Jack Osman, professor of health science at Towson University, isn't bothered that most Thanksgiving menus feature sweet potatoes smothered in brown sugar, nuts and marshmallows. What bothers him is that many people serve them only on holidays.

"Sweet potatoes in their natural state are just about the perfect vegetable," says Osman, 59, a nutrition expert and one of the country's leading authorities on sweet potatoes. Far from being the fat-laden vegetable we tend to think of, a sweet potato is actually one of the most nutritious vegetables around, he says.

High in beta carotene, which fights cancer, lacking any fat or cholesterol, and packed with generous doses of vitamins E and C and other nutrients, sweet potatoes are not only delicious prepared dozens of different ways, they're downright good for you. Just hold the fat and sugar.

"If we claim that we're interested in improving our health and nutrition, then we should be incorporating sweet potatoes as a regular part of our diet," says the fit and youthful-looking Osman, with a note of frustration creeping into his otherwise soothing voice. "My major goal is to get Americans to eat sweet potatoes twice a week."

Osman's own conversion came during a sabbatical in the 1990s when he started growing sweet potatoes. With a substantial crop on his hands, he read everything he could find on the subject and experimented with countless recipes. The more he knew, the more impressed he was with the vegetable's nutritional benefits.

These days, Osman cultivates about 12 of the more than 5,600 known varieties of sweet potatoes. With his wife, Beverly, he sponsors a fall dig-your-own festival on their 63-acre Pennsylvania farm. And Osman travels the country lecturing on sweet potatoes' nutritional properties.

He also addresses that old question of what's the difference between sweet potatoes and yams.

"The sweet potato (Ipomoea batatas) is a storage root in the morning-glory family," he says with a patient sigh. "It's not a modified potato and it isn't related to yams in any way."

Yams, he says, are starchy tubers native to Africa; they grow in places like New Guinea and the Caribbean, not North America. The canned "yams" that are sold in stores and end up on our Thanksgiving plates are actually sweet potatoes, most likely from North Carolina, where 40 percent of America's sweet-potato crop grows.

The holiday favorite has long been grown in this part of the world, according to Sue Johnson-Langdon, executive director of the North Carolina Sweet Potato Commission. Native Americans grew them, Christopher Columbus made note of them in his logbooks, and George Washington cultivated them at Mount Vernon.

So why aren't Americans eating them like crazy? Yearly consumption of sweet potatoes has fallen, down from an average of 20 pounds per person in the 1930s to less than four pounds in the 1990s.

Osman notes a number of reasons for the decline, including year-round availability of dozens of different vegetables, the sweet potato's association as a holiday dish, and lingering perceptions of it as a poor man's food.

Still, he is optimistic that with better education and marketing efforts, Americans will gradually return to consuming the orange-fleshed delicacy with greater frequency. As proof, he notes that a Pennsylvania supermarket chain, Saubel's, has started regularly baking and selling a low-fat, chocolate sweet-potato pie.

So, given the many recipes available, what kind of sweet-potato dish will grace Osman's own Thanksgiving table? To begin with, a sweet-potato centerpiece, featuring several oversized specimens from his garden. And then perhaps an orange-liqueur casserole with mandarin oranges. And, of course, a sweet-potato dessert.

Maybe even two. After all, it is a holiday.

Jack Osman's Sweet Potato, Apple-Raisin-Mandarin Orange Delight

Serves 6

1 pound sweet potatoes

1 to 2 tablespoons vegetable oil

3 apples, cored, peeled and sliced

one 8-ounce can mandarin oranges, juice reserved

1/2 cup brown sugar

1/2 cup raisins

1 teaspoon cinnamon

1/2 cup walnuts or pecans (optional)

Slice sweet potatoes, peeled or unpeeled, into 1/4 -inch-thick slices. Heat pan with oil over medium heat and add sweet potatoes. Fry on both sides for 5 to 7 minutes, reducing heat to low.

Add apples and juice from mandarins, then brown sugar. Stir and simmer. Add raisins and cinnamon, cover and simmer until apples are soft, about 5 minutes. Add mandarin slices and nuts, if using. Serve hot.

Dr. Jack's Chocolate Sweet-Potato Pie

Serves 8

1 1/4 cups of slow-baked sweet potatoes, pureed in a food processor (see note)

1/2 cup or slightly more of light-brown sugar (to be adjusted, depending on sweetness of the sweet potatoes)

1/2 cup evaporated milk

6 ounces of low-fat vanilla yogurt (or peach yogurt), room temperature

1 package instant chocolate pudding

2 tablespoons chocolate syrup (plus reserve to drizzle design on top)

1 egg, room temperature

4 egg whites, room temperature

3 tablespoons flour

1 tablespoon butter

1 teaspoon vanilla extract

2 teaspoons cinnamon

8- to 9-inch graham-cracker crust

For all ingredients except crust and reserved syrup: Measure and combine into a food processor. Blend for 1 to 2 minutes. Distribute evenly into graham-cracker pie shell. For the first 12 minutes, bake at 450 degrees. Reduce temperature to 350 degrees and bake an additional 55 minutes or longer, until set. Remove from oven and place on a cooling rack.

Before serving, drizzle design on top using reserved chocolate syrup.

Note: Slow-baked sweet potatoes (300 degrees) allow for the enzymes to work longer converting the starch to a sweeter sugar form.