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Background: Breast milk, with its complex, individual and over time adapting composition, is considered the optimal source of nutrition for infants during the first months of life. Two possible contributing factors to the benefits of breastfeeding compared to infant formula-feeding are the differences in iron and lactoferrin (Lf) concentrations between breast milk and infant formula. The overall purpose of the LIME (a Swedish acronym) study was to add knowledge on how to reduce the gap in health and development between breastfed and formula-fed infants. The aim of this double-blinded controlled trial, and doctoral thesis, was to investigate how added bovine lactoferrin and reduced iron concentration in infant formula affect health and development.

Methods: Recruitment took place from June 2014 to June 2018. With equal gender distribution, healthy term Swedish formula-fed infants (n=180) were randomly assigned, from 6 weeks to 6 months of age, to receive a low iron formula (2 mg/L) with bovine Lf (1.0 g/L) (Lf+, n=72), a low iron formula without Lf (Lf-, n=72) or a control standard formula with 8 mg/L iron and no Lf (CF, n=36). Additionally, 72 breastfed infants were recruited as a reference (BF) group. Blood samples were drawn at 4, 6, and 12 months. Primary outcomes were cytokine levels and iron status. Secondary outcomes were growth, gastrointestinal symptoms, infection-related morbidity and treatments, antibody response to vaccines and cognitive development.

Findings: All explored outcomes were unaffected by Lf fortification and the two low iron groups (Lf+ and Lf-) were combined and compared to the CF group. At 6 months of age the TGF-β2 levels were lower among the low iron groups and more similar to the BF infants. No other significant differences in cytokine levels were observed. There was a trend of lower geometric mean of ferritin at 4, 6, and 12 months for the combined low iron groups compared to the CF group (67.7 vs 88.7, 39.5 vs 50.9, and 20.5 vs 25.1 μg/L, respectively, p=0.054, p=0.056, and p=0.082). No similar trends or significant differences were found for any of the other iron status indicators, except for hepcidin at 12 months with lower levels in the low iron group compared to CF (37.8 vs 49.4 ng/mL, p=0.027). Overall, infants fed low iron formula had iron status indicators closer to the breastfed reference group and the prevalence of iron deficiency (ID) and iron deficiency anemia (IDA) was generally low with no significant differences among the intervention groups.

There were no clinically relevant effects of the interventions on growth, gastrointestinal symptoms, infection-related morbidity, vaccine antibody response or neurocognitive development.

In secondary analyses, the present study confirmed previous results of higher cognitive scores among breastfed infants compared to formula-fed and observed an unexpected lower IgG response to vaccines against Hib and Diphtheria.

Conclusion: Adding bovine lactoferrin did not affect any of the clinical outcomes explored. Lowering infant formula iron concentration from 8 to 2 mg/L minimally reduced iron stores to levels closer to breastfed infants but did not increase the risk of ID/IDA during the first year of life. Consequently, 2 mg/L is a sufficient level of iron fortification during the first six months of life in a population with low risk of ID. Both adjustments are considered safe with no observed adverse effects.