1. Mn and Cu intake and retention in twenty full-term infants and six preterm infants were studied on the basis of 72 h balances. The age of the infants was 2–16 weeks and the gestational age of the preterm infants (triplets) 34 and 36 weeks. Three nutrition schemes were pursued: breast-fed, formula-fed with unsupplemented adapted formula and formula-fed with trace element supplementation.
2. The mean Mn concentration of all breast-milk samples (n 2339) was 6·2 μg/1. The two formulas had similar Mn concentrations (77 and 99 μg/1) but had different Fe, Cu (121 and 619 μg/1), Zn and I contents. The mean Cu concentration in mother's milk was 833 μg/1.
3. The following mean daily Mn intakes and retentions (μg/kg) respectively were measured: breast-fed fullterm 1·06 (sd 0·43) and 0·43 (sd 0·65), formula-fed full-term 14·2 (sd 3·1) and 2·8 (sd 4·8), formula-fed preterm 15·0 (sd 2·2) and 0·06 (sd 5·87). The results for Cu were 114·5 (sd 22·3) and 88·0 (sd 46·5) μg/kg in breast-fed, 19–8 (sd 4·2) and 4·6 (-11·5–9·6) in the unsupplemented formula-fed and 106·4 (sd 18·9) and 55·5 (sd 20·3) in the supplemented formula full-term infant group. No significant influence of the trace element contents of the formulas on the relative retention of Mn or Cu was found.
4. Young preterm infants, and to some degree young full-term infants, often had negative Mn balances caused by a high faccal excretion. The formulas with a Mn concentration below 100 μg/l gave a sufficient supply of Mn. Preterm infants fed on the unsupplemented formula had a marginal Cu supply and their first balances were negative (-3·8 (sd 1·8) μg/kg).
5. In accordance with the estimated safe and adequate daily dietary intakes (recommended dietary allowances), formula-fed infants receive much more Mn than breast-fed infants and their absolute retention is higher.
6. Cu from breast-milk had a significantly better biological availability than that from cow's milk formula. If retentions similar to those in breast-fed infants are intended, we conclude, therefore, that cow's milk formula should be fortified with Cu up to a level of at least 600 μg/l.