Does iron in formula cause constipation?

When my son was born, he was given formula with the regular amount of iron at the hospital. After we brought him home, he became constipated. Our family practitioner told us to try a low iron formula. We did so and the constipation went away. My son is now eight weeks old and seems very healthy. Should I try to switch him back to the formula with full iron?

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Robert Steele

Robert W. Steele, MD, is a board certified pediatrician at St. John's Regional Health Center in Springfield, MO. He graduated from medical... Read more

Your questions are excellent ones because they bring into perspective some of the myths that are passed down not only from parent-to-parent but doctor-to-doctor as well. The origin of the link between iron fortified formula and constipation is difficult to find. Presumably it stems from the fact that adults who take iron supplementation in larger quantities can have changes in their stool color. Some of these adults who experienced these changes also felt the iron made them constipated. From there, the assumption was made that any amount of iron could cause constipation including any that might be found in formula. This is an understandable misunderstanding (if you will) since there are many infants who do develop constipation due to certain infant formulas. However, research has proven this assumed link of iron in formulas and constipation to be false.

The definitive study concerning this issue was published back in 1985 when researchers took almost 300 children and for three months gave half of them iron supplementation and the other half a placebo or fake medication (with parental permission of course). Parents were unaware which the child was taking. They asked the parents to keep a diary concerning vomiting, diarrhea, constipation, and fussiness. What they found was there was no difference in any of these things except constipation, and in this case the infants taking the placebo had more constipation than those taking the iron.

Babies who were born at full term have enough iron that was passed through the placenta during the final trimester to last four to six months. So, minimal iron supplementation is required during this time. The problem is when infants are placed on low iron formulas early in life, by the time six months rolls around most forget to switch back to regular iron fortified formula. Then the child is at risk for significant anemia over the next several months. Therefore, I tend to agree with the advice of not putting infants on low iron formulas. The extra iron during the first six months does no harm, it does not cause any additional gastrointestinal symptoms including constipation, and it insures the child will be on the correct formula at four to six months when this extra iron is essential. If you choose not to switch to a regular formula, your son should definitely receive an iron supplement by the time he is four months old.

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