BOSTON —  Jonah Pesner is looking ahead to his crucial carb-loading, fuel-up meal on the night before running his first Boston Marathon. On the menu: matzoh.

It's not the usual choice for marathoners loading up on carbohydrates to drive their run, but Pesner, a rabbi, has limited options.

Passover begins just two days before the April 21 marathon, and the holiday's strict dietary rules mean Jewish runners can't eat bread and pasta, the normal staples in the days before the big race.

Besides matzoh, which is unleavened bread, Pesner plans to pound down foods such as potatoes during a rare "carb-load seder" the night before the race.

Pesner never considered breaking the dietary rules for the sake of the race, which he is running with his wife for an autism charity.

"For me, running the marathon is a very spiritual quest," he said.

The marathon is always held on Patriots Day, a state holiday that falls the third Monday in April, and often comes within the weeklong Passover holiday.

Marathon organizers try to be sensitive to religious concerns, but major changes to suit various religions aren't practical, said Marc Chalufour, spokesman for the Boston Athletic Association, the marathon's organizer.

"You've got 25,000 runners and you obviously want to be sensitive to the needs of all of them," Chalufour said. "But you can't make a change to accommodate some of the runners at the expense of the majority."

The dietary restrictions for Passover forbid eating leavened foods, such as bread, cake, beer or pasta, which have yeast or other fermented grain products.

The prohibition is traced to the roots of the holiday, which marks when God sent an angel to kill first-born Egyptian sons, but spared the houses of the Israelites. Soon after, Pharaoh freed the Jews, who fled in such a hurry that the dough they took didn't have enough time to rise.

Jews usually hold a Passover seder, a meal with religious rituals, in their homes on the first two nights of the holiday, which is usually observed for eight days.

The level of observance varies. An Orthodox Jew, for instance, does not work or drive on the first two and the last two days of Passover, so he or she would not run a marathon on those days.

It's not an issue for Pesner, whose liberal Reform branch generally suggests followers hold a seder on just the first day of the holiday, though the dietary rules are observed the entire week.

Pesner, 39, acknowledges he has questions about the effects of his diet on his race. Matzoh is known to have a binding effect on the digestive tract.

"It's definitely a concern," Pesner said, chuckling.

Sandy Karpen, a real estate agent from Scottsdale, Ariz., said he and his wife, Sharon, are changing their tradition of attending seders the first two nights of Passover to accommodate their training. The second seder is the day before the race, and Karpen and his wife wanted to rest, rather than attend a seder on what is typically a long night.

Their rabbi from the Conservative Jewish tradition advised them that Jews may fulfill their obligation by observing only the first day, and said they could do the same.

The 17-time marathoner admits to some guilt about straying from his lifelong tradition, but has no regrets.

"I guess sometimes you're looking for justification for what you're doing," he said. "My rabbi said it was acceptable to do, and that was good enough for us."

Karpen, 49, and his wife ate fish and potatoes before their last long runs as sort of practice.

"The last thing you want to do is change your diet or change anything you've been doing throughout your cycle," he said. "You never want to experiment the day of the race."

Wayne Cohen, from Houston, figures that on the day before the marathon, he'll have egg whites and fruit for breakfast, rather than pancakes, and salmon with potatoes for dinner, instead of a carb-filled pizza.

But Cohen, 51, has decided he'll break Passover rules on the morning of the race, when he's planning to eat oatmeal without water and likely some pieces of bagel. Cohen has run about two dozen marathons, and decided he doesn't want to mess with his normal race day routine.

And he's not feeling guilty about it.

"I've pretty much convinced myself I would be a hypocrite if I said it would," he added. "It's not like I've been perfect in my religious beliefs.

"I'm beyond that," he said. "I'm not going to worry."