You suggested a quick recipe for lentil soup that was both tasty and low in fat. With colder weather here, I'd like to make some. Can you provide the recipe again?

ANSWER - Gladly. We're fans of soup and are pleased to promote it as a healthful centerpiece of meals. Chop a medium onion and a stalk of celery with a tablespoon of oil. Add a one-pound package of rinsed lentils and an eight-ounce can of crushed tomatoes or tomato sauce. Add water to a level twice the height of the lentils and simmer until tender, about an hour or so. Then add a bunch of chopped escarole and simmer five minutes, or until greens are just cooked. Season with salt and black pepper.Divided among six people and served as a main dish, a portion would contain about 300 calories. Serve along with French bread and a salad, skim or low-fat milk and fruit for dessert, and you'll be hard-pressed to find a winter dinner that's lower in cost or easier to prepare.

To vary the flavor without adding appreciably to the caloric count, use beef or chicken stock as the liquid. Or brown a clove or two of minced garlic along with the onion or celery. Or add a sprinkling of Parmesan cheese at the time you serve the soup.

QUESTION - I was reading the nutrition-labeling panel on a container of yogurt and was surprised to note that vitamin D wasn't listed. Why?

ANSWER - According to labeling regulations, vitamin D does not have to be included. And although it's derived from milk, yogurt isn't an especially good source of the vitamin. It's true that we rely on milk as a dependable source of vitamin D. But fresh from the cow, the amount of the vitamin that milk contains is both small and variable. How much is present depends on what the cow has eaten and on how much time she's spent in the sunlight. Besides, since it's a fat-soluble vitamin, what D there is gets removed along with the fat in low-fat and skim-milk products.

The reason we can turn to milk as a D source is that most milk is treated in one of two ways. It may be exposed to ultraviolet light, which converts a sterol in the milk to vitamin D, producing a limited amount. Or in order to produce greater D richness, a concentrate is added to bring levels in milk up to 400 International Units per quart. But vitamin D is not added to yogurt.

QUESTION - I try to keep my family's cholesterol in check. I recently purchased some mayonnaise marketed as "cholesterol-free," but everyone complained about the taste. I then rechecked the label on regular mayonnaise, and the amount of cholesterol seemed so small that I wonder whether it's worth bothering with the cholesterol-free type at all. What do you say?

ANSWER - We say it's more important to get accustomed to using only a small amount of mayonnaise, which is high in fat and calories, than it is to worry about the cholesterol it contains. True, eggs are an ingredient in regular mayonnaise, where they preserve the emulsion essential to its texture. However, the amount needed to fill this role actually is quite limited.

A single egg yolk, which provides about 215 milligrams of cholesterol, can emulsify as much as two cups of oil. Thus, a tablespoon of mayonnaise would contain fewer than 10 milligrams of cholesterol. So don't worry about cholesterol in mayonnaise. Instead, focus on cutting down on mayonnaise because of calories and fat.

QUESTION - What is "autolyzed" yeast?

ANSWER - These are yeast cells that have been broken down into their component parts by enzymes working inside the cells. The resulting autolysate may be processed further, into yeast extract, by removing the cells' walls and other bitter-tasting compounds. Food technologists can vary the conditions under which the autolysis takes place to create a variety of different flavors ranging from "cheesy" to "savory."

Both autolysates and yeast extracts are used as flavor enhancers in a long list of foods, including cheese products, apple pies, meat pies, hot dogs, soups and snack foods.

1990, Washington Post Writers Group