* Nike down after flagging weakness in N. America sales
* Health stocks drag, real estate top gainer on S&P
* Celgene falls after follicular lymphoma regimen fails trial
* Indexes down: Dow 0.11 pct, S&P 0.05 pct, Nasdaq 0.08 pct (Updates to close)
By April Joyner
NEW YORK, Dec 22 (Reuters) - Wall Street's major indexes dipped on Friday in low trading volume before the holiday weekend as several blue-chip stocks slipped, including Nike.
Nike Inc shares fell 2.3 percent after the company forecast muted growth in current-quarter revenue, reflecting its struggles in the North American market.
UnitedHealth Group Inc was down 0.8 percent after the health insurer agreed to buy Chilean healthcare company Banmedica SA for $2.8 billion.
Investors are winding down ahead of Christmas on Monday, when the market will be closed.
"It's been a strong week," said Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia. "Whether the market is up a little bit or down a little bit is not indicative of larger trends ... It's easy to push things around when not many people are trading."
Indeed, major Wall Street indexes were on track to end the week higher, buoyed by a historic overhaul of the U.S. tax code.
President Donald Trump signed Republicans' massive $1.5-trillion tax overhaul into law on Friday and also approved a short-term spending bill that averts a possible government shutdown.
The Dow Jones Industrial Average fell 28.23 points, or 0.11 percent, to 24,754.06, the S&P 500 lost 1.23 points, or 0.05 percent, to 2,683.34 and the Nasdaq Composite dropped 5.40 points, or 0.08 percent, to 6,959.96.
For the week, the Dow rose 0.42 percent, the S&P gained 0.29 percent and the Nasdaq added 0.34 percent.
Wildly volatile bitcoin plunged below $12,000, losing around a third of its market value in five days, before rebounding to above $14,000. Companies that have been riding the bitcoin wave were hit hard by the cryptocurrency's slump.
Long Blockchain Corp, Overstock.com Inc, Riot Blockchain Inc and Marathon Patent Group Inc tumbled between 2 percent and 15 percent. But even with Friday's losses, their share prices are higher now than before the companies ventured into bitcoin.
Data on Friday showed U.S. consumer spending went up in November and shipments of key capital goods orders increased for the 10th straight month, confirming strong economic momentum.
The benchmark S&P has climbed about 20 percent this year and is on track for its best performance since 2013 on solid corporate earnings, strong economic fundamentals, upcoming cuts to corporate tax rates and hopes of looser regulations.
Real estate led the S&P 500 in gains, with a 0.7 percent rise. Health was the biggest decliner, falling 0.3 percent.
Celgene Corp shares fell 1.4 percent after the company's follicular lymphoma regimen failed in a clinical trial.
Advancing issues outnumbered declining ones on the NYSE by a 1.04-to-1 ratio; on Nasdaq, a 1.30-to-1 ratio favored decliners.
Volume on U.S. exchanges was 4.81 billion shares, compared to the 6.98 billion average for the full session over the last 20 trading days.
Additional reporting by Sruthi Shankar in Bengaluru; Editing by Lisa Shumaker and Nick Zieminski