Junk food is a pejorative term for food containing a large number of calories from sugar or fat with little dietary fiber, protein, vitamins or minerals. The term can also refer to high protein food like meat prepared with saturated fat. Food from many hamburger joints, pizza places and fried chicken outlets is often considered junk food.
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In a study published in the European Journal of Clinical Nutrition, the frequency of consumption of 57 foods/drinks of 4000 children at the age of four and a half were collected by maternal report. At age seven, the 4000 children were given the Strengths and Difficulties Questionnaire, with five scales: hyperactivity, conduct problems, peer problems, emotional symptoms and pro-social behavior. A one standard deviation increase in junk food was then linked to excessive hyperactivity in 33% of the subjects, leading to the conclusion that children consuming excess junk food at the age of seven are more likely to be in the top third of the hyperactivity scale. There was no significant correlation between junk food and the other scales.