Washington ? Many Americans are like a loaf of bread — soft, with one side round. Their choice of bread may be part of the reason.

Some researchers say white bread and other refined grains seem to go to the gut and hang out as belly fat.

“Waist circumference was very much associated with this high-refined grains pattern,” said Katherine Tucker, an associate professor of nutritional epidemiology at Tufts University in Boston. She and other scientists are studying what happens to the bodies of people who eat lots of refined bread.

The researchers have tracked the eating habits of a group of healthy, largely middle-age people in Baltimore. The focus is on 459 people with a variety of eating habits. Some prefer refined grains; others favor whole grains, fruits and vegetables.

Refining removes the fibrous bran and oil-rich germ, leaving the sweeter endosperm, the whitish-colored meat of the kernel.

The Tufts researchers say calories from refined grains like to settle at the waist.

The belt size of the white bread group expanded about one-half inch a year, which probably put some of the research subjects into a larger size of pants over the three years they were tracked, Tucker said. At the end, the white bread group had three times the fiber group’s gain at the gut.

It is not surprising that the waists expanded, said Dr. David Ludwig, director of the obesity program at Children’s Hospital in Boston.

Ludwig was not connected to the Tufts study, but his research involving younger adults had found something similar. One of the factors he checked was the waist-to-hip ratio — whether people’s torsos were more tapered or more round. People who ate less fiber were rounder.

Shancell Ward, of Washington, D.C., stacks white bread on shelves at a grocery store in Washington. Scientists say white bread and other refined grains seem to go to the gut and hang out as belly fat.

Waist size is important for health as well as looks.

A person with a bigger gut has a higher risk of heart disease than a person who weighs the same but who does not carry extra weight around the belly.

Why that is and why refined grains would send more calories to the gut are a mystery. The Tufts researchers, who published their data in June of 2003, are seeking answers.

Their theory is that it is linked to the ease in which the body breaks down carbohydrates in the endosperm into simple sugars. When sugars flood the body, insulin levels rise to help pull the sugars out of the bloodstream and store them in cells, often as fat.

“I think abdominal fat cells may be more sensitive to insulin’s effects than other fat cells in the body,” said P. Kristen Newby, lead author of the Tufts study.

For consumers, white versus whole grain is not necessarily an either-or choice. About 70 percent of households have a white bread eater, and about the same percentage have a whole wheat eater, according to a February report by the Mintel Group, a market research firm in Chicago.

Curbing carbohydrates as a way to rein in the insulin response is an important rationale for popular carb-curtailing diets such as the Atkins and South Beach plans. As a result, dieters are giving up refined grains.

Outside a Giant supermarket in Reston, Va., Pete Krone of Oak Hill, Va., said the South Beach diet took him from the farthest-out notch on his belt to the closest-in, and is pushing him toward a smaller belt. Cutting back on refined grains was a big part of it, he said.

“The South Beach diet gets rid of what’s known as the bad carbs — the high-sugar carbs, which would be white bread, potatoes, starches, pasta — and gradually replace it with wheat bread, (and) cereals that are higher fiber,” Krone said. “What went first was the fat around my waistline.”

While it appears that people who eat more refined grain products can wind up needing bigger belt sizes, more studies will be needed to prove it, Newby said.