* Caterpillar hits 5-year low after sales forecast cut
* Health stocks on track for 5th day of losses
* Investors await Yellen speech at 5 p.m. ET
* Indexes down: Dow 0.64 pct, S&P 0.54 pct; Nasdaq 0.72 pct (Updates to late afternoon, adds commentary)
By Sinead Carew
Sept 24 (Reuters) - Key U.S. stock indexes fell in afternoon trading on Thursday, weighed by tumbling healthcare stocks and shares of market heavyweight Caterpillar after a cut in its sales forecast added to worries over sluggish global growth.
Eight of the 10 major S&P sectors were lower, with the health sector's 1.4 percent fall leading the declines. The Nasdaq biotech sector was down 2.8 percent. Both indexes were on track for a fifth straight day of losses.
While the market pared losses somewhat in late afternoon trading, investors were cautious ahead of a speech by Federal Reserve Chair Janet Yellen, which could provide clues regarding the timing of an increase in U.S. interest rates.
"You need to see better China data and more certainty about the Fed," said Scott Wren, senior global equity strategist at Wells Fargo Investment Institute in St. Louis, Missouri.
"We're trading in these wide ranges due to continued uncertainty and Yellen's speaking tonight. We tell our clients expect a lot of these 20-30 point range days."
Yellen, who is due to deliver an inflation speech at 5 p.m. ET (2100 GMT), had cited concerns about slowing global growth as a key reason for holding off from a long-anticipated Fed rate hike last Thursday.
Shares in Caterpillar, the world's biggest mining and construction equipment maker, were down 6 percent at $65.94, making it the biggest drag on the Dow. It said it could cut up to 10,000 jobs as it faces challenging conditions in key regions and the mining and energy sector.
"The (Caterpillar) news is not helping matters, it's emblematic of a weaker global economy," said Joseph Quinlan, chief market strategist for U.S. Trust, in New York.
At 2:41 p.m. ET, the Dow Jones industrial average fell 104.46 points, or 0.64 percent, to 16,175.43, the S&P 500 lost 10.54 points, or 0.54 percent, to 1,928.22 and the Nasdaq Composite dropped 34.36 points, or 0.72 percent, to 4,718.38.
Healthcare stocks have been under pressure since Hillary Clinton, the leading U.S. Democratic Presidential candidate, vowed to stop "price gouging" by drug companies.
Among the 10 key S&P sectors, only energy and utilities eked out gains.
Earnings of S&P 500 companies are expected to decline 3.9 percent in the third quarter from a year ago, Thomson Reuters data shows.
The CBOE Volatility index, known as Wall Street's "fear gauge," rose 5.7 percent to 23.40. Its long-term average is 20.
A gauge of U.S. business investment plans fell slightly in August, jobless claims barely rose last week, and new single-family home sales rose more quickly than expected in August.
Declining issues outnumbered advancing ones on the NYSE by 2,039 to 958, for a 2.13-to-1 ratio on the downside; on the Nasdaq, 1,752 issues fell and 991 advanced for a 1.77-to-1 ratio favoring decliners.
The S&P 500 posted no new 52-week highs and 69 new lows; the Nasdaq recorded 16 new highs and 174 new lows. (Additional reporting By Tanya Agrawal in Bengaluru; Editing by Bernadette Baum)