One month of "slobbing it" has weight gain effects that can last for years, research has shown.
Scientists asked volunteers to gorge on fast food and be less active for four weeks, expecting them to put on weight but then return to their normal size.
Instead, after a promising start to their weight loss efforts, the participants kept getting bigger.
After two and a half years they were still more than three kilos heavier than when the study began.
A comparison "control" group who did not change their lifestyle showed no similar increase in weight.
"The long-term difference in body weight in the intervention and control groups suggests that there is an extended effect on fat mass after a short period of large food consumption and minimal exercise," said study leader Asa Enersson, from Linkoping University in Sweden.
The aim of the research was to study the long-term effects of sedentary "couch potato" lifestyles.
Eighteen volunteers with an average age of 26 were asked to spend a month eating at least two fast-food meals a day to boost their calorie intake by 70%.
At the same time they had to reduce their activity levels to no more than 5,000 steps a day, as measured by a pedometer.
Over the four-week period the participants gained an average of 6.4 kilos.
Most of the extra weight was lost six months later when they had returned to their normal eating and exercise habits.
But the positive trend was short-lived. One year on the group's average weight was 1.5 kilos higher than at the start. After two and a half years their weight gain had risen to 3.1 kilos.
The findings were published in the online journal Nutrition & Metabolism.
They indicate that even a short period of over-eating and lack of exercise can change a person's physiology, making it harder to lose weight and stay slim.
"The change of fat mass was larger than expected when compared to the controls," said Ms Enersson. "It suggests that even short-term behavioural changes may have prolonged effects on health."