One of every eight youngsters under age 12 is hungry, according to a new report that is the most comprehensive look yet at childhood hunger in America.

Millions more children are estimated to be at risk of hunger, meaning a full one-quarter of the nation's children from birth to age 12 are suffering from food-shortage problems, said the study conducted by the Food Research and Action Center, a nutrition advocacy organization based here.It called for greater spending on existing government food aid programs.

The danger is not just poor nutrition. Children who don't get enough to eat are more apt to be tired, irritable, unable to concentrate and prone to headaches and other illnesses that keep them out of school, the study said.

"These children often seem invisible," it said. If their problems go unattended, these hungry children will be less productive as adults and "our society will be less competitive in the world marketplace."

The three-year, million dollar, door-to-door survey looked at seven areas of the United States where the characteristics of the low-income population reflected the low-income population of the country overall.

The people surveyed came from families whose annual income was 185 percent of the poverty level or less. Since the government's poverty level in 1990 for a family of four was $12,700, such families at 185 percent of poverty would earn $23,495.

The 185 percent figure was used because it is the one used to determine eligibility for free school lunches and food stamps.

The surveyors asked questions of families in urban areas of Connecticut, Michigan, Minnesota and New York and rural areas of Alabama, California and Florida. Based on those answers, they classified some families as hungry and others as at risk.

Families with hungry children are poor; their incomes are an average 25 percent below the poverty line, the study found. While these families spent nearly one-third of their income on food, that amounted to only 68 cents per person per meal.

Most of the family income went for housing, the surveyors found, with the poorest families spending more than 60 percent on shelter. In contrast, the study noted, the typical American household spent 22 percent of its gross income on shelter in 1987.

The surveyors found that hungry children were two to three times more likely than other low-income children to have such health problems as unwanted weight loss, fatigue, irritability, headaches and inability to concentrate.

These youngsters were absent from school almost twice as much as other children, said the report.

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The report's recommendations

- Increase the appropriation for the Supplemental Food Program for Women, Infants and Children.

- Expand the school breakfast program and protect the school lunch program from budget cutbacks.

- Expand the federal Child and Adult Care Food Program and Summer Food Service Program for Children so that youngsters who are not in school won't go hungry.

- Improve access to food stamps.