The 13 million rural poor have worse diets, lower levels of blood nutrients, more abnormally small babies and more infant mortality than better-off Americans, and often also rank below the urban poor, according to a study released yesterday by Public Voice, a consumer advocacy group.

Executive director Ellen Haas told a Capitol Hill news conference that the rural poor are "a nearly forgotten segment of our society." She was joined by Senate Majority Leader Robert J. Dole (R-Kan.) and House Majority Whip Thomas S. Foley (D-Wash.), who expressed deep concern over the findings.

The study, financed by the Ford Foundation, found that the average rural American received $635 in cash, food and other income benefits from government programs based on need, compared with $1,406 for metropolitan residents.

Dole, calling the study far more objective than other hunger studies, said lower rural participation may come about because rural people with low incomes sometimes own assets that exclude them from benefits.

The study showed that 13.5 million rural residents were below the government's official poverty line in 1983 compared to 12.9 million urban residents. Most of the rural poor were not farmers.

The statistics showed:

* 12.3 percent of the rural poor consumed less than two-thirds of the recommended daily allowance of protein, compared with 6.7 percent of the rural nonpoor and 6.2 percent of the nonpoor nationally. Forty-eight percent of the rural poor took in less than two-thirds of the recommended amount of Vitamin A, compared to 42 percent of the rural nonpoor and 38 percent of the nonpoor nationally. Similar disparities were found for other nutrients.

* 44.8 percent of rural poor children had below-normal blood levels of Vitamin A compared with 28.2 percent of nonpoor overall; disparities were also found for zinc, iron and Vitamin C.

* 7.3 percent of rural poor children were abnormally short for their ages compared to 2.8 percent of nonpoor children nationally, and 3.6 percent of rural poor children were abnormally light in weight for their heights, compared to 3.4 percent for nonpoor children nationally.

* Infant mortality rates in the 85 poorest rural counties rose to 16.29 per 1,000 live births in 1983, compared with 11.13 in all other counties.