The blue-chip Dow Jones Industrial Average closed over the 22,000 mark for the first time on Wednesday, but investor fears about the sustainability of the gains took the shine off the round number milestone.

The rally lost momentum during the day's trading and despite the recent run up, helped by strong earnings from Apple Inc and Boeing Co, some technical indicators were flashing warning signs.

"The market gain has been built on a narrow group of issues. That typically is not indicative of great health," said Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia. "I would not be shocked ... if we saw a pullback."

And with the Dow industrials at a record high, Dow theory suggests that the Dow Transportation Average index should also hit a record in order to confirm the market's march higher.

But that index trails the Dow industrials' year-to-date performance by almost 10 percentage points and is nearly 6 percent below its own July 14 record high.

Also, overall market breadth, or the number of winning stocks relative to losers, is weakening even as the major U.S. indexes hover near record highs.

That means the broad gains have been driven by advances in a declining number of companies, and market watchers fear they could be hard to sustain.

The number of 52-week lows among NYSE- and Nasdaq-traded stocks is at its highest since late June while the number of 52-week highs has dropped sharply since mid-July.

Apple, McDonald's Corp and UnitedHealth Group Inc have each added more than 200 points to the index.

The Dow is a price-weighted index, meaning names like Apple, with its $157 price tag, and Boeing, which closed Wednesday near $238 per share, will generally have more of an influence over the index than components like the roughly $25 per share General Electric Co.

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