Case study

Complementary feeding during the first six months of life. Yes or no?

baby eating

For years, children’s healthcare centres in the Netherlands advised young parents to feed their baby only breast milk for the first six months of life. Exclusive breastfeeding was thought to reduce the risk of infants developing food allergies (such as cow’s milk, egg and peanut allergies) and eczema. “A substudy of our KOALA study found that the reality is a bit more nuanced.” says Carel Thijs, associate professor in the Department of Epidemiology at Maastricht University. The guidelines have since been updated.

Carel Thijs is the initiator of and lead researcher on the KOALA study. “KOALA is a longitudinal cohort study of the health and development of approximately 2,800 children born between 2000-2003. We’ve been following these children, who are now young adults, from pregnancy. During pregnancy, during the first years of the children’s lives and during their teenage years, their parents have been completing questionnaires about their health, diseases and all kinds of relevant factors such as diet and lifestyle. The children, now 17 to 20 years old, recently completed a questionnaire themselves. We’ve also taken blood, breast milk and stool samples at various points in time. I initiated this study more than twenty years ago because I was interested in the development of allergies and asthma, partly in the context of the hygiene hypothesis. Simply put, the hypothesis suggests that our immune systems don’t get enough exposure to stimuli due to better hygiene. It doesn’t learn to distinguish between relatively harmless and more harmful stimuli (like microbes and food components), resulting in overreactions and allergies.”

Subproject: complementary feeding during the first six months of life

“The aim of the KOALA study is to gain a better understanding of what causes allergies, eczema and asthma”, explains Monique Mommers, a fellow researcher, biologist and epidemiologist who is an assistant professor in the Department of Epidemiology at Maastricht University. “It looks at factors such as exposure to microorganisms (vaccinations, gut microbiota, infections), diet, exercise, lifestyle and genetics. The cohort includes parents and children who follow the ‘standard way’ (including vaccinations etc.) as well as parents who, for example, hold a more anthroposophical worldview, who choose not to vaccinate their children and are more cautious about the use of antibiotics. The KOALA study has produced a wealth of data over the course of more than two decades. Subprojects developed over time, one of which was research into complementary feeding during the first six months of life.”

Koala cohort

KOALA study (Child, parents and health: lifestyle and genetic constitution)
Period: 2001-now
Project website: 
- English: KOALA Birth Cohort Study | KOALA study 
- Nederlands: KOALA | KOALA study 

Carel Thijs

Exclusive breastfeeding: yes or no?


“For a long time, parents were recommended to feed their baby only breast milk for the first six months of life”, says Thijs. “The idea was that hypersensitivity to certain types of food could be prevented by not exposing children to them until a later age. We wanted to find out if this was true. The data from the KOALA study provide insight into the health of children who – for a certain period of time – were exclusively breastfed; fed a combination of breast milk and formula; exclusively formula-fed; or fed combinations involving complementary feeding such as fruit and vegetable purees or porridge. We carefully analysed when, what and how much these infants had been fed. We expected to find that infants who received cow’s milk-based formula before the age of six months were more likely to develop a cow’s milk allergy. Or, depending on the complementary foods they were offered, a peanut allergy or a chicken egg allergy. But we actually found that the opposite was true.”

Findings confirmed

“Our study was among the first of its kind”, continues Thijs. “Its remarkable findings were further investigated and confirmed in various randomised controlled trials in England and Australia, among other countries. The Dutch guidelines have since been updated to reflect these findings; they now recommend that children start receiving complementary foods from the age of four months. The Netherlands Nutrition Centre provides guidelines for introducing complementary foods from four months onwards. This is particularly good news for working mothers, who often find it difficult to exclusively breastfeed for six months. Introducing complementary foods earlier makes breastfeeding more manageable, giving children more time to enjoy the benefits of breast milk for their immune systems.”

Solid foods and the gut microbiome

The findings have also inspired a follow-up study, says Mommers. “The LucKi Gut study is looking into the impact of introducing solid foods on gut microbiota development in young children. The children in question were born from 2016 onwards, so they’re growing up with the updated Netherlands Nutrition Centre guidelines. Our focus is on the development of gut microbiota and allergies, based on the concept of tolerance development as a counterpart to the previous notion of how allergies develop. The LucKi Gut study examines stool samples from babies and compares them to those of participants in a similar study in Canada. The results show that introducing a wider variety of foods leads to greater gut microbial diversity. The study has also revealed significant cultural differences with regard to first foods for babies. In Canada, for example, children are introduced to meat and eggs at a younger age. Further research is required to determine the effects of a more varied diet on the gut microbiome and allergy development.”

Text: Margo van Vlierden
Translation: Emdash

May, 2022

LucKi Gut study

LucKi Gut study
Project website:

Monique Mommers

Involved partners