Excess amounts of "junk" food and little exercise can cause liver damage in just one month, scientists claim.
Experts from Linkoping University in Sweden carried out the study on 12 men and six women and found that people who ate junk food twice a day experienced varying degrees of damage to their liver, and also put on a lot of weight.
They all started out the trial healthy and slim, but were then given at least two fast food meals a day, while their levels of exercise and alcohol intake were also restricted.
At the end of the study, which is published in the journal Gut, 17 of the participants had increased their body weight by between 5% and 15%, with one putting on 12kg after just two weeks.
Levels of an enzyme, alanine aminotransferase (ALT), were also measured in the blood, which can indicate liver damage.
Large increases in ALT occurred after just one week on the fast food diet, and over the study period rose from 22.1 units per litre to 69.3 units per litre among the group.
Evidence of insulin resistance, which increases the risk of diabetes, was also recorded.
The researchers said: "The most important finding of this study was that during the study period 13 subjects on the high caloric diet developed pathological ALT, according to the updated definition of healthy ranges.
"In most subjects this elevation was evident within the first week."