OPELIKA, Ala. - When 17-year-old Tianna Summers puts a fork full of fresh lima beans in her mouth in the school lunchroom here, she is eating a vegetable seldom seen in any other American school.

"Junk food is not the center of our universe," Summers said, polishing off a meal of barbecued pork, the lima beans and a salad.

But in most of the country, it is.

A school lunch often looks like an exercise in fat loading, with a supersize soft drink from a vending machine, followed by a candy bar from another machine. The meal is more in keeping with a fast-food menu than what the Department of Agriculture says is nutritious.

This yawning discrepancy between what students should eat and what most of them pile onto their trays has become a central issue in the national debate over why Americans are growing obese. For the first time in five years, Congress will take up the school lunch issue this winter, writing legislation that will affect the diet of 27 million public school children, in elementary through high school.

The problems are immense, and any solution is prey to an array of interests vested in the $10 billion annual federal school nutrition programs. Among the interests are the soft drink and food service industries, agribusiness and farmers.

In six other schools visited recently, in New York City and Montgomery County, where hundreds of students were eating lunch, only five of them took a green vegetable with the main course. Faced with bad-tasting canned green beans provided free by the federal government, children in New York City and Montgomery County opted out.