The co-primary objectives of this study are: 1) to investigate the effect adding a synbiotic (combination of a pre-and probiotic) to iron-fortified follow-up formula (FUF) on iron bioavailability, and 2) to investigate the effect adding Human Milk Oligosaccharides (HMOs) to iron-fortified FUF on iron bioavailability in 10-14 month-old Thai children.

Iron stores of healthy, full-term neonates are supposed to cover iron needs for the first 4-6 months of life. After 6 months of age, requirements for absorbed iron are high, as infants need to build body iron stores and triple their body weight before the first year of life. Iron concentrations in mature breastmilk are low (~0.3 mg/L) and cannot adequately cover the high iron requirements of children between 6 and 24 months of age. Follow-up formulas (FUF) are fortified milk-based products intended for infants above 6 months of age and young children aged 1-3 years, and are widely used as a liquid part of the weaning diet to supplement the diet of young children with macro- and micronutrients. Prebiotics and probiotics are increasingly added to foods for infants and young children due to emerging evidence on possible health benefits. We recently showed that the addition of prebiotic galacto-oligosaccharides (GOS) to an iron-containing micronutrient powder increased iron absorption in Kenyan infants. To our knowledge, the effect of the combination of pre- and probiotics (synbiotic) or human milk oligosaccharides (HMOs) on iron absorption from an iron-fortified FUF has not been investigated.

Thai children (10-14 months old) will be assigned to receive FUF fortified with isotopically labelled iron as FeSO4 1) with synbiotic, 2) HMO, and 3) without added prebiotics as a reference, in random order. Besides the iron source and synbiotic or HMO, the test FUF will be identical in terms of macro- and micronutrient composition. For the secondary objectives of this study, which will have a single-group, before-after design, each infant will receive the test FUF with added synbiotic for 25 days on gut comfort and quality of life outcomes.

Inclusion Criteria:

• Written informed consent has been obtained from the parents/legally acceptable representative
• Child aged 40-56 weeks (± 3 weeks)
• Child exhibits no clinical signs/symptoms of chronic disease or acute illness
• Capillary Hb ≥70 g/L
• Anticipated residence in the area for the study duration
• Z-scores for weight-for-age and weight-for-length both >-3, if the infant is severely underweight or severely malnourished.
• Singleton, full-term gestation birth (≥37 weeks)
• Birth weight ≥2.5 kg and ≤4.5 kg

Exclusion Criteria:

• Child exhibits clinical signs/symptoms of chronic infectious, metabolic, genetic illness or other disease including any condition that impacts feeding or growth
• Child received vitamin and mineral supplements in the 2 weeks prior to enrollment
• Child is exclusively breastfed
• Child received antibiotic treatments in the 4 weeks prior to enrollment
• Parents or caretakers not willing/not able to comply with the requirements of the study protocol
• Child has allergy or intolerance to cows' milk protein or lactose, or severe food allergies
• Child is participating in any other interventional clinical trial that would interfere with study outcomes