Before he became Adviser to the American Nation on weight loss, sex and diet, Richard Smith was, not to split hairs, adrift, undistinguised, a failure.

Now he is not a failure. He is 39, weighs 205, has bright alert eyes and takes time along life's highway to smell the home fries.

He has the quiet easy movements of a contended man. He is for all pratical purposes handsome.

Never in a whole life of sweating in a public relations firm could he have hoped to accomplish so much for mankind and for himself as now he has. "My mother never let me have soda pop,"he said yesterday in an intimate disclosure of the ultimate origins of such important works as "The Bronx Diet," The Dieter's Guide to Weight Loss After Sex" and a couple of convenient calendars giving day-to-day pearls. And yet it was all an accident. c

He wrote a funny piece for a magazine. Then for Cosmopolitan, another magazine that paid even better than the first. Then publishers wanted a book, then another book and another. Now he can buy all the soda pop he wants.

Like Montaigne, he gives endless advice couched in the disguise of wisdom:

If it looks like enough, it will be enough.

Sex is so pleasurable it is often compared to Southern cooking.

Outside every thin person there's a regular-size person screaming to get in

Pound cake is coffee-soluble. Genoasalami is not.

Foods that combine poorly include fudge and whiskey, beets and Lorna Doones.

Really taste your food. It may take several portions.

Avoid blue food.

A midnight snack is merely getting the jump on breakfast.

Avoid foods that rust.

Tender dalliance with a fruitcake will more than compensate for the cold shoulder from your dog.

Greasy pork chops are a vital source.

Stop when you've eaten enough to lastuntil the next time.

"look at those mints," he said, entering a Mayflower Hotel restaurant and demonstrating his mint grope, which any connoisseur knew was the result of endless practice over the decades.

"i have managed to lose 60 pounds," he went on, ordering a salad which he canceled, in favor of something called a Double Beef, on the theory salads have a lot of calories, and should as a rule be avoided like botulism.

"i grew up in Liberty, N.y., and saw duty at the Adirondacks resorts as a youth. Nobody goes to them now; it's cheaper to go to Europe. But those sorts were the true Tarawa of Americaneating.

Whe I arrived in New York at the age of 18 I worked for Pepsi-Cola. As soon as I turned the corner in my truck, I'd jump out and open some nice warm Pepsi. If you can get it in the mouth, eat it. There's something wrong with people who don't eat everything, isn't there?

"now the bottom-line price required to get me to eat a soft-boiled egg is $10,000," he went on, pouring his beer over ice which he thinks makes the beer go so much farther, you know, "so I don't mean there can't be one or two things you simply don't like to eat.

"but I had a date with a woman who said, 'I should tell you I'm a lacto-vegetarian.' I said, "What's the soonest we can end this date?"

It's understood that his success withbeautiful female vegetarians is only moderate. It was speculated also that Smith can be an oral type.

"what gets me," he said, investigating his filet of beef, "are the ones who rule out a whole class of food," and on days when he wishes to feel creepy and horrified he drops into a health-food store to see all the folks with gray skin

His books, which are mighty funny, give absurd or witty or profound advice in the same generally asinine format of serious diet books. There are charts, handy tips for maintaining the diet while traveling (never eat a sandwich you find dropped on a sidewalk in Rangoon) and careful explanations of physiological processes ("why We Wake Up.")

Along the way he issues profound thoughts which have raised him far above most diet masters to guruesque heights:

"A day without sex is like a night without sex," he once said.

His celebrated Bronx Diet (which seized the imagination of an entire borough) is based on a revolutionary new insight: To lose weight, eat less. Also, you eat to acquire calories. (virtually all other diets deny both truths.)

"I learned early in life to eat fast. At those resorts, like Grossinger's (where he worked extremely briefly) you learned fast, too. I could drop a strawberry tart, recover it before it hit the floor and pop it in my mouth before straighteing up," he said, with the contented glow of one who is a master and knows it, having disciplined the body to coordination -- grace under pressure.

If 11 people at a table ordered steaks, you placed an order for 12. The extra one you ate between the kitchen and the dining room. I can eat a steak in four bites while walking through the linen storage room. I was a grown man before I knew you could eat sitting down.

I'm not committed to another book, I'm free at the moment, on a manic high, because when I have a deadline I fall into depression."

"Depression is good for you," he was reminded sternly. "Without it, you would never have produced your oeuvre inyour particular genre."

"True enought," he said with another beer and wondering about dessert.

"you hear about some woman or other who makes pie crust so light it just floats away? It's not for me. I like those crusts so heavy and so rich it makes no differnece if there's any actual pie inside them. Three bites and you bust out. That's what a pie should be." (he halted the cart and took apple.)

"I'm glad you think the books have helped suffering humanity. We have to take care of ourselves. No point trusting the largesse of others. Many's the time I've been saved by a Twinkie pie. I always eat before going to dinner; you never know what you may get, if anything.

"i had some friends, gourmet French cooks, who are of course the very worst kind, and they gave me a dab of trout with some artful sauce. Always take along a Twinkie in your pocket. You can duck in the bathroom and thus sustain life till you get out. I remember thinking all through that trout dinner about where the hell it was that I'd seen an all-night pizza place.

"often I've seen a poor health-food girl and thought a Miss Drake's cupcake could save her. I approach anydinner invitation as if I were going to Tulsa for Sunday night. Ever been there on Sunday night? Well, you take precautions."

Sometimes science has been slow to perceive the truths that Smith divines without research.

"blueberries prevent aluminum-siding salesman," he may say, years before science gets around to corroborating through double blinds what Smith sees like a clear-eyed babe.

No more beer. Thank you. No more pie. Need to keep up the strength, however. He likes the food of San Francisco. He dreams he might live on Nob Hill with a cable car running from his bed to his favorite Chinese restaurant.

"may do a book on remedial sex and weight loss." he said. Or remedial weight and sex loss. Whatever.

Watching out for sandwiches on sidewalks or lobby floors he waved and set off for his room, grabbing only six mints (a non-Olympic showing indeed) on the way out. But then as he said, it had been a tough interview, leaving him weakish. Once he gets to his room and his twinkie he's usually okay again.