Swedish researchers say a diet of burgers, fries and soft drinks can damage the liver and increase waistlines, but may also boost levels of good cholesterol, a key measure of cardiovascular health.

In an experiment inspired by the movie Supersize Me, in which filmmaker Morgan Spurlock lives on McDonald's meals for a month, the researchers asked 12 men and six women to participate in a similar experiment.

The 18 participants — all in their 20s, lean and in good health — spent a month eating two meals a day at fast-food restaurants while living a sedentary lifestyle. The participants were told not to exceed 5,000 steps per day, with the goal of increasing their body weight by five to 15 per cent. All but one of the participants reached that goal, while five boosted their body weight by the maximum of 15 per cent.

The fast-food diet increased not only waistlines, however, but also an increase in alanine aminotransferase (ALT), an enzyme that indicates liver damage and a future risk of developing Type 2 diabetes. High ALT levels are often linked to hepatitis C or the consumption of large quantities of alcohol.

In their study, published online on Feb. 14 in the British Medical Association journal Gut, the researchers reported that participants' ALT levels jumped after the first week of the experiment. By the end of the month, most participants' levels had quadrupled.

"The results scared me," lead researcher Frederik Nystrom, a doctor at the University Hospital of Linkoping, told AFP. "One of the subjects had to be withdrawn from the study because he had 10 times the normal ALT levels."

For 11 of the 18 participants, the enzyme rose to levels that would normally reflect liver damage, the researchers said, although no such damage occurred.

Two of the participants developed liver steatosis, a condition in which fat cells build up dangerously in the liver. The condition is associated with a risk of developing Type 2 diabetes.

The researchers said their findings prove that high ALT levels can be caused by food alone, which could prove important for doctors diagnosing patients showing high levels of the enzyme but no symptoms.

"We suggest that in clinical evaluation of subjects with elevated ALT physicians should include not only questions about alcohol intake, but also explore whether recent excessive food intake has occurred," wrote the researchers, noting that even a "short over-indulgent holiday" could boost the levels.

Bad for the liver, good for cholesterol

However, the researchers also found that a month-long fast-food binge can actually increase levels of HDL cholesterol, a result Nystrom called "counter-intuitive" in an interview with AFP.

HDL cholesterol, or good cholesterol, can help carry fatty acids and excess bad cholesterol — which can cause coronary artery disease — from the body's tissues for processing to the liver.

"The study showed that the increase in saturated fat correlated with the increase in healthy cholesterol," Nystrom said.

Nystrom told AFP the researchers' findings were consistent with the so-called "French paradox," that is, how the French can consume a diet rich in fats like butter, cream, cheese and meat, but have generally low levels of heart disease and hypertension.

The cholesterol findings have not yet been published.