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PEDIATRICS Vol. 110 No. 3 September 2002, pp. 545-552

Sex Differences in Iron Status During Infancy

Magnus Domellöf, MD, PhD^*, Bo Lönnerdal, PhD, Kathryn G. Dewey, PhD, Roberta J. Cohen, PhD, L. Landa Rivera, MD, Olle Hernell, MD, PhD^*

^* Department of Clinical Sciences, Pediatrics, Umeå University, Umeå, Sweden
Department of Nutrition, University of California, Davis, California
Medicina Infantil, San Pedro, Sula, Honduras

--> Background. It is commonly assumed that there is no difference in iron status between male and female infants, despite a lack of studies addressing this question.

Objective. To study sex differences in different measures of iron status in infants.

Methods. At 4 months of age, 263 term, breastfed infants (121 Swedish and 142 Honduran) were randomized to receive iron supplements or placebo until 9 months of age. Blood samples at 4, 6, and 9 months of age were analyzed for hemoglobin (Hb), mean cell volume (MCV), zinc protoporphyrin (ZPP), plasma ferritin, and transferrin receptors (TfR).

Results. At 4, 6, and 9 months, boys had significantly lower Hb, MCV, and ferritin and higher ZPP and TfR than girls. At 9 months, boys had a 10-fold higher risk of being classified as having iron deficiency anemia. The differences at 9 months in MCV (71.6 vs 75.1 fL) and ZPP (59 vs 49 µmol/mol heme) remained significant after controlling for iron supplementation, site, growth variables, and other possible confounders. For ferritin, there was a remaining sex difference at 9 months among Swedish (29 vs 53 µg/L) but not Honduran infants. For Hb and TfR, sex differences at 9 months were larger in unsupplemented infants, especially in those with a birth weight of <3500 g.

Conclusions. There are substantial sex differences in Hb and other indicators of iron status during infancy. Some of these may be genetically determined, whereas others seem to reflect an increased incidence of true iron deficiency in boys.

Key Words: sex differences • gender • hemoglobin • MCV • ferritin • transferrin receptors • zinc protoporphyrin • breastfeeding • human infant • iron deficiency anemia

Abbreviations: Hb, hemoglobin • MCV, mean cell volume • ZPP, zinc protoporphyrin • TfR, transferrin receptors

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Received for publication Aug 6, 2001; Accepted Mar 18, 2001.

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