AT lunchtime in a grand old house surrounded by maples just beginning to turn, the fragrances wafting from the kitchen where Teresa Heinz Kerry taught her three sons to cook were so tempting it was difficult to concentrate on the expressionist paintings from the 1960's on the walls of the living room.

The source of the tantalizing aromas was one of Mrs. Heinz Kerry's favorite dishes, a fish soup. It was simmering on a Wolf stove in a kitchen filled with all the modern accouterments of the serious cook: a huge center island with a granite top, a Sub-Zero refrigerator; copper pots hanging from the ceiling. A rosemary bush sat on a table inlaid with old blue and white Portuguese tiles.

Shortly into an hourlong interview, it was evident that beneath the glossy chestnut hair, the expensive clothes and the cultured European and British-accented English, is a woman who knows how to cook. Even the living room, filled with family photographs — from both marriages — and comfortable, slightly threadbare sofas, gives a hint of the importance of food in this 90-year-old house just outside Pittsburgh: a pile of Gourmet magazines is neatly stacked under an antique table.

Campaigning leaves Mrs. Heinz Kerry no time in the kitchen these days, so it is her cook who has made the soup and a Portuguese poundcake. But the recipes are hers, handwritten in a large red loose-leaf notebook. "We've made my interpretation of a wonderful fish soup with haddock," she said of the contents of the pot. "I used to make it with halibut or striped bass, but I don't eat much halibut or tuna or swordfish because of the mercury."

The wife of Senator John Kerry, the Democratic candidate for president, talks as knowledgably about food safety as she does about classic French sauces. She cites the relationship between human health and what cattle feed on. She says that if she were younger, she would go into fish farming to produce more environmentally friendly, healthful stock and that she would raise rabbits because of they are high in protein and low in fat.

"I used to have 80 head of cattle," she said. "I used to feed them a little grain, but now they are all grass fed with alfalfa because of studies that show that animals that feed on grass have high levels of C.L.A.'s," conjugated linoleic acids or nutrients that are believed by some New Zealand scientists, she said, to reduce heart attacks and cholesterol levels.

The depth of her knowledge about food safety and nutrition comes from extensive reading and from consultations with experts. Some of it was personal: when Senator Kerry was operated on for prostate cancer last year, she spoke with an oncologist at Johns Hopkins who told her that green tea, tomatoes and cruciferous vegetables may help prevent the disease.

But her interest may also stem from her relationship with her father, who was an oncologist in Mozambique, where Ms. Heinz Kerry was born and brought up. She remembers going out in the bush with him. She said: "He would ask patients, `What are you eating?' Are you making a pig of yourself?' "

Mrs. Heinz Kerry would have been a doctor herself, she said, but she wanted to raise her children and did not think she could do both. Asked if she could successfully juggle the demands of the job of first lady and the charitable foundations she intends to continue to run, she said yes. If her husband is elected, she said, she will use her position to help reduce the epidemic of childhood obesity.

"We can't be a nation content to have young children at 7 and 8 suffering from adult onset diabetes," she said. "That's not right; it's a preventable thing. I am very concerned about junk food in the schools." When reminded that the Reagan administration called ketchup a vegetable, she laughed. "My late husband got a good giggle out of that," she said.

One of the projects she envisions dedicating herself to in the White House is to improve the food in the public schools by working with food manufacturers and chefs to "get ideas for things that are appetizing but healthy." Speaking of the exclusive contracts many schools have with soft drink companies, she said: "I understand why school boards have co-dependency with certain manufacturing companies. The point is, it starts with the manufacturing companies who can do things differently." She recalled how the makers of baby foods, under public pressure, removed the MSG and salt.

On the campaign trail she focuses on prevention when she talks about health care. "Schools and communities have to commit to the idea of being healthy," she said at a town meeting last week. "We have to start looking at the food, the drinks and sports as nice, good healthy living. It could save $50 billion to $60 billion if we could stop diabetes."

As a stay-at-home mother, she practiced what she preached. She did all the cooking for the family when her children were young and before her first husband, John Heinz, the ketchup heir, ran for Congress. (Senator Heinz died in a plane crash in 1991.) She wouldn't let them eat junk food. "I told my children this is my house, and I'm the witch," she said. "And every house has a witch. I had some leverage because I made everything myself — breads, jam, soups.

"I taught my boys to cook and make bread," she continued. "I missed the hard breads from home, so I used to go to the co-op and buy grain and grind it.

"None of my children are afraid of cooking," she added. "They can make anything and they enjoy good food."

After she married Senator Kerry, she expanded his limited culinary repertory. "When I married him, he did a very good olio with pasta," she said, "and a very good vinaigrette. And he made the best meringues in the world and the best chocolate mousse, just like my mother's. I taught him to make risotto and to make soups, to cook when I was not around.

"His diet before we were married was pasta with olio or tomato sauce, a salad and then ice cream, and I told him that even though he was thin, it was not the best way to eat."

She did not learn the craft at her mother's knee, or from the family cook. But when she came home from boarding school, she would head for the kitchen "to look and just taste all my favorite things, so when I went to graduate school in Europe, and I missed all the things I love, I asked my mom if she could send me eight recipes of things I liked." Everything else, she said, she learned through experimentation. "I never cook from cookbooks," she said. She is famous in her family for her scones.

The campaign has taken a toll not only on her cooking but on her weight. "I've gained so much weight now because I eat late and I'm so tired, it's just very hard," she said. "For someone who has always been thin, either you become neurotic about it or give up and say, when this is over I'll deal with it. I'll cut out wine.' "

For a trip to Colorado, where she joined her husband to celebrate her 66th birthday, she packed several good bottles. "The wine cellar is not bad at the farm," she wrote in a note to me.

If her husband becomes president, she said, she will fulfill the traditional role of first lady. Asked if she would cook in the White House, she said: "I know there is a tiny little kitchen somewhere there, and I know I'll make my sauces unless I can teach the big chef to make the sauces the way I like them: they have more garlic in them than most people like."

To the frequent question of whether she would be more like Laura Bush or more like Hillary Rodham Clinton, she has a stock answer. "I say neither. I am going to be me, and they say `How are you going to be me,' and I say, `I don't know because I don't know what it's like there, but I will be me because that's the only thing I know how to do.'

"Laura Bush is Laura Bush, and she's fine for her husband, and that's their marriage. And Hillary was Hillary, and that's their understanding and her career, and she made no bones about it. So that's fine, but I have no interest in a career.

"I like to bring people together so we don't waste opportunities and resources and keep doing the wrong things when we know better. Corporate America makes great things and things that can hurt us. They have to be part of the solutions. There's nothing to say you don't make a profit by doing good."

What role model would she choose if she got the job of first lady? Mrs. Heinz Kerry is reflective and diplomatic. "I was always impressed by Betty Ford and what she went through and how full of integrity she was, and how brave.

"I think Mrs. Reagan was a role model of my mother's generation, intelligent, very supportive of her husband. I am very different from my mom but I admired her devotion."

It is John Adams's wife, Abigail, whom she admires without qualification for "trying to keep things going when her husband was in Paris and her child was in Russia," she said. "It made me think of my own mother, because I was separated from my mother when I was young, and I know how hard it was for my mother to have all her children away. I value that strength."