Autism spectrum disorder (ASD) refers to a range of conditions characterized by challenges with social skills, repetitive behaviors, speech and nonverbal communication. ASD begins early in childhood and impacts the individual throughout their lifetime. While the causes of ASD are not fully understood, research suggests that both genetic and environmental factors play a role. One area of interest has been whether breastfeeding affects a child’s risk of developing autism. Here we review the evidence on links between breastfeeding and autism.
Key takeaways on breastfeeding and autism
– Some studies have found an association between breastfeeding and reduced risk of autism, while others have not found a significant link. Overall the evidence is mixed at this point.
– It’s theorized that components in breast milk like fatty acids, immune factors and probiotics may help shape the developing brain and microbiome in ways that lower autism risk. However, more research is needed.
– If there is a protective effect of breastfeeding, it is likely small. Autism has complex origins and many other genetic and environmental influences. Breastfeeding alone should not be viewed as a way to prevent autism.
– Breastfeeding has many other health benefits for both mother and baby, so it is recommended by health organizations regardless of autism considerations.
Overview of autism spectrum disorder
Autism spectrum disorder encompasses a group of developmental conditions characterized by difficulties in social communication and interaction, as well as restricted interests and repetitive behaviors. Symptoms of ASD begin to appear during early childhood, typically before age 3.
Some common signs of ASD in young children include:
– Delayed speech and language development
– Lack of eye contact
– Difficulty relating to others or participating in reciprocal conversation
– Excessive focus on certain objects or topics
– Repetitive behaviors like flapping hands or rocking
– Adverse reaction to routine changes
– Unusual sensitivity to sensory stimuli
ASD is considered a spectrum disorder because it affects individuals differently and to varying degrees. There is no “typical” case of autism. Symptoms can range from mild to severe and may be accompanied by language delays, intellectual disability or other health issues. Some people with ASD, particularly those with high-functioning autism or Asperger’s syndrome, have strong cognitive abilities.
Prevalence of autism
Around 1 in 44 children in the United States are diagnosed with ASD, according to estimates from the Centers for Disease Control and Prevention (CDC). Rates of autism diagnosis have risen dramatically since the 1990s, but it is unclear how much of this reflects increased awareness and screening compared to an actual increase in incidence. Studies show autism is over 4 times more common in boys than girls.
Causes and risk factors
The causes of ASD are complex and not fully understood. Current research suggests autism likely develops from a combination of genetic and environmental factors that affect early brain development.
Known genetic factors behind ASD include inherited mutations, structural changes in DNA and differing numbers of DNA segments. Environmental risk factors that have been linked to autism include:
– Advanced parental age
– Prenatal viral infections
– Air pollution exposure
– Certain prescription drugs taken during pregnancy
– Complications during birth
However, autism cannot be explained solely by genetics and environment. In most cases, it is still unclear exactly how these influences might interact to shape an individual’s neurodevelopment.
Breastfeeding overview
Breastfeeding provides infants with complete nutrition specially adapted to their needs. The major components of breastmilk are water, carbohydrates, proteins, and fats. It also contains numerous other bioactive compounds, including antibodies, enzymes, hormones, growth factors and beneficial bacteria.
The nutrients in breastmilk help support optimal growth and development during the infant and toddler stages. The non-nutritive components provide immune protection and shape the gut microbiome. For example, antibodies called immunoglobulins passively protect babies against infections until their own immune systems mature.
The World Health Organization recommends exclusive breastfeeding up to 6 months of age, with continued partial breastfeeding alongside solid foods through at least 1 to 2 years. However, rates and duration of breastfeeding currently fall short of these guidelines in many countries including the United States.
Health benefits of breastfeeding
Extensive research shows breastfeeding provides short-term and long-term health advantages for both mother and child.
