KOSHER food can stack on the kilos. So says the rabbi who has launched a diet challenge to Australian Jews: lose 1000 kilograms in 12 weeks.

Rabbi Mendel Kastel is publishing a diet online this month that comes with kosher recipes, a kosher conversion guide, personalised menus, exercise plans and tools to set goals and track weight.

"Bagels, chopped liver, matzo balls, schmaltz herrings - there are things in a kosher diet that make us put on weight," he said. "Rather than a white bagel, try a wholemeal one."

As head of the Rabbinical Council of NSW, Rabbi Kastel has sent a message out to rabbis across Australia to challenge their communities.

Corporate sponsorship in the form of a dollar for every kilogram lost will be directed to the Jewish House, a crisis housing agency.

"We aim to lose about a kilo a week. So we're looking for 100 people to lose 10 kilos each. And the more, the better," he said.

Rabbi Kastel said he did not know if Jews were fatter than other Australians, but he said they needed to be aware of the importance of eating well, drinking moderately and getting regular exercise.

The rabbi has been road-testing the program for the past five weeks and said he has shed six kilograms. "Just 20 kilograms to go."

Under Jewish law people should eat only kosher food. Meat, for instance, must come only from animals that chew the cud and have cloven hoofs, such as cows. And they must be killed in a particular way, removing as much blood as possible.