OCEAN CITY'S three-mile boardwalk has always been a pretty good barometer of national fads. If O.C. has it in June, odds are it'll be selling in Peoria by the fall.

Except, that is, when it comes to food. It seems beach vacationers have long preferred the traditional to the trendy -- witness the block-long lines of sunburned tourists waiting hungrily for buckets of Thrasher's fries or boxes of Fisher's popcorn.

But for 1990, signs are that a little trendiness is at last intruding.

While the briny sea breeze remains rich with the aroma of peanut oil, burned sugar, corn syrupand fried batter, the boardwalk is finally offering up an alternative -- food that's good for you. New promenade stands are doing brisk business selling such healthy food stuffs as fruit cups, fat-free frozen yogurt and fresh-squeezed fruit juice.

"We're hoping to have the second biggest line on the boardwalk this summer, after Thrasher's," said Pat Vega, manager of TCBY yogurt, which opened this summer.

Vega said that people are drawn to the frozen yogurt because it is different from standard boardwalk fare. "Not everyone wants the french fries and hamburgers."

Added John Grosskettler, whose Flying Fruit Fantasy sells fruit shakes and yogurts, "I'm definitely seeing a trend toward more sensible eating, and we're popular because we're one of the only healthy places on the boardwalk."

Even the venerable Dolle's Candyland, saltwater taffy-makers since 1910, has this year added a sugar-free taffy.

"It's proving pretty popular," owner Rudolph "Bunky" Dolle said. "More people are asking about the ingredients now than they ever did before."

Still, not to worry: Dolle's biggest seller remains the 80-year-old family recipe, sugar and all. And Dolle himself says it's sometimes hard to resist traditional boardwalk favorites.

Other merchants agree, pointing out that even the most health-conscious people forget about the mundane things such as diet while at the beach. On a crowded summer night, to step on the boards is to enter the circus of boardwalk life, and food is clearly part of the fantasy. To breathe the air is to taste the cuisine -- Fisher's popcorn, Dayton's chicken, Ponzetti's pizza, Thrasher's french fries.

"We were sitting here the other day, watching everyone go by," said Rob Cole, general manager of the Alaska Stand, a boardwalk institution since the 1930s, "and the owner said, 'You know about 90 percent of the people have some kind of food in their hands.'

Thrasher's, which opened in 1929, sells only one product -- french fries, cooked in peanut oil -- yet always maintains a line a half-block long. Each summer, Thrasher's sells enough potatoes to fill five train carloads, according to owner Charles Jenkins Jr.

"It's an old tradition with our family to come here," said Fay Thomas-Morley, a vacationer from Belleville, Pa. "We stood in the line here in February, in the snow, to eat Thrasher's fries."

More recent boardwalk traditions include Love's Lemonade, Bull on the Beach and Belgium Funnel Cakes -- although Irma Jester, 90, whose husband had a waffle stand in the 1920s at the Pier, challenges the generally accepted idea that funnel cakes only came to the boardwalk 10 years ago.

"We sold the same thing they call funnel cakes back in 1920," she said, "only we called them waffles. They were 15 cents."

During the boardwalk's early days, around the turn of the century, food stands sold mostly candies and ice cream. Saltwater taffy, which originated in Atlantic City in the late 1800s, has been in Ocean City since at least 1910. It was in the 1920s, Jester recalls, that cotton candy and candied apples came along. Chocolate covered bananas were added in the '40s.

With many boardwalk treats, the joy is still that they're not good for you, they're just good. And, you can only have them once or twice a year.