Q. Since recent bypass surgery I have been trying hard to limit my intake of fat. I really do not like plain skim milk, but I know it is a potent source of calcium and other nutrients. To make it taste better, I have been adding a little chocolate syrup. Now a friend tells me that the syrup contains some fat. Is this true? And what about cocoa?

A. If added in reasonable amounts, you will get little fat from the chocolate syrup used to flavor milk. It contains less than one gram of fat per ounce (about two tablespoons) of syrup. To put that figure into perspective, a teaspoon of margarine contains nearly five grams of fat.

Having said that, we must caution that the ounce of syrup has about 90 calories, as many as you would get from a whole cup of skim milk. So if weight is a problem, you will certainly want to consider measures to "stretch" the syrup, such as diluting it with a little artificially sweetened coffee to create a mocha milk drink that saves half the calories.

Now about cocoa. In producing cocoa, some fat is removed from the seeds or "nibs," making it lower in fat than chocolate. So-called "high-fat" or breakfast cocoa contains a minimum of 22 percent fat. While that might seem like a lot, the amount used to make a cup of cocoa is fairly small, just a little over a tablespoon. Thus if made according to the directions on the box, the cocoa powder would add a little over one gram of fat to the drink. Finally, the prepared cocoa mixes, to which you add only milk, contain a bit less fat.

Q. Why does the same amount of oil have more calories than either butter or margarine?

A. The one-word explanation is "water." Oil is 100 percent fat while butter, as churned from cream, still contains 15 percent water, and margarines have water added during the manufacturing process. Thus a tablespoon of oil provides 125 calories, while a tablespoon of butter or margarine has just 100. But 25 calories is a small difference and should not influence your decision to use oil rather than butter or margarine.

Q. I am having an open house and would like to serve some hors d'oeuvres that are low in fat and calories for guests who want to watch their weight. Do you have any suggestions?

A. An important step in cutting the calories in hors d'oeuvres is to provide low-calorie "vehicles" for dips and spreads in place of calorically dense chips and fat-laden crackers. The best choices are the array of raw vegetables that have grown so popular in recent years. These are also practical, because vegetable slices that are spread ahead of time suffer less loss of quality than do crackers, which tend to become soggy.

Beyond that, there are several excellent dips and spreads as well as bite-sized hors d'oeuvres that will not dash the dieter's good intentions. One simple and surprisingly good choice is skewers of small vegetables, such as tiny onions, cherry tomatoes, mushrooms and green-pepper chunks brushed lightly with a mixture of soy sauce and oil and put under the broiler for a minute or two.

Stuffed mushrooms, always a favorite, can be prepared in low-calorie versions. One tasty stuffing calls for poached white fish mixed with a little egg to hold it together and is seasoned with parsley, scallions, tarragon and white wine.

Delicious bite-sized cheese balls can be made using drained cottage cheese mixed with a small amount of a strongly flavored cheese like roquefort. They can be dressed up by rolling them in finely minced parsley and chives. Cottage cheese can be turned into a creditable dip by mixing in minced cucumbers, radishes and carrots, and seasoning with garlic, celery seed and white pepper.

Roasted chunks of chicken breast which have been previously marinated in a lemon vinaigrette are another good pick. As an option for those who can afford the extra calories, you can serve an Indonesian peanut dunking sauce along with them.

The calories in your favorite dunking sauce can be sharply reduced by using yogurt, especially the low-fat variety, in place of sour cream. Similarly, substituting ricotta cheese made with part skim milk in recipes for spreads made with cream cheese can cut calories considerably. A tablespoon of the ricotta contains about 25 calories, while the same portion of cream cheese has twice that amount.

Finally, we suggest that you re-examine your old recipes for hors d'oeuvres. With some simple modifications of the type we have mentioned, they may fit your needs quite well.