That's the conclusion of a study of more than 3,000 white and black American adults. Participants reported their fast-food dining habits for 15 years, starting when they were 18-30 years old.
"Appropriate action would be to reduce portions to normal sizes, and to sell burgers of lean meat, whole-grain bread or buns, fat-reduced mayonnaise, more vegetables, lower-fat fried potatoes, and reduced-sugar soft drinks," writes Arne Astrup, in an accompanying editorial in The Lancet.
In the study, those who ate fast food more than twice a week gained 10 more pounds during the study than participants who ate fast food less than once a week. They also doubled their insulin resistance, a sign of early diabetes.
It's no secret that many Americans struggle with their weight. Nearly two-thirds of American adults are overweight or obese, according to University of Minnesota researcher Mark Pereira, PhD, and colleagues.
Diabetes is also widespread. More than a million new cases are diagnosed each year, says the American Diabetic Association.
A little more than 6% of American adults have diabetes. It's even more common among older adults. About 18% of people aged 60 or older have diabetes, according to the American Diabetic Association. Diabetes can lead to heart attacks, stroke, blindness, and kidney failure.
The researchers don't exclusively blame fast food. They also note other lifestyle choices. For instance, frequent fast-food eaters who were white said they drank more alcohol, were less physically active, watched more TV, and ate a less healthy diet. The same was not true for black participants.
The bottom line: Opt for smaller portions and the healthiest fast-food items, say the researchers.