Insurer UnitedHealth Group Inc posted better-than-expected second-quarter profit on Tuesday, helped by growth in its plans for elderly and low-income Americans.

The largest U.S. health insurer by market value also raised the low end of its 2009 earnings forecast.

Net income rose to $859 million, or 73 cents per share, from $337 million, or 27 cents per share, a year earlier, when the company took a big charge for a legal settlement.

Analysts on average expected 70 cents a share, according to Reuters Estimates.

Revenue rose 6.8 percent to $21.7 billion.

Revenue in the company's Ovations unit serving seniors rose 13 percent to $8 billion. Enrollment in its Medicare Advantage plans jumped nearly 20 percent to 1.74 million.

At its unit serving Medicaid plans for low-income Americans, revenue shot up 45 percent to $2 billion, spurred by strong enrollment gains.

UnitedHealth spent 83.6 percent of its premiums on medical costs, up from 83.2 percent a year earlier, but within company expectations. Wall Street closely watches the ratio to gauge overall profitability.

The Minneapolis-based company forecast 2009 earnings of $3 to $3.15 per share, lifting the low end of its range by 10 cents.

UnitedHealth said on Monday that it would acquire the northeast U.S. operations of Health Net Inc for about $510 million to expand its presence in the region.

UnitedHealth is the first health insurer to report second-quarter results, although Congress' efforts to overhaul the U.S. healthcare system may overshadow the industry's earnings season.

(Reporting by Lewis Krauskopf; Editing by Lisa Von Ahn)