An activist group yesterday released eight pages of practical advice on how to maintain a low-fat, low-cholesterol, anti-heart disease diet that the Reagan administration had decided not to publish.

Entitled "Eating the Moderate Fat and Cholesterol Way," the advice repeats what many doctors counsel: eat less fatty meat and high-fat dairy products, fewer eggs and fewer saturated, mainly solid, fats.

Most nutritional scientists consider this good advice to help avoid heart and blood vessel disease, and a National Academy of Sciences panel said three weeks ago that the same kind of diet might help avoid cancer.

John Ochs, press aide to Agriculture Secretary John R. Block, said yesterday, "We felt there was not the scientific evidence for the U.S. Agriculture Department or the government to publish" the advice, which was written during the Carter administration.

To spokesmen for the Center for Science in the Public Interest, which issued the material, the decision represents not caution but suppression.

They pointed out that Block announced last September that he would issue a publication called "Food/2," containing the moderate-fat and cholesterol chapter.

"Less than a week later," they said, "he reversed that decision after meat, egg and dairy interests complained that the booklet would harm sales of their high-fat products."

At about the same time, the Agriculture Department also decided that it would no longer give away but would sell, for $2.25, another Carter administration endeavor, a pamphlet entitled "Nutrition and Your Health: Dietary Guidelines for Americans."

Those guidelines--issued by the Agriculture and Health, Education and Welfare departments in February, 1980--warn unequivocally to "avoid too much fat, saturated fat and cholesterol."

Three months later, the Food and Nutrition Board of the National Academy of Sciences disagreed. It said such advice was needed only for persons with a family heart disease history or too much cholesterol in their blood.

Cholesterol is a fatlike substance that clogs arteries and helps cause heart trouble. The body continually produces it, but overeating and, especially, eating saturated fats tend to build up the supply.

USDA's Ochs denied that the department was suppressing the "Food/2" fat and cholesterol information.

The American Dietetic Association, the country's registered dieticians, asked for the text so they could distribute it "and we gave it to them and said we'd be happy to see them publish it," he said.

However, James Breeling, the dietetic group's executive director, said he was given the information only after filing a Freedom of Information request. He said the text would be published only if his group felt it would sell well.