BAGELS have become a staple of the healthful eater as well as the dieter, taking the place of Danish pastries, croissants and even muffins for those pursuing fewer calories and less fat.

But those virtuous eaters are in for sticker shock. Even without a slathering of butter or cream cheese, some bagels sold in New York contain more than 500 calories each.

Call it bagel inflation. When bagels made their debut in New York at the beginning of the century, they weighed just two or three ounces. The United States Department of Agriculture has the quaint idea that bagels are still about the same weight as they were back then and provide about 150 to 200 calories each. But there's nothing uniform about bagels today.

In the beginning, bagels were small. They became hard in six or seven hours and were affectionately known as cement doughnuts. They were served with salty lox and a very thick layer of cream cheese, known as a shmeer, which helped moderate the saltiness of the lox.

Since Americans like everything big, bagels ballooned when they started to become as popular as doughnuts. Even traditionalists like Bagel Oasis in Queens have begun making bagels bigger, because everyone else does.

"We used to make 2 1/2-ounce bagels that had a crust on all sides," said Mike Edelstein, a partner in the bakery. "They had a crust that was hard," he said, knocking a counter for emphasis. "But now we've come down to making a more neutral bagel, not as tough, and it weighs four ounces."

Bagel Oasis still makes bagels by hand and still boils them before baking. But the company is an exception. It's hard to find a handmade bagel or a bagel that isn't all puffed up from the addition of dough conditioners.

For the first half of the century in New York, only members of the Bagel Bakers' Local 338 could make bagels. Today, anyone with $100,000 to $200,000 can buy a bagel bakery package, which includes cooking equipment and just about anything else you might need to start up, except the desire to make bagels the traditional way. It's not much different from buying a complete package to go into the coffee bar business.

Bagels were once flour, water and malt. Today, they may contain sugar or honey or even, if the word on the street is to be believed, Crisco or saccharin. And plain bagels have given way to blueberry bagels and cheese bagels and raisin-nut bagels and pizza bagels. When the calorie and fat tally for cheese and nuts is added, the healthy halo of the bagel disappears.

To find out where the standard bagel stands in relation to the contemporary New York bagel, plain bagels were collected from 15 bagel shops and markets in New York City. Most of the bagel shops make their own, but some buy from suppliers.

The bagels were delivered to Certified Analytical Group Inc. in Corona, Queens, for a weigh in. Most of them weighed at least half again as much as the standard Agriculture Department bagel. Some are three times as much -- it's easy to find a bagel that weighs six or seven ounces.

In the test, two bagels were bought from each of 10 shops, weighed and analylized for calories. An average of the two was calculated. In addition, bagels were bought from five other shops and simply weighed.

For those who believe bigger is better, Bagel on the Square was the winner. The average weight of a bagel there was seven ounces, with 552 calories per bagel. Pick a Bagel was not far behind, with a bagel that weighed on average just under seven ounces and contained 518 calories. Ess-a-Bagel and Dean & Deluca also had 500-plus bagels and Nosh a Bagel came in at 472 calories. The others analyzed were in the 300-plus range. The lightest bagel, three ounces, was from Dunkin' Donuts.

In general, it is safe to conclude that all plain bagels of the same size will have approximately the same number of calories -- about 75 to 80 calories an ounce -- and between one and two grams of fat. Unless, of course, someone adds Crisco.

Should the bagel lover decide on a modest amount of cream cheese -- say, two tablespoons -- that means another 100 calories.

A slice of smoked salmon? Don't ask. Weighing In (Shmeers Not Included)

TWENTY plain bagels were analyzed for weight and calorie content by Certified Analytical Group Inc., Corona, Queens. The weight listed below is an average of the two bagels tested from each place. Some of these shops have more than one location in New York.

BAGEL ON THE SQUARE 7 ounces, 552 calories.

PICK A BAGEL ON SECOND 6.7 ounces, 518 calories.

DEAN & DELUCA (made by A & S Bagels): 6.4 ounces, 507 calories.

ESS-A-BAGEL 6.4 ounces, 503 calories.

NOSH A BAGEL 6.2 ounces, 472 calories.

H & H 4.5 ounces, 347 calories.

ZARO'S 4.3 ounces, 355 calories.

ZABAR'S (made by Columbia): 4.3 ounces, 338 calories.

BAGEL OASIS 4.1 ounces, 323 calories.

E.A.T. 3.9 ounces, 307 calories.

The average plain bagel has around 75 to 80 calories an ounce. Bagels from five other stores were simply weighed. The lightest of all the bagels tested was from Dunkin' Donuts, at just 3 ounces. One from Hot Jumbo Bagels came in at 3.5 ounces. D'Agostino's bagel was 3.8 ounces, and Moishe's bagel weighed 4 ounces, as did Balducci's. To learn the approximate caloric content of any bagel, simply multiply the weight by those figures.