* Regeneron, Amgen lead health care sector
* Twitter surges 7 pct after investor raises stake
* Yum Brands hit by China woes, drags McDonald's
* Indexes up: Dow 0.29 pct, S&P 0.37 pct, Nasdaq 0.41 pct (Updates to afternoon)
By Noel Randewich
Oct 7 (Reuters) - U.S. stocks rose in a volatile session on Wednesday, led by a rebound in biotechnology stocks, putting the S&P 500 on track for its sixth day of gains out of the last seven.
Materials shares were higher, helped by rising gold and silver prices.
Investors' focus is slowly turning to earnings, and Yum! Brands was the S&P's biggest loser after reporting weak sales out of China, which accounts for more than half of its revenues.
Yum! slumped 19 percent, while McDonald's, which also has a large exposure to China, fell 0.5 percent to $101.12.
The S&P health index, up 1.1 percent, was the biggest gainer. A selloff in healthcare and biotech stocks had weighed on the market on Tuesday.
"On days where investors are looking for fallen angels, healthcare helps fill that equation today," said Art Hogan, chief market strategist at Wunderlich Securities in New York.
Healthcare was led by Regeneron Pharmaceuticals and Amgen, both up more than 4 percent. Express Scripts said it reached deals to cover two costly new cholesterol drugs produced by the two companies.
The S&P materials index rose 1 percent and the S&P energy index was up 0.5 percent even after U.S. crude settled down 1.5 percent.
Gold and silver futures have been climbing in recent sessions and are up since the end of September, giving a boost to materials shares.
Gains in U.S. indexes, even after the reversal of an oil rally, suggest that the worst might be over for commodity prices hit by concerns about China's economy, said Warren West, principal at Greentree Brokerage Services in Philadelphia.
"We're not so worried about commodities, we think they're finding a bottom here," West said.
The Dow Jones industrial average rose 0.44 percent, to 16,864.12, the S&P 500 gained 0.51 percent, to 1,990.03 and the Nasdaq Composite added 0.63 percent, to 4,778.49.
S&P 500 companies are now expected to report a 4.4 percent fall in third-quarter profit, the biggest decline in six years, according to Thomson Reuters data.
Twitter jumped 7 percent after Saudi Arabian billionaire Prince Alwaleed bin Talal and his investment firm raised their stake to more than 5 percent.
Advancing issues outnumbered declining ones on the NYSE by 2,222 to 799, for a 2.78-to-1 ratio. On the Nasdaq, 2,020 issues rose and 771 fell.
The S&P 500 index showed eight new 52-week highs and no new lows, while the Nasdaq recorded 42 new highs and 29 new lows. (Reporting by Tanya Agrawal and Abhiram Nandakumar; Additional reporting by Devika Krishna Kumar; Editing by Savio D'Souza and Nick Zieminski)