Benefits for the infant include:
– Boosted immune system
– Protection against infections
– Reduced risk of asthma, allergies, obesity and sudden infant death syndrome (SIDS)
– Potential increased intelligence
For mothers, breastfeeding is associated with:
– Faster postpartum weight loss
– Reduced risks of breast and ovarian cancers
– Decreased risk of postpartum depression
– Delayed return to fertility after giving birth
These well-established general benefits have motivated studies probing whether breastfeeding could also reduce risk for specific conditions like autism.
Breastmilk composition and relevance to autism
Breastmilk delivers a complex mix of nutrients, antibodies and beneficial microbes that act in synergy to promote healthy development. Researchers have looked at how its unique composition could theoretically help lower odds of disorders like autism.
Some components of interest include:
Long-chain polyunsaturated fatty acids (LCPUFAs)
Docosahexaenoic acid (DHA) and arachidonic acid (ARA) are omega-3 and omega-6 LCPUFAs crucial for brain growth and neural functioning. They accumulate in the brain and retina during late pregnancy and the first years of life. Premature infants and those unable to be breastfed have reduced DHA and ARA levels.
Several studies have found lower levels of these fatty acids in individuals with autism compared to controls. While not proving cause-and-effect, this suggests neural deficits in ASD could perhaps relate to LCPUFA deficiencies during key developmental windows. Human milk naturally contains these essential fatty acids.
Oxytocin
Breastfeeding triggers release of the hormone oxytocin in both mother and infant. Oxytocin promotes social bonding and trust. Some children with autism have difficulties detecting social cues, so oxytocin could potentially help shape socio-emotional brain circuits early in life.
However, direct proof for long-term behavioral effects of breastfeeding-associated oxytocin is currently lacking.
Probiotics
Breastmilk transmits a distinctive mix of bacteria to populate the infant gut. This “seeding” helps train the developing immune system and fine-tune metabolic pathways. Imbalances in the gut microbiome have been implicated in ASD. Probiotics in breastmilk could potentially help steer the microbiome down a trajectory that lowers risk. But concrete evidence tying together these mechanisms is still minimal.
Immune factors
Breastmilk provides immunoglobulins, cytokines and cells that defend against infection while training the infant immune system. Immune challenges during sensitive windows could theoretically disrupt early brain development. So the immune protection conferred by breastfeeding represents another plausible route for reducing neurodevelopmental disorder risks.
While these breastmilk components show biological promise for affecting autism risk, research has not yet firmly established cause-and-effect relationships. Most ideas remain speculative at this point. But they provide foundations for ongoing human studies.
Review of studies on breastfeeding and autism
To evaluate whether the theoretical benefits above translate into real-world impacts, we can examine what epidemiology studies have found regarding links between breastfeeding patterns and autism prevalence.
Studies finding reduced risk
Some cohort studies have reported an association between increased breastfeeding duration and decreased odds of a child developing autism:
– A 6-year study following over 1000 children in Boston found each additional month of breastfeeding was linked with a 7% reduction in autism risk.
– In a Danish cohort of over 140,000 children, those breastfed for less than a month had a higher autism hazard ratio compared to those breastfed for at least 6 months.
– A 2013 meta-analysis compiled data from 8 studies, finding a 15% lower autism risk in children who were ever breastfed compared to those never breastfed.
– A more recent 2019 meta-analysis combined 18 studies and found breastfeeding for 6-12 months was associated with a 17% reduction in odds of autism versus shorter breastfeeding periods.
Null studies
On the other hand, some analyses have not found a statistically meaningful association between breastfeeding duration and ASD:
– A 2018 Spanish study of 2700 children saw no link between breastfeeding for under 6 months versus 6 months or more and autism risk.
– In a 2020 study following over 8000 children in the Netherlands, the authors found no significant association between duration of breastfeeding and autism odds after adjusting for confounders.
– A 2022 study utilizing data from nearly 400,000 children born in Sweden reported no correlation between increased length of breastfeeding and risk of childhood autism diagnosis.
Potential confounding factors
One challenge with observational studies is accounting for variables that could influence both breastfeeding behavior and autism odds. For example:
– Older, more educated and higher income mothers tend to breastfeed longer but also differ in other ways that could impact child development outcomes.
– Mothers perceive signs of autism risk and may consequently breastfeed siblings for longer periods.
– Underlying infant health issues could cut breastfeeding short but also relate to neurodevelopment.
Well-designed studies try to statistically control demographic, socioeconomic, perinatal and other factors. But residual confounding is difficult to fully exclude in this type of research.
Key limitations of current research
While some studies suggest breastfeeding could offer modest protection against autism, there are good reasons the evidence remains inconclusive:
– Small individual study sample sizes with limited statistical power
– Variability in study design and adjusted risk models
– Challenges controlling for confounders that could influence both breastfeeding and autism odds
– Lack of randomized controlled trial data
– Potential biases like retrospective recall of breastfeeding duration after an autism diagnosis is made
– Inability to pin down which specific components of breastmilk might drive any protective effects
Given these limitations, current research is not sufficiently robust to make definitive conclusions about the relationship between breastfeeding and autism risk. Larger, more rigorous longitudinal analyses controlling for confounders are needed.
Is breastfeeding duration important?
If breastfeeding does affect autism risk, could longer duration be key, or is any amount helpful?
Some patterns tentatively suggest a dose-response relationship where longer breastfeeding periods correlate with lower autism odds. In particular, breastfeeding for 6 months or more shows the strongest protective associations in meta-analyses.
However, findings are mixed, since some individual studies have found:
– Similar autism rates when comparing those who were never breastfed to those who breastfed for less than 6 months
– Lower autism prevalence with any breastfeeding versus none, but no further risk reduction beyond 3-6 months
Pinpointing an optimal duration is also complicated by potential two-way relationships. Mothers may instinctively breastfeed for shorter periods if an infant already seems to be exhibiting developmental delays or early autistic symptoms.
Overall current evidence does not confirm that specific breastfeeding lengths are required to impact autism risk. More research is warranted on potential dose-response effects.
Does breastfeeding exclusively confer benefits?
Many studies dichotomize breastfeeding as exclusive versus partial. Exclusive means breastmilk only, while partial includes mixed feeding with formula supplementation. This distinction is relevant because components like probiotics that theoretically influence brain development would be reduced by formula use.
However, research results remain mixed on whether exclusive breastfeeding confers extra protection against autism compared to partial:
– A previously mentioned Danish study found exclusive breastfeeding for 6 months or more was associated with the greatest autism risk reduction (27%), followed by partial breastfeeding (17%).
– But other analyses have observed similar lowered autism odds for any breastfeeding regardless of exclusivity.
– One research group found no duration or exclusivity effects, and speculated autistic infants might have inherent difficulties establishing exclusive breastfeeding.
Like with duration patterns, current evidence is too preliminary to determine whether breastfeeding exclusivity matters regarding impact on autism outcomes.
Does type of feeding matter more than breastmilk itself?
Another key question is whether any influence of breastfeeding on neurodevelopmental disorders relates intrinsically to bioactive components of breastmilk, or if it simply reflects differences in feeding method and maternal contact.
Formula-feeding and bottle use lack skin-to-skin contact and visual cues associated with nursing at the breast. Researchers have conjectured that subtleties like eye gaze synchronization during breastfeeding could support socio-emotional and language development relevant to autism risk.
However, studies explicitly comparing feeding method are rare. Existing analyses tend to focus on breastfeeding duration while treating formula supplementation as a confounder. Experiments able to disentangle the separate contributions of breastmilk properties versus mode of feeding are needed to address this open question.
Role of gut microbiome mediators
Another avenue for future research is drilling down into the potential mediating roles of gut bacteria transferred through breastmilk.
Differences in microbial composition between breastfed versus formula-fed infants are well established. Could gut dysbiosis be one bridge explaining why breastfeeding might alter neurodