Latest research has found that a month-long diet of fast food and no exercise can undoubtedly lead to dangerously high levels of liver-damaging enzymes. However, the researchers were also surprised to find that junk food consisting of burgers, French fries and sodas also boosted so-called good cholesterol that is essential element for heart's health.

The Swedish researchers who studied 18 lean, healthy students also added that consumption of such fast foods is very harmful for liver, not to mention the waist size.

In the experiment, the students were asked to gain 5 to 15 percent of their body weight in a month by eating at least two daily meals at fast-food restaurants and adopting a sedentary lifestyle. Their fast-food diet featured hamburgers and other foods high in saturated animal fat.

On average, the students gained 14 pounds and added 2.6 inches to their waistline. Their body fat percentage was up by 3.7 percent during the study. After studying their blood samples, it was found that there was an increase in levels of the liver enzyme alanine aminotransferase (ALT). High ALT levels can be a sign of liver damage.

The study, published in the advance online edition of Gut, also found that excessive fast food diet might have some health benefits too, apparently from fat. Healthy HDL cholesterol also increased over the four-week period.

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.