Babies who were breastfed at 1 and 6 months had specific gut microbiome compositions, which the researchers say may affect immune system development.
The research team, including Dr. Christine Cole Johnson, chair of the Department of Public Health Sciences at Henry Ford Hospital in Detroit, MI, says the findings further support the "hygiene hypothesis" - the idea that early childhood exposure to pathogens affects later-life risk of disease.
"For years now, we've always thought that a sterile environment was not good for babies. Our research shows why. Exposure to these micro-organisms, or bacteria, in the first few months after birth actually help stimulate the immune system," says Dr. Johnson.
"The immune system is designed to be exposed to bacteria on a grand scale," she adds. "If you minimize those exposures, the immune system won't develop optimally."
Other studies have supported this claim. In June 2014, for example, Medical News Today reported on a study published in the journal Allergy and Clinical Immunology, in which researchers found exposing babies to bacteria and allergens in the first year of life may reduce the risk of allergies, wheezing and asthma later in life.
Breastfed babies 'at lower risk of pet-related allergies'
In this latest research - consisting of six studies - Dr. Johnson and colleagues set out to determine whether maternal or birth factors, as well as breastfeeding, affect the composition of gut bacteria - or the gut microbiome - in infants, and whether these compositions influence their risk of developing allergies or asthma.
In addition, the team assessed whether specific compositions of gut bacteria influenced the development of regulatory T cells (Treg) - white blood cells that regulate the immune system.
To reach their findings, the researchers analyzed data from the Wayne County Health, Environment, Allergy and Asthma Longitudinal Study (WHEALS), which investigates how environmental and biological factors influence the development of allergies and asthma in early life.
The researchers analyzed stool samples collected from babies at 1 and 6 months following birth.
The results of their analysis revealed that a mother's race/ethnicity, an infant's gestational age at birth, prenatal and postnatal tobacco smoke exposure, the presence of pets in the home and whether a baby was born via Cesarean section or vaginal delivery influenced an infant's gut microbiome composition.
They also found that babies who were breastfed at 1 and 6 months had specific gut microbiome compositions, compared with babies who were not breastfed, which the researchers say may affect immune system development. In addition, babies who were breastfed at 1 month were at lower risk of pet-related allergies.
The researchers also identified a specific gut microbiome composition among children with asthma who experienced flare-ups or night-time coughing within the first year of life.
What is more, they found - for the first time - that an infant's gut microbiome composition was associated with levels of Treg cells.
Commenting on their findings, Dr. Johnson says:
"The research is telling us that exposure to a higher and more diverse burden of environmental bacteria and specific patterns of gut bacteria appear to boost the immune system's protection against allergies and asthma."
In May 2014, MNT reported on a Danish study in which researchers claim longer breastfeeding duration is associated with higher levels of "friendly" gut bacteria in infants.