It is lunchtime downtown and carryouts and delis are jammed. Holding dessert in one hand and money in the other, Washingtonians pay for towering vanilla swirls studded with pieces of peanut-butter cookies, strawberry cones drizzled with wet walnuts in syrup, Styrofoam cups full of hot fudge and M&Ms.

Frozen yogurt is being served.

It's being served more and more -- and in lots of other places besides K Street carryouts. Nationwide, there are estimated to be 11,000 frozen yogurt outlets, an increase from 1985 when there were fewer than 3,000. Most frozen yogurt is sold as soft serve -- whether at luncheonettes or shopping malls -- but sales of hard-pack pints and quarts in supermarket freezer cases have skyrocketed as well. Between 1985 and 1990, sales of these products increased from $8.3 million to $82.8 million.

But the dessert Americans are increasingly choosing is not always the guilt-free treat they so desperately seek. Depleted of some of its live active cultures and dressed up with flavorings and toppings, frozen yogurt is far from nutritional nirvana.

"Consumers are continually looking for the magic. 'What can I eat and not feel guilty?' I think frozen yogurt falls right into that. We love to fool ourselves," said Hope Warshaw, a nutrition consultant in Newton, Mass.

Warshaw added that frozen yogurt can be "a wonderful alternative to ice cream, especially the superpremium ice creams," but that "it comes down to the quantity you choose to eat and the form in which you choose to eat it."

When it comes to soft-serve frozen yogurt, the question of quantity can get tricky because there are no common rules for how big a "large" is or how little a "small" is. At a handful of Washington's yogurt counters, a "small" ranged from 4 to 6 ounces and a "large" ranged from 8 to 12 ounces.

In addition, many carryouts that dispense nutrition information about their yogurt products give the serving sizes in 1-ounce portions. Thus, what may appear to be a skinny snack, if not calculated properly, can pack more fat and calories than it initially seemed. For example, I Can't Believe It's Yogurt's "original" yogurt contains 27 calories per ounce and one gram of fat. A large order -- 12 ounces -- contains 324 calories and 12 grams of fat.

When it comes to supermarket products, many frozen yogurts list serving sizes in 3-ounce portions, rather than the 4-ounce servings used for ice milk and ice cream. That means that a pint contains 5 1/3 servings. Nancy Wellman, president of the American Dietetic Association, said she has to con- stantly remind people that a serving of ice cream isn't a pint.

It also means that when compared by 4-ounce servings, some frozen yogurts contain the same amount of fat and calories as ice milk. Still, unadorned, frozen yogurts don't approach the calories and fat of superpremium ice creams such as Ha agen-Dazs or Frusen Gla dje'. For example, a 4-ounce serving of vanilla frozen yogurt averages about 120 calories and 3 grams of fat. The same size serving of a superpremium vanilla ice cream has double the calories and more than five times the amount of fat.

Whether at home or from a carryout, splurging on toppings can sour a good thing. Two tablespoons of nuts, M&Ms, or chopped peanut-butter cups can add anywhere from 100 to 150 calories to an order. In a small order of yogurt, the topping could easily pack more calories than the main course.

"Frozen yogurt is not a perfect food in that it overcomes abuse, but it's still pretty darn good," said Dan Corrigan, vice president of marketing for I Can't Believe It's Yogurt, the country's second-largest yogurt franchise. "A 300-calorie dessert is still pretty reasonable," he added.

While the live cultures in yogurt have been touted as curing everything including cancer and promoting long life, their only proven benefit, according to Robyn Flipse, spokeswoman for the American Dietetic Association, may be to restore bacteria in the colon. When prolonged regimens of antibiotics, taken to cure infections, destroy the natural flora of bacteria in the stomach, these can be reestablished by the live active cultures in yogurt, Flipse said.

Whether there is enough bacteria in frozen yogurt to be meaningful is unclear. In the 1970s, the first frozen yogurts had bacterial counts comparable to the refrigerated variety but many consumers rejected the resulting tart taste, according to industry experts. The acidity that caused the tartness was reduced, thus lowering the bacterial counts.

Even now, says Jay Gould, vice president of marketing for Colombo Inc., the country's largest manufacturer of frozen yogurt, it "still has the perceived yuk factor. A lot of people won't try the product. People think it will taste like refrigerated yogurt."

Yet some frozen yogurts don't even contain live active cultures. The milk from which they are made is pasteurized, then cultured, then pasteurized again, which kills the beneficial bacteria. Tom Balmer of the International Ice Cream Association said re-pasteurization increases shelf life and stops the culturing process, which halts the acidic taste.

For this reason, the ice cream trade group has asked the Food and Drug Administration to require inclusion of live active cultures as part of the definition of frozen yogurt. Currently, there are no government standards as to what frozen yogurt has to contain to be called frozen yogurt, or the fat level for "lowfat" or "nonfat" frozen yogurt.

But Manfred Kroger, a professor of food science at Penn State University who teaches dairy technology, believes that by homogenizing frozen yogurt into a sweet, mild product, it has been corrupted. Industry "made Wonder Bread an American wonder," he said. "Now, unique bakeries are cropping up all over. We killed something we had 40 years ago. Let's not do it to frozen yogurt."