UPDATE 5-Humana earnings jump, but sees lower 2010 profit

* Q3 EPS $1.78 vs $1.77 Wall Street estimate

* Sees 2010 EPS $5.05-$5.20 vs $6.15 for 2009

* Pressure on Medicare margins, but enrollment up

* Sees commercial enrollment down about 140,000

* Shares fall 1.7 percent (Adds comments from U.S. Senate Majority Leader's spokesman)

By Lewis Krauskopf

NEW YORK, Nov 2 (Reuters) - Health insurer Humana Inc (HUM.N) forecast a drop in profit next year due in part to pressure on margins as it sees lower U.S. government reimbursement for its Medicare plans for the elderly.

Humana, which also posted a 65 percent jump in third-quarter profit on Monday, is one of the largest providers of Medicare plans so investors have been concerned about a hit to earnings as the government squeezes rates.

Humana shares fell 1.7 percent. Its 2010 forecast was largely in line with Wall Street expectations, when excluding asset writedowns related to the end of a military contract, said Stifel Nicolaus analyst Thomas Carroll.

"They put it at a level in line with current expectations," Carroll said. "They talk about decent enrollment and revenue growth but also feeling some margin pain."

Humana, whose report comes as Congress debates health reform legislation, excluded from its forecast any potential impact from the health overhaul, such as an industry tax.

The insurer expects a big jump in Medicare Advantage enrollment -- adding as many as 240,000 members -- countered by a decline of about 140,000 members in commercial plans serving employers as the weak economy leads to fewer workers with coverage.

Humana expects an operating Medicare profit margin of about 5 percent next year, down from about 6.5 percent this year.

"They really managed to mitigate the margin hit a lot," said Sanford Bernstein analyst Ana Gupte. "And they're really dramatically increasingly Medicare enrollment."

Third-quarter net income was $301.5 million, or $1.78 per share, compared with $183 million, or $1.09 per share, a year earlier, when the company was hit by investment losses.

Analysts on average expected $1.77 per share, according to Thomson Reuters I/B/E/S.