Growth and Body Composition in Breastfed Infants - Study on Age of Introduction of Complementary Foods in Iceland

At approximately 5 months of age, all infants in Iceland are invited to receive routine care (health checkup, immunization etc.) at the well-baby clinics at the primary health care centers in their neighbourhood. Nurses at well-baby clinics in and around Reykjavik cooperating with the study investigators screen all infants for eligibility according the eligibility criteria. Mothers of eligible infants receive oral and written information about the study and if interested, a written consent is obtained and study investigators are informed about the potential participant.

When the infant is around 5.5 months of age, the mother is contacted and a home visit by study investigators scheduled. In the home visit, more detailed information about the study is given and equipment for predose urine samples are given. The date for the first part of the study (day 0) is scheduled.

Day -1 and/or day 0: Mothers sample predose urine sample(s): 2x4 ml for group "Exclusive Breastfeeding A" and 1x2 ml for groups "Exclusive Breastfeeding B" and "Partial Breastfeeding".

Day 0 (home visit by investigators): Infant weighed, drinks 2.5 g/kg body weight + 1 g doubly labeled water or 0.7 g deuterium.

Mothers sample postdose urine samples: 6x4 ml for group "Exclusive Breastfeeding A" (days 1, 2, 3, 5, 6, 7) and 2x2 ml for the other groups (days 0+5 hours and day 2).

Mothers of infants in groups "Exclusive Breastfeeding B" and "Partial Breastfeeding" keep a 3-day weighed food record assessing their infants' diet including breast milk (by test weighing).

Day 7 (home visit by investigators): 2x10 ml breast milk samples (fore and hind milk) are obtained, questionnaires answered by mothers: background questionnaire, food frequency questionnaire on mother's habitual diet, the Baby Eating Behaviour Questionnaire (BEBQ) and the Infant Behaviour Questionnaire-Revised (IBQ-R).

At 9 months of age mothers keep a 3-day weighed food record assessing the infants' diet including breast milk if the infant is still breastfed (by test weighing).

At 12 months of age the primary outcome measure is assessed. Day -1 or day 0: Mothers sample predose urine sample: 1x2 ml. Day 0 (home visit by investigators): Infant weighed, drinks 1.2 g deuterium. Mothers sample postdose urine samples: 2x2 ml (days 0+5 hours and day 2). Mothers keep a 3-day weighed food record assessing their infants' diet including breast milk if the infant is still breastfed (by test weighing).

Mothers answer the Child Eating Behaviour Questionnaire (CEBQ).

Inclusion criteria

• Icelandic mother
• Singleton birth
• Gestational age 37-42 weeks
• Birth weight >2500 g
• Living in Iceland until 12 months of age
• Only groups "Exclusive Breastfeeding A" and "Exclusive Breastfeeding B": Exclusively breastfed at 5.5 months of age (≤5 formula feedings and ≤5 water feedings since birth allowed)
• Only group "Partial Breastfeeding": Receiving at least 100 g / 100 ml of complementary foods (formula or solids) at 5.5 months of age

Exclusion criteria

• Diseases or defects likely to affect growth or body composition
• Not breastfed at 5.5 months of age