The belt buckle has settled into a looser notch and the scales tell a terrible tale. It is time, you think, to turn wine into water, to put aside gingerbread and eggnog and wave farewell to friends until you can greet them 10 pounds hence.

But it really is time to give an elegant diet dinner. Announce it as such, and you can look extravagant by serving a trifling amount of truffle (8 cal./oz.), a dab of caviar (74-90 cal./oz.), or a sliver of smoked salmon (50 cal./oz.) while, in reality, you are filling up your guests with pasta (216 cal./cup) or eggs (50 cal./oz.).

Scramble enough eggs for everyone and top each serving with a spoonful of sour cream (25 cal./tbs.) and a small bit of fresh caviar.

Beat eggs in a bowl, add thin slices of black truffle and let them sit covered for a couple of hours. Then scramble the eggs and serve them.

The distinctive flavor of white truffles is not helped by cooking, so they should either be gently heated in butter and then added to fresh pasta, which has been cooked and tossed in a mild butter or cream sauce, or the truffle can be shaved over the pasta just before serving.

The smoked salmon can be incorporated into an omelette or served on the side with thin slices of French bread (67 cal./slice) spread with unsalted butter.

Following are sources for fresh caviar, truffles and smoked salmon:

Sutton Place Gourmet, 3201 Mexico Ave. (363-5800), has white truffles, 2 oz. for $69.79, black truffles 7/8 of an oz. for $29 or 7 oz. for $190. Fresh Severga caviar (a lower grade) is $99.79 for 7 oz. and fresh Beluga is $199.79 for 7 oz.

Cannon's Seafood, 1065 31st St. NW, (337-8366) has fresh caviar at $24 an oz. and Nova Scotia smoked salmon at $18.80 per lb.

The French Market, 1632 Wisconsin Ave. NW, (338-4828) is carrying fresh Washington state caviar which they describe as being "as good as second-grade Iranian or Russian caviar, though not quite as salty." It sells for $65 for 7 oz. or $125 for 14 oz. They also have Nova Scotia smoked salmon at $14 a lb., Scottish smoked salmon at $25 to $30 a lb., and double smoked salmon at $18 a lb. Black truffles are $41.75 for a 1-oz. can or $15.50 for 3/7 of an oz.; white truffles are also $41.75 for a 1-oz. jar and $16.50 for 1/2 oz. Instead of leaving them in the brine, they suggest you might store the truffles in cognac.

Neam's Market, 3217 P St. NW, (338-4694) has Nova Scotia smoked salmon at $13.98 a lb., and tins of black truffles are about $30 for 7/8 of an oz.

Larimer's, 1727 Connecticut Ave. NW., (332-3366) has fresh Beluga caviar for $420 a lb. or $210 for 8 oz. Nova Scotia salmon is $14.50 a lb. and tins of black truffles are $24.50 for 7/8 of an oz. or $48.50 for 1 3/4 oz.

Wagshal's Delicatessen, 4855 Massachusetts Ave. NW., (363-5698) has Nova Scotia salmon for $18 a lb., black truffles for $147 for 7 oz. and fresh Beluga caviar at $125 for a 4-oz. jar.