What's the best way to prepare your body for a fast?

"Use the same eating strategy you would for a marathon," says Rabbi Richard J. Israel, who is asked this question every year at this time as Jews prepare to go without food and water for 26 hours in observance of Yom Kippur (Sept. 17 this year).

A seasoned "ultra-marathoner" (50 miles, rather than the usual 26), Israel discovered several years ago that the pre-race "carbo-loading" regimen popular with athletes helped him through his annual Yom Kippur fast.

"Since filling the body with carbohydrates builds endurance for a race, I figured it could build endurance for a fast," says the 53-year-old director of the B'nai B'rith Hillel Foundations of Greater Boston.

Before sundown on the eve of Yom Kippur, "I ate a normal meal, but emphasized carbohydrates like noodle or potato dishes, rather than the traditional fats and proteins of chicken soup. It worked great, and I told others about it. Now they swear by it."

This pre-fast strategy, stresses Israel, "doesn't prevent people from experiencing the fast. Our discomfort is intended to help us reflect on our human frailty. But that does not mean Jews are required to make themselves as miserable as possible.

"If you are not eating or drinking for 26 hours there is no chance you will forget you are fasting. But it is important for you to be able to focus on some soul-searching and prayer, rather than on your complaining stomach."

Among his other fasting suggestions:

* "Camel-Up" at your pre-fast meal. "Lack of food is not the problem, lack of fluid is . . . The solution, therefore, is to super-hydrate beforehand."

* Wean yourself from caffeine. "If you drink a lot of coffee or cola drinks, taper off during the week or so before the holiday . . . Caffeine-withdrawal symptoms are less of a problem when you are eating and drinking than when you are fasting."

* Reenter cautiously. After the fast be careful not to gorge yourself, unless you don't care about putting on weight. Fasting slows the metabolism, so "the calories you take on right after a fast will stay with you a lot longer than those acquired when your metabolism is once again functioning at full speed."

As to running on the Day of Atonement, "It goes against the spirit of the day," says Israel. But since Jewish holidays begin and end at sundown, it is possible to observe Yom Kippur and still get a daily workout.

"I do a long run beforehand," says Israel, who hasn't missed a day of running in seven years. "Then I break my fast with a run. It's wonderful to go several miles feeling so light."

More Fast Advice. Because Irish Nationalist hunger striker Bobby Sands refused food for 66 days before he died of starvation, "We've unfortunately had very good experiments in how long someone can live without food," says Dr. Aaron Altschul, professor emeritus and director of the Diet Management/Eating Disorders program at Georgetown University Hospital.

A day-long fast should pose no health hazard to most people, says Altschul, who advises anyone with certain medical conditions, such as diabetes, to check with a doctor before fasting.

But while he sanctions brief fasts for religious purposes, Altschul is "totally opposed" to popular programs that promote fasting to lose weight or cleanse the body of toxins. "It's an extremely poor and potentially dangerous way to try to lose weight. As for eliminating toxins, the body's very good at cleansing itself without being deprived of necessary nutrients."

He does recommend occasional "minimal eating days"--800 nutritionally balanced calories--for those who want to shed pounds or prepare for a holiday. These mini-fasts can be done once a week, as often as two to four times a week, or for a few days before a big feast.

Sixty percent of these 800 calories should be carbohydrates, 20 percent protein and 20 percent fat.

"Everybody's got exactly the wrong idea," he says, about complex carbohydrates like noodles and rice. A gram of carbohydrate provides 4 calories; a gram of protein, 4 calories; a gram of fat, 9 calories, and a gram of alcohol, 7 calories.

"So if you want a food of low caloric density you pick something low in fat and high in water--like rice, potatoes, even bread. By contrast, foods of high caloric density are high in fat and low in water, such as most cheeses, nuts and red meats.

"Virtually anything in moderation is okay. But that can be the trouble with fasting . . . obsessions are really the problem. It's much better for people to learn to live with less food. We like to see people stay in the habit of eating."