People who gorged on fast food for four weeks and did little exercise put on an average of 6.4kg of weight.
Two years later, signs of increased body fat were still apparent, says the Linkoping University study.
The Swedish researchers studied a group of 18 adults with an average age of 26.
During the study, the details of which were published in the journal Nutrition & Metabolism, the 18 participants had their physical activity limited to 5,000 steps per day, considered to be tantamount to a sedentary lifestyle.
The study suggests that even a short period of excessive eating and a lack of exercise can potentially change a person's physiology - making it harder to lose and keep off weight.
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