* Regeneron, Amgen lead health care sector
* Twitter surges 8 pct after investor raises stake
* Yum! Brands hit by China woes
* Indexes close up: Dow 0.73 pct, S&P 0.8 pct, Nasdaq 0.9 pct (Updates to close)
By Noel Randewich
Oct 7 (Reuters) - U.S. stocks ended stronger after a volatile session on Wednesday, led by a rebound in biotechnology companies that pushed the S&P 500 to its highest level in three weeks.
Materials shares also rose, helped by rising gold and silver prices.
Investors' focus is 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! Brands slumped 18.83 percent after the restaurant operator cut its full-year profit forecast, citing a slower-than-expected recovery in its key China market and the drag from a strong U.S. dollar.
The S&P health index, up 1.47 percent, was the biggest gainer. A selloff in healthcare and biotech stocks had weighed on the market on Tuesday.
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.
"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. "What was a drag over past few days is now the leader."
The S&P materials index rose 1.34 percent and the S&P energy index was up 1.33 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 0.73 percent to end at 16,912.29.
The S&P 500 ended 0.8 percent higher 1,995.83 after trading down 0.18 percent earlier in the day.
The Nasdaq Composite added 0.9 percent to 4,791.15.
After Wednesday's rise, which was its sixth in seven sessions, the S&P is down 3.09 percent for the year. The Dow is off by 5.11 percent.
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. The index is trading around 15.5 times expected earnings, in line with its long-term average.
Twitter jumped 8 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 2,385 to 670 On the Nasdaq, 2,061 issues rose and 776 fell.
The S&P 500 index showed eight new 52-week highs and no new lows, while the Nasdaq recorded 48 new highs and 34 new lows.
About 8.3 billion shares changed hands on U.S. exchanges, above the 7.4 billion average for the previous 20 sessions, according to Thomson Reuters data. (Additional reporting by Tanya Agrawal, Abhiram Nandakumar; and Devika Krishna Kumar; Editing by Savio D'Souza and Nick Zieminski)