THE WATERFRONTS, the hand laundries, the tailor shops have all been ladders of success for immigrants to America. Now it is the pizza parlor, or so it seems around Washington. pepperoni-and-cheese keeps company with kebabs and aushak. And some of our most interesting ethnic restaurants hide behind stacks of cardboard pizza boxes.

Ikaros in Georgetown was a pioneer: pizza and souvlaki. Since then we have had pizza and Greek salad at Vesuvio's; pizza and riijstaffel, at least briefly, at Magna Graeca; pizza and homemade Turkish yogurt at Atilla's. Hobo's on New Hampshire Avenue has had pizza and Afghan aushak. The Lebanese Taverna's pizza has been eclipsed by some of the best Lebanese food Washington has seen. In Adams-Morgan you can have pizza and Cuban food; downtown pizza can be found with Korea's bulgogi, and at Culmore Pizza in Virginia, if you order ahead you can get with your pizza Iran's ash reshte, a lamb's head or a brain sandwich.

The phenomenon provides Washington culinary adventurers a chance to taste foods that probably could not otherwise command a large enough audience to survive. And it allows, as we saw at Culmore Pizza, a family to feed the kids at one table on their favorite fast food while the adults at the next table relive their gastronomic memories of home.

But what do we export in terms of food reminiscences? Four Swedish girls of our acquaintance, each of whom had been visiting the U.S. for a year or so, returned home during this spring and summer. Each of them carried one indispensable souvenir of the U.S.A.: a popcorn popper.

And what are we eating, those of us who stay closer to home? As of 1978, according to surveys by USDA and "Woman's Day" magazine, a lot of us were not eating breakfast -- 14 to 38 percent, especially those ages 19 to 35. Even so, according to the USDA, women in that age group typically packed in 1,600 calories a day, men about 2,475 calories.

According to Pennsylvania State University's research, the average American ate the same amount of food in 1980 as in 1960, but the mix has changed. Corn sweeteners were up 385 percent, low-fat milk up 282 percent, soft drinks up 176 percent and cheese up 116 percent. Americans were eating more chicken but less veal and lamb, less whole milk and less fresh potatoes. In the meantime, the consumption of frozen potatoes rose from 2.6 pounds per person in 1960 to 18.1 pounds in 1979.

The increases attributed to health consciousness are cheese and lowfat milk. But if Americans are lowering the fat content of their milk for health reasons, they are also increasing their use of all sweeteners, among them corn sweeteners, and consuming more soft drinks, an average of 37 gallons apiece in 1980.

After all, we are the people who put saccharin in our coffee and drink it with a doughnut.

In any case, we have long been a snacking society. And several readers have reminded us of that as they took us to task for our definition of the word "gedunk" in our story on cooking in a nuclear submarine. Our Navy sources for the story, admittedly all very young men, related that gedunk is junk food, named after the sound of money in a vending machine. Our more mature readers remember hearing the term 50 years ago, one in a Harold Teen comic strip where it was a soda confection. Another knew the 1950s Navy snack shop as "the gedunk stand," and said that before 1950 gedunk referred to where one bought ice cream. Even some dictionaries refer to gedunk as soda fountain confections.

And so we are forced to admit: We have never been in the Navy. Or even on a cruise ship. Our minuscule sailboat hasn't been out of the garage all summer. We have, of course, eaten our share of gedunk, but never wanted to acknowledge it with an investigation into its etymology.