Miami Beach, Fla. ? Bulletin from the medical front: Eating fast food and sitting hours on end in front of the TV are really terrible for the waistline.

As obvious as this might sound, there actually is little firm scientific data to prove the point, especially against fast food. But now a large study released Saturday shows just how bad super-sized burgers and nonstop tube time can be for one’s health.

“Fast food emphasizes primordial preferences for salt and fat,” said researcher Mark Pereira. “This may promote overeating.”

The project did turn up one surprise: While a lot of fast food is clearly bad for white people, it seems to make no difference at all for blacks.

The reason for this is unclear, but the researchers said the data suggest that black people’s diets are often just as unhealthy at home as they are at the drive-through window. So a lot of fast food doesn’t make matters any worse than they already are.

The analysis, conducted by Pereira of Boston’s Children’s Hospital, was based on a 15-year follow up of 2,027 whites and 1,726 blacks between ages 18 and 30 in Chicago, Minneapolis, Birmingham, Ala. and Oakland, Calif. He presented the data at a meeting of the American Heart Assn.

The key findings:

• Whites who eat fast food twice or more a week have a 50 percent greater risk of obesity than do those who eat this way once or less.
• Their risk of abnormal glucose control, an inability to break down sugar efficiently that often foreshadows diabetes, is double.
• Whites who eat fast food more than twice a week and also spend at least 2 1/2 hours a day watching television have triple the risk of both obesity and abnormal glucose control, compared to those who eat out once or less and watch less TV.

“It’s clearly the composition of fast food meals that we feel plays a role, with a lot of saturated fat and low quality carbohydrates, white bread and lots of soda,” said Pereira. “And what you are not getting is also critical, including fiber and more healthful types of fats. It’s a dietary pattern that is the opposite of what’s recommended for health.”

And of course, there are all those calories. A supersize fast food meal may exceed 1,600 calories, more than many people should eat in a day.