Breast milk provides optimal nutrition to the developing infant, and that has prompted both the American Academy of Pediatrics and the World Health Organization to increase the recommendation for exclusive breastfeeding to age 6 months. As a part of the diet, the AAP recommends breast milk for at least 1 year, while the WHO advises continuing for 2 years or more. Breast milk provides nutrition with polyunsaturated fatty acids that have also been shown to help prevent allergies. In susceptible infants, the development of allergic symptoms was modified by the intake of n-3 long-chain polyunsaturated fatty acids through breast milk. Another study demonstrated the substantial reduction in risk of childhood asthma as assessed at age 6 years, if exclusive breastfeeding is continued for at least the first 4 months of life. Breast milk protects against the infant’s susceptibility to infections as well as against future development of allergic diseases, in part due to its fatty acid content. Breast milk not only provides nutrition that helps to balance immunity in infants, it also directly impacts the development of the Th1 response.
Evidence points toward the importance of breast milk in the maturation of the infant’s immune system, helping with immature Th1 function. During the education of the immune system in infancy, maternal milk provides signals to the immune system that generate appropriate response and memory. Although infants’ Th1 response is somewhat inefficient, breast milk compensates for this relative inefficiency by providing considerable amounts of secretory IgA antibodies and lactoferrin. These secretory IgA antibodies bind the microbes at the infant’s mucosal membranes, preventing activation of the pro-inflammatory defenses while lactoferrin both destroys microbes and reduces inflammatory responses. Breast milk also contains various cytokines, including IL-1, IL-6, IL-12, TNF-alpha, IFN-gamma, and IL-8, that help defend against gastrointestinal and respiratory infections. A recent study found high levels of immune-related miRNAs that were stable under acidic environments in breast milk for the first 6 months of lactation. The dietary intake of miRNAs by infants can have a profound impact on the development of the infant’s immune system. Breast milk clearly imparts important factors for the proper maturation of the infant’s immune system.
Breast milk also provides a significant amount of Th3 tolerance factors and anti-inflammatory compounds that help regulate immune responses and inflammation. Breast milk is rich in TGF-beta, IL-10, erythropoietin, and lactoferrin, which can help reduce the excessive inflammatory response to stimuli in the infant’s intestine. In research conducted with mice, the presence of TGF-beta and an allergen conveyed protection from allergic asthma. Some studies suggest that breast milk may even protect against type 1 diabetes, multiple sclerosis, and rheumatoid arthritis.