The researchers found that 20 percent of obese toddlers have iron deficiency, compared to 7 percent of normal-weight toddlers. Lack of iron reduces the amount of oxygen carried through the body by the blood and can cause anemia.
Experts blamed parents who let toddlers drink cow's milk and juice from a bottle, instead of weaning them and introducing iron-rich foods such as meat, beans, eggs, spinach and fortified breads.
Toddlers still fed from bottles tend to drink too much milk and juice, which are low in iron, and don't get enough solid food, said Dr. Jane Brotanek of the University of Texas Southwestern Medical Center in Dallas, a study co-author.
'What you put in your baby's bottle can affect your child's future,' she said.
The researchers also found that children who attend day care centers are about 50 percent less likely to have iron deficiency than children who aren't in day care. Day care providers may pay more attention to nutrition, Brotanek speculated.
The study appears in the September issue of the journal Pediatrics.
It is the first to discover a link between obesity and low iron levels in preschoolers. Iron deficiency can cause mental and behavioral delays, so the findings underscore the importance of healthy eating habits in children.
Dr. Geoffrey Allen, a specialist in blood diseases at Chicago's Children Memorial Hospital, said the findings are a reminder of the importance of childhood tests for iron deficiency.
'It's amazing how anemic children can be and still run around and play,' Allen said. 'It can be hard to detect mild and moderate anemia.'
The researchers analyzed data from 960 U.S. toddlers who had three blood tests for iron deficiency. The information came from 1999-2002 data in the National Health and Nutrition Examination Survey conducted by a branch of the U.S. Centers for Disease Control and Prevention.