Too much fast food could put people on a fast track to diabetes, a new study suggests.

Just one month on a fast food diet was enough to alter the ability of fat cells to respond to insulin, researchers from Linköping University in Sweden reported online April 30 in Molecular Medicine. The inability to respond properly to insulin, called insulin resistance, is a hallmark of type 2 diabetes.

Cell biologist Peter Strålfors of Linköping University got the idea to put people on a fast food diet from the 2004 documentary Super Size Me, in which a man eats a steady diet of McDonald’s food and grows heavier  and increasingly ill. Strålfors recruited 18 lean young people to go on a fast food binge. At the beginning of the experiment, the volunteers averaged a trim body mass index of 22.4. Body mass index, or BMI, is calculated from a person’s weight and height and indicates the degree of body fat, in most cases. A BMI between 18.5 and 24.9 is considered normal.

To follow a high-calorie diet, volunteers ate two fast food meals a day for a month. They also restricted physical activity to 5,000 steps a day — half the recommend amount of daily exercise. Before the experiment began, the researchers extracted fat from under the skin of the volunteers’ bellies. Or tried to — the volunteers were so lean that researchers were able to get enough fat for analysis from only six people.

Bingeing on fast food about tripled the volunteers’ daily calorie intake, the team reports. By the end of the month, volunteers had gained an average of 10 percent of their body weight, increasing BMI to 24.3, still within the normal range. On average, the volunteers gained about 12 to 15 pounds, 7.5 pounds (3.4 kilograms) of which was fat.

Fat cells collected from the volunteers at the end of the month showed a moderate amount of insulin resistance, the researchers found. The bingers were more insulin resistant at the end of the month than are most healthy people twice their age, although still not as resistant as diabetics, Strålfors says.

Other studies have shown that losing weight increases insulin sensitivity. “Just the process of changing has effects at both ends,” he says.

A vast increase in calorie intake and rapid weight gain are extremes that probably don’t reflect real-world situations, says Marc Hellerstein, a metabolic researcher at the University of California, Berkeley. And fat, or adipose tissue, is not the main tissue affected by insulin resistance. Muscle and liver are affected more, he says.

Still, even given what he sees as the study’s weaknesses, Hellerstein called it an interesting experiment that shows that weight gain, even in the short-term, can affect metabolism. The study did not address whether more exercise might have protected against some of the increase in insulin resistance in the face of weight gain.

After a month of bingeing on fast food, the volunteers returned to their former diets, exercise habits and, eventually, leaner BMIs, Strålfors says. The volunteers’ insulin sensitivity also returned to normal and he says he doesn’t expect any lasting harm to come from the month-long fast food bender. The researchers hope to extend the study to include more volunteers and get a better picture of how gaining weight changes the body’s response to insulin.

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