Coca-ColaCo., PepsiCoInc. and Dr PepperSnapple Group Inc. have struggled to reverse the decline in sodaconsumption in the U.S., where shoppers increasingly reach for water, coffee,and other drinks.

Now they have a bigger worry: soda revenue.

Soda firms havefocused on new zero- or low-calorie sweeteners.

As U.S. consumption steadily slipped over the pasteight years, the beverage giants typically were able to raise prices enough tokeep soda revenues from America's favorite drink growing. But soda sales atU.S. stores declined in the second half of last year—including during theholidays, when partygoers normally pay up to gulp more.

Now industry analysts wonder if the downturn insales is here to stay.

"The question from here is if that is the newnorm,'' Steve Powers, a beverage analyst at Sanford C. Bernstein, said of thelatest store sales numbers.

Soda companies raised prices aggressively in 2011after commodity costs surged. Prices were increased a bit in late 2012, butvolumes fell even more sharply.

Sugary bubbles have become a lightning rod in the U.S.for consumer health concerns, such as diabetes and obesity. Meanwhile, babyboomers are aging, and soda's traditional target market—youth—is often turningto water, energy drinks and coffee instead.

Soda sales declined 0.6% last year through Dec. 30to $28.70 billion at U.S. stores tracked by Symphony IRI Group. In volumeterms, sales dropped 1.8%.

The pace of decline got worse later in the year.Sales, in dollar terms, skidded 2.5% in the 12 weeks ended Dec. 30 from a yearearlier, and were down 2.8% when counting just December, according to themarket-research firm, after soda makers raised prices, further damping demand.By volume, sales fell 3.55% in the 12-week period and 4.9% for December.

The data don't include sales of soda inrestaurants, vending machines, and some other venues. Industry insiders saytaking those outlets into account, overall soda sales revenue likely roseslightly last year—but barely.

While Coke, Pepsi, and Dr Pepper Snapple have allaggressively expanded their portfolios to include faster-growing products likesports drinks and fruit juices, a prolonged drop in U.S. soda revenues wouldrepresent a serious blow. Soda represents nearly 25% of the U.S. beveragemarket. Its massive scale has also guaranteed profit margins for decades.

About 60% of Coke's revenue in the U.S. is derivedfrom carbonated soft drinks, compared with about a quarter at PepsiCo. Morethan 70% of sales at Dr Pepper Snapple, the No. 3 player, are from soda andabout 90% of its revenue is from the U.S. Unlike Coke and PepsiCo, though, ithardly sells any cola, which has suffered steep declines.

Coke and PepsiCo together spent about $20 billionin 2010 to acquire their biggest U.S. bottlers, increasing U.S. exposure andthinning profit margins.

Last week, citing falling soda volumes, Bernsteincut its recommendation on Dr Pepper's stock to hold from buy and trimmed itsearnings and share-price estimates for Coke and PepsiCo. Stifel Nicolaus alsotrimmed its 2013 earnings estimate for Coke.

The companies say their fortunes are far from grim.Soda is posting healthy growth in many parts of the world, providing a boostfor Coke and PepsiCo, which draw about 60% and 50% of their revenue fromabroad, respectively.

Their newer drinks also are profitable and growingstrong, they say. Last year Coke acquired control of coconut water brand Zicoand dipped its toes in U.S. dairy for the first time, buying a stake in themaker of Core Power, a workout recovery shake.

Sales of PepsiCo's Naked juice brand rose about 25%last year, and tea and coffee sold through joint ventures with Lipton and Starbucksare posting healthy growth.

"I think we can all be optimistic about thebusiness we're in,'' Sandy Douglas, Coke's global chief customer officer, saidlast month.

PepsiCo is investing hundreds of millions ofdollars in marketing to turn around its U.S. soda business after losing marketshare to Coke. In 2010, Diet Coke unseated Pepsi as the No. 2 domestic soda byvolume, behind Coca-Cola. Investor calls to split PepsiCo's better-performingsnack business from its beverage business could return if there is no sign ofimprovement.

Coke launched new television ads this week tocounter consumer concerns about obesity and moves by officials to restrict sodasales. New York City plans to cap portion sizes for soda at many retailestablishments in March.

The ads argue that soda shouldn't be singled outfor weight gain and encourage Americans to have "fun" burning offcalories through dancing and other activities.

The soda companies also are working to developzero- or low-calorie natural sweeteners that better mimic the taste offull-calorie sodas. But the going has been slow, keeping diet soda's share ofthe overall soda market at around 30%. One candidate, based on the steviaplant, can leave a bitter aftertaste in some sodas, particularly cola.

Indra Nooyi, PepsiCo's chiefexecutive, said her company has made "enormous progress'' the last twoyears as it experiments with sweeteners and is 90% closer to a breakthrough."Unfortunately, the last 10% is the toughest part,'' she acknowledged inDecember at a conference hosted by Beverage Digest, a trade publication.

Last year, PepsiCo rolled out nationally PepsiNext, an artificially sweetened, mid-calorie version of its flagship cola. Morerecently it tweaked its artificially sweetened, zero-calorie Diet Pepsi toimprove shelf life. Coke began testing naturally sweetened, low-calorieversions of Sprite and Fanta in some U.S. markets last summer. Dr PepperSnapple is rolling out artificially sweetened, 10-calorie versions of 7-Up,Sunkist and three other sodas this year after launching a 10-calorie version ofDr Pepper in 2012.

But such efforts have yet to turn soda's fortunes.Pepsi Next and Dr Pepper 10 each have less than a 1% market share and Coke'slast big diet cola launch, Coke Zero, was in 2005. Some industry observersthink soda companies haven't done enough on other fronts to win back drinkers.

"They're so focused on a sweetener event thatthey've neglected more traditional innovation like flavors and functions,''said Mark Swartzberg, a beverage analyst at Stifel Nicolaus.

Write to Mike Esterl at mike.esterl@wsj.com

A version ofthis article appeared January 19, 2013, on page B1 in the U.S. edition of TheWall Street Journal, with the headline: Is This the End of the Soft-Drink Era?.