It was the first time an association had been found between obesity and iron deficiency in children as young as 1, the researchers said, and they said junk food may be to blame.

“The reasons for the strong association in this age group are unclear and need to be elucidated,” Dr. Jane Brotanek of the University of Texas and other researchers cautioned in their study, published in the journal Pediatrics.

“Dietary practices may play an important role since diets high in calories but poor in micronutrients may lead to both iron deficiency and overweight” children, they added.

Iron deficiency anemia in infancy and early childhood can impair learning, hamper school achievement and lower scores on tests of mental and motor development, the researchers said.