vereating for just four weeks can increase fat mass and weight over two years later , a Swedish study shows . Researchers recruited 18 healthy and normal - weight people with an average age of 26 . For a four - week period , participants increased calorie intake by 70 % ( mostly by eating fast food ) and limited daily activity to a maximum of 5000 steps per day ( considered sedentary ) . Not surprisingly , weight and body fat of the participants went up significantly during the study and then decreased after the study ended . Participants are believed to have returned to the diet and lifestyle they had before the experiment . However , two and a half years after the experiment , the mean weight gain for participants was 6.8 lbs with a standard error of 1.2 lbs