By Cynthia Lin 
 

Treasury prices gained Thursday as investors kept their optimism about the U.S. economy in check despite a solid headline reading on last quarter's growth.

In midday trading, benchmark 10-year notes gained 6/32 in price to yield 2.301%, according to Tradeweb. Two-year notes rose 1/32 to yield 0.473%, while the 30-year bond advanced 10/32 to yield 3.031%. Bond yields decline when prices rise.

Those gains came despite the U.S. reporting that gross domestic product grew 3.5% in the third quarter, surpassing expectations. Investors felt details to the report were less rosy, with typically volatile spending on defense boosting the reading, while the price index continued to show a lack of inflation.

"The internal mix of the GDP report was not as good as the headline suggests," said Andres de Lasa, a government bond trader at Pierpont Securities, adding that prices are reverting a bit higher after Wednesday's decline.

Mr. De Lasa said last session's declines came as investors placed new bets against Treasurys, particularly around the two- and five-year maturities, after a policy statement by the Federal Reserve showed increased optimism about the U.S. economy. While the central bank maintained that it will wait a "considerable time" before raising rates, for investors, the latest statement moves policy one step closer to tightening.

"If growth continues at this pace--we think it will--the first Fed tightening could easily come in the spring, especially if wage gains start to pick up," said Ian Shepherdson, chief economist at Pantheon Macroeconomics.

Reflecting jitters about when the central bank might increase rates, the Treasury's seven-year note auction Thursday afternoon drew a light crowd. The $29 billion offering booked enough bids to cover the sale 2.42 times--the lowest cover ratio since November 2013. Buyers demanded a 2.018% yield on the notes, which was higher than the going market rate at the time of the sale, reflecting soft demand.

Still, the lack of inflation in the U.S. and around the world remains a concern for economy watchers and a key factor keeping bond buyers around.

Within Thursday's GDP report, the price index for personal consumption expenditures rose at a 1.2% annual rate in the third quarter, from 2.3% in the second quarter.

Sharon Stark, fixed-income strategist at D.A. Davidson Co., said weak inflation numbers of out Europe also added to support for Treasurys. Germany's consumer-price inflation unexpectedly slowed this month, rising 0.7% annually from 0.8% in September.

"Treasury yields could trade lower into year-end just because you get episodes of geopolitical unrest and disappointing numbers," Ms. Stark said, calling the 3.5% third-quarter GDP growth result "unsustainable." Her year-end forecast for the 10-year yield is 2.20% to 2.25%.

Muted inflation readings have also put Treasury Inflation Protected Securities, or TIPS, under pressure. That has narrowed the gap between yields on 10-year Treasurys and 10-year TIPS, which reflects investors' average annual inflation expectation for the coming decade. That gap recently stood at 1.91%, from 2.25% at the start of the year.

But Unlike Ms. Stark's lower-yield forecast, many analysts still believe strong U.S. growth will eventually trump concerns about the global economy and lift the U.S. 10-year yield toward 2.75% in the coming months.

Economists at Capital Economics are among those who see rising yields, expecting the Fed to increase interest rates in March and forecasting a 2.80% yield on 10-year notes at the end of that month.

Write to Cynthia Lin at cynthia.lin@wsj.com