But investigators were also stunned to find that a relentless regimen of burgers, fries and soda also boosted so-called good cholesterol, seen as a key measure of cardiovascular health.
How the study was done
Researchers in Sweden asked 12 men and six women in their twenties, all slim and in good health, to eat two meals per day at McDonalds, Burger King or other fast-food restaurants over four weeks. The volunteers were also told to refrain from exercising.
The goal was to increase body weight by 10 to 15 percent to measure the impact of an abrupt surge in calorie intake.
Blood samples were taken before, during and after the experiment to monitor levels of an enzyme called alanine aminotransferase, or ALT, a potential marker for liver damage often seen among heavy drinkers and patients with hepatitis C.
Levels of ALT increased sharply after only one week, and quadrupled on average over the entire period, said lead researcher Frederik Nystrom, a doctor at the University Hospital of Linkoping. "The results scared me," he said. "One of the subjects had to be withdrawn from the study because he had 10 times the normal ALT levels."
Findings startle researchers
For 11 of the 18 subjects, ALT rose to levels that would normally reflect liver damage, even among individuals who did not drink any alcohol, although no such damage occurred, he said.
Two of the individuals had liver steatosis, or fatty liver, in which fat cells build up dangerously in the liver, he said. Steatosis is associated with the risk of developing Type 2 diabetes, which has taken on epidemic proportions, especially in industrialised countries.
Published in the British Medical Association's journal Gut, the study "proves that high ALT levels can be caused by food alone," said Nystrom.
That signs of liver damage were linked to carbohydrates was another key finding, he said. "It was not the fat in the hamburgers, it was rather the sugar in the coke," he said.
'Fast food diet may have benefits too'
But the most startling result implies that an intensive fast food diet might have some health benefits too, apparently from fat. "We found that healthy HDL cholesterol actually increased over the four-week period - this was very counter-intuitive," Nystrom said.
HDL, sometimes called "good cholesterol," seems to clean the walls of blood vessels, removing excess "bad cholesterol" that can cause coronary artery disease and transporting it to the liver for processing.
Nystrom has yet to publish the cholesterol findings, but said they were consistent with the so-called "French Paradox."
For nearly two decades, scientists have wrestled to explain how the French can consume a diet rich in fats - from abundant butter, cream, cheese and meat - yet have generally low levels of heart disease and hypertension.
"The study showed that the increase in saturated fat correlated with the increase in healthy cholesterol," he said.
The young Swedish guinea pigs ate at least two fast-food meals a day, and terminated the study once they had gained a maximum of 15 percent in weight. On average, they tipped the scales 6.5 kilos more, but one ballooned by 12 kilos.
Study inspired by movie
Nystrom got the idea for his study from the 2004 Oscar-nominated documentary "Supersize Me," in which filmmaker Morgan Spurlock asked doctors to monitor him over a 30-day period in which he ate at McDonalds morning, noon and night.
Doctors were so alarmed by changes in his blood chemistry - including skyrocketing levels of ALT - that they begged him to halt his experiment.
"I wasn't just inspired by the movie, I copied it to the best of my ability," said Nystrom. The movie helped spur a change of tack by fast-food corporations to include healthier options on their menus. On their websites, McDonald's and Burger King highlight salads and low-fat products - alongside the classic burgers and colas – and offer guidance on balanced diets and a healthy lifestyle. – (Sapa) - (February 2008)