"I DIDN'T THINK it would work," says 82-year-old Virgil De Vault. "When my dentist sent me to the orthodontist, he said he didn't have any octogenarians, but he'd try. I thought that for me, 80 years old, going with teen-age braces would look kind of funny."

Not an unreasonable thought. But braces are glistening from the most unexpected mouths these days. The American Association of Orthodontists estimates that in the last 10 years, the number of adults in orthodontic treatment has doubled and that 20 percent of orthodontic patients are adults.

And some rather well-known adults they are, too. Nancy Kissinger was flashing a metal-incrusted smile around Washington last year. The Redskins' Mark Moseley and general manager Bobby Beathard are still entangled in the wires and hooks.

De Vault wore braces for five months and now says, "I should have done this 20 years ago." But with the new hardware come challenges adults don't usually concern themselves with. Spinach, for example.

"No one will tell you if you have a big piece of spinach in your mouth," says Lisa Steward, assistant conference manager at the Atomic Industry Forum in Bethesda. "And you're sitting there, smiling and smiling."

Chances are you don't think of spinach as a dangerous substance. But then, maybe you don't need to, because maybe you don't have braces.

Steward and her colleague Renee Cook now smile proudly, the railroad tracks gone from their teeth. But they remember what it was like to be an adult in braces. What it was like was embarrassing.

"Japan was really bad for that," says Cook. "The seaweed--you're constantly getting it caught in your braces. You can't cut food up and stick it in the back of your mouth when you're eating with chopsticks. And at the airport in Tokyo, the braces set off the metal detector. I had to submit to a total body search."

"And then there are the flying rubber bands," Steward says. "I was at dinner and someone said, 'What's that on my cheek?' I said, 'I don't see anything. There must be a bug in here.' What are you going to say when something flies out of your body and hits someone?"

So. Braces offer adults a seemingly endless array of new ways to embarrass themselves.

"When you're talking, sometimes your lips get stuck on the metal," says Moseley, whose upper teeth were encased in metal several months ago, just after his teen-age daughter got out of her braces. "I kind of picked a bad time to do it since this off-season I was doing so much speaking, TV and commercials. It feels like you've got a mouth full of gum all the time, but most people say just ignore it."

But some things are hard to ignore.

"You've got the boys at the Baskin-Robbins trying to pick you up," says Jean Shapiro, a graphic designer. "Braces knock 10 years and about 10 pounds off. I think people think they're cute."

And "there's a lot of kidding," says Robert Hallahan, director of the news bureau in the public affairs department of the National Association of Broadcasters. As he talks, his braces glistening under the bright office light, fellow workers stick their heads into the office and offer comments.

"We tell him he should tune in to Radio Marti."

"Did you ask him what it did for his sex life?"

"Show her the 'Braces are Beautiful' T-shirt we got you."

"If he spits on you, he doesn't mean it."

Hallahan brushes off the gibes with what can literally be called a sparkling smile.

"You really develop some one-liners to come back," Cook says, " 'It makes you look younger. Maybe I'll keep them on forever.' I was determined I wasn't going to not smile."

And smiling may be the only defense against some of the minor annoyances adults encounter when they journey into the adolescent-oriented world of orthodontia. Cook's orthodontist gave her rubber bands in a little bag decorated with giraffes and elephants. Steward remembers trying to squeeze her seven-month-pregnant form into her orthodontist's diminutive chairs. And Robert Hallahan was surprised to learn that adults make up nearly 85 percent of his orthodontist's practice.

"In his office, there's a history of the White House," Hallahan says. "Each time I go, I read a little more of that. My fellow patients are sitting there reading 'Superman.' "

Which raises the inevitable question every adult in braces must face: "Why now?"

"People assume first that you're doing it for cosmetic purposes," says Cook. "They think you're pretty vain to be doing it at this advanced age."

But few of the thousands of adults in braces have donned the metal for purely cosmetic reasons. Dentists now expect most Americans to go through life with their own teeth, and some adult orthodontia is a tool to prevent the gum problems that force so many into dentures.

Many of the people who now must answer to such taunts as "tinsel teeth" and "metal mouth" were prime candidates for braces when they were young, but couldn't afford it, and so they waited until they could pay for treatment themselves. And braces don't come cheap. Treatment usually costs $2,000 to $2,500, and can be even more for severe problems--or for the new braces that fit behind the teeth and are not visible.

"When I go to the orthodontist they ask, 'Are you following the schedule?' " says Hallahan. "I say, 'Of course, I'm following the schedule. I'm following it because it would cost me too damn much not to.' Over three years my orthodontist said it would cost as much as a car. I think what he had in mind was an Eldorado."

Then there are the people who passed their prime orthodontia years before everyone knew eyelids could be lifted and teeth rearranged.

"I grew up in a part of the country where people didn't know about that sort of thing," says Ted Ferrell, a district staff manager at C&P Telephone. "You just thought you're born that way and that's the way you were."

Ferrell had "lived with" a severe congenital jaw problem until a year ago, when his dentist told him new orthodontic and surgical techniques could help him. Since then he has had corrective surgery and braces, and he now wears a retainer.

"People are a little bit suspicious," he says. "Men aren't supposed to do this for cosmetic purposes, and they suspect that's why you're doing it. I guess I have to say I'm gratified that it made some difference in that respect, but I wouldn't have done it just for that reason."

But many adults have found that their metallic smiles inspire as much respect as suspicion.

"People show quite a bit of admiration," says Cecilia Jakovich, administrative assistant to Rep. Katie Hall (D-Ind.). " 'Oh, that was so courageous of you to get them,' they say."

Of course, the jokes continue. Metal mouth. Railroad tracks. These phrases have not disappeared from our vocabulary. But whatever the temptation, as dentist and orthodontic patient Eliot Alpher says, "You don't take a Cabinet-level official and make fun of him because he's wearing braces."

Even if he has a little piece of spinach caught in his metalwork.