesday beat analyst estimates for second-quarter underlying earnings, as strong income growth at their asset management units helped boost their results from a year earlier.

Core earnings from Manulife's global wealth and asset management business jumped nearly 50% to C$356 million ($284 million) in the three months ended June 30 from the same period last year, while Sun Life increased asset management earnings by 20% to C$311 million.

Earnings at Manulife, Canada's top insurer, were also supported by 7.6% growth in its Asian operations, helping offset weakness in Canada and the United States, where core earnings fell 7% and nearly 21% respectively.

Sun Life, however, posted a 34% increase in underlying earnings in its U.S. business due to lower death and health claims. Asia and Canada lagged with smaller increases, of 6% and 3% respectively, hampered by expenses in Asia and higher death claims in India, along with lower investment activity in Canada.

All of Canada's biggest life insurers have benefitted from strong asset management performance, with Great-West Lifeco also reporting earnings that beat analyst estimates on Tuesday, thanks to its U.S. business, stronger equity markets and higher fee revenue.

Manulife shares have added 6.5% this year and Sun Life 15%. Great-West Lifeco shares rose 0.8% on Wednesday, following its results on Tuesday after markets closed, bringing gains this year to 25.2%.

Manulife posted profit excluding one-off items of 83 Canadian cents a share, from 78 cents a year earlier, beating estimates of 77 Canadian cents.

Sun Life's underlying profit increased to C$1.50 a share from C$1.26 a year earlier. Analysts had expected C$1.47. ($1 = 1.2543 Canadian dollars) (Reporting By Nichola Saminather, editing by Chris Reese and Richard Pullin)