A key farm official in Japan's ruling Liberal Democratic Party, pleading for more time to ease his country's barriers to agricultural imports, said yesterday that Japan may not be able to increase its purchases of American beef because of health and religious reasons.

Attempting to explain a slackening of Japanese interest in buying U.S. beef, Tsutomo Hata, chairman of the LDP's Agricultural Committee and a former agriculture minister, cited Buddhist restrictions on eating meat and the Japanese traditional diet that emphasizes fish over meat.

He also said the Japanese have a "much, much larger" digestive system than Americans. That, he said, makes it harder for them to eat beef.

(On other trade issues, Japanese officials have argued that their nation and people are different and therefore cannot always use non-Japanese products. One Japanese trade official, defending import barriers for foreign skis, said last year that Japanese snow is different from snow in the rest of the world.)

Hata, a member of Prime Minister Noboru Takeshita's inner circle, made his comments during a steak luncheon given by the Congressional Beef Caucus as part of a campaign to get Japan to drop barriers to beef imports.

He was immediately challenged by Sen. Phil Gramm (R-Tex.), one of Congress' most steadfast supporters of free trade, who said the problem was not health, the size of the Japanese digestive system or religion.

"The basic problem is your protectionism," Gramm said. "Open your markets and let people see if your intestines are too long, let them see if the teachings of Buddha" will keep the Japanese from eating beef.

"Japan is a great trading nation that is not acting like one. Great trading nations are not protectionist. Your protectionism cheats your workers and consumers," continued Gramm, who interrupted Hata when the Japanese party official tried to reply.

The exchange was symptomatic of the increasingly bitter tone that U.S.-Japanese trade relations are taking. It was especially marked because of Gramm's position in the Senate as an ideological free trader who said he will vote against the trade bill because it is too protectionist.

While Hata said Japan needs more time to prepare its highly protected agriculture sector for an end to import barriers, U.S. Trade Representative Clayton K. Yeutter said time had run out. The U.S. position is that the quota agreement signed in 1984 was final, and that the past four years should have been enough time to make the transition.

"I believe the government of Japan has had enough time to adjust for the elimination of quotas on beef and citrus," Yeutter said.

But Hata, who took part in the first negotiations to ease Japanese barriers on farm products 12 years ago with then-U.S. trade representative Robert Strauss, said, "Rapid and drastic change all of a sudden will be a very difficult matter for politicians."

Despite the quotas and high tariffs, Japan is the largest overseas market for U.S. beef. Nearly $500 million worth of U.S. beef was bought in Japan last year.

But Sen. Max Baucus (D-Mont.), host of the Beef Caucus luncheon, said U.S. exports would "increase significantly" if the barriers were removed. He cited Agriculture Department estimates that beef is the farm commodity with the greatest potential for growth in the Japanese market.

Farm imports are a sensitive cultural and political issue in Japan, where preserving some self-sufficiency in food is considered a critical issue by a people who remember how they almost starved after World War II. At the same time, farm constituencies are the backbone of support for Japan's ruling party.

The Reagan administration already is at loggerheads with Japan over another farm issue: A recent decision by an impartial panel that Japan's restrictions on other agricultural imports violates international trade rules.

Japan, however, refused to accept the ruling of the panel formed under the compact that polices international trade, the General Agreement on Tariffs and Trade, saying the Takeshita government needs more time to work out a domestic consensus on the issue.