(FORTUNE Magazine) – I recently went by Paul McGlothin's house in Westchester County, N.Y., to watch him eat breakfast. At 5:45 in the morning he began his first course by measuring out exactly eight ounces of unsweetened cranberry juice on his Tanita 1458N digital scale. McGlothin had been up since four, when he had a glass of water with lemon juice. He told me he had consumed exactly zero calories since yesterday at lunch. I thanked him for letting me come by so early in the morning. "It's not early for me!" he said. "I've been up working for two hours. I'm lucid. I'm feeling great. And the last time I ate was 17 hours ago!"

McGlothin, who declines to give his exact age but says he is in his mid-50s, is the head of a 40-person advertising company called Arts for Business. He is also a serious practitioner of caloric restriction (CR)--an extreme diet that purports to dramatically extend human life. While it's not yet scientifically proven to work in people, tests have shown that it does work in critters like mice (see www.calorierestriction.org for more information). The idea: If you eat 30% fewer calories than is natural for your body type, you'll live 30% longer. For most of us, CR sounds like a living hell. But how awful is it, exactly? Is living to 100 worth the torture?

To find out, I spent a day with McGlothin, who has been on CR for ten years. (He works at home in the mornings and heads to the office in the afternoons.) He eats between 1,900 and 2,000 calories a day--the equivalent of one Whopper with cheese, large fries, and a chocolate shake --and is, as you might expect, a very skinny guy. At almost 6 feet tall, he weighs 133 pounds and has a 30-inch waist. When we shook hands, I noticed that the band on his Ironman watch--pulled all the way to the last notch--stuck out like a ripcord.

Here's what he consumed after the lemon water and cranberry juice:

6 A.M. Tea made from orange peels, lemon peels, dandelion, burdock, ginger, and a Chinese green tea packet.

6:15 A.M. A cup of brewer's yeast. McGlothin pours 20 grams of the powder into a glass of tea, mixes it into a tan paste, and eats it with a spoon: "I love the taste of this stuff!" I have a cup and find it to be surprisingly tolerable, despite a somewhat gruelish consistency.

6:30 A.M. A 72-gram piece of salmon, about as big as a king-sized candy bar.

7:00 A.M. A bowl of blanched oranges, nonfat kefir, and two tablespoons of lecithin. (Bland fruit in a bitter, yogurt-like substance with little globs of dried soybean oil mixed in. Yummy.)

8:18 A.M. Two cups of soup made from beets, onion, green peas, broccoli, and mustard seeds that McGlothin made the night before. He enters the calories into a program on his laptop called NutriBase Clinical Nutrition Personal Food Manager. (Mustard seed, six grams: 28 calories; broccoli, 70 grams: 20 calories.)

11:21 A.M. Lentils and walnuts mixed with Very Veggie, an organic vegetable cocktail from concentrate, and sprinkled with the contents of a Curcumin 95 capsule (an antioxidant).

11:42 A.M. Blueberries and walnuts mixed into kefir.

11:48 A.M. More lentils. (McGlothin's wife, Meredith Averill, who follows the diet too, admits that she's "coveting more salmon" but resists the urge.)

12:05 P.M. Yet another bowl of lentils, this time with olive oil and garlic.

12:20 P.M. A bowl of sweet potatoes lightly steamed, "kind of al dente," with kefir.

12:42 P.M. Soup with organic spinach, mushrooms, and beets.

1:05 P.M. A few more walnuts.

And that's it for the day, except for a couple of pieces of sugar-free chewing gum. Abstaining from food after lunch helps keep him alert, McGlothin says: "How many people fall asleep after lunch and are really doing nothing at work after that?" I know at least one.

I ask him about some common complaints about life on CR, such as feeling cold all the time. "I always feel fine in the summer, and that's the season when people have strokes," says McGlothin. Does he have feelings of depression? Eating salmon makes him feel "optimistic" and "positive." What about a reduced sex drive? "Not a problem in our house!" he says. "Though testosterone definitely goes down, but that's cancer-protective." He adds that his heart rate, body fat, and blood pressure are "about those of an Olympic swimmer."

As McGlothin practices it, CR is difficult, certainly, but not quite as outrageous as I'd expected. He seems determined rather than loony, and he has lots of interesting things to say about what foods are good for conference calls (whey) or for multitasking (chicken). It is enough to make me want to go on a diet myself. Almost.

FEEDBACK gdavid@fortunemail.com