Even short periods of bingeing on junk food can leave the body more prone to gain weight for years to come, they found.
Turning to chocolate or ice cream to help mend a broken heart could cause the waistline to bulge years later, they suggest.
The first group gained an average of a stone over the month.
Swedish researchers weighed and measured 18 slim, healthy and active men and women who were asked to almost double their calorie intake for one month.
The volunteers, aged in their early 20s, ate at least two junk-food meals a day and did very little exercise.
Much of the extra fat was stored on the hips, giving credence to the saying, 'a moment on the lips, a lifetime on the hips'.
Dr Asa Ernersson, of Linkoping University in southern Sweden, said: 'The change in fat mass was larger than expected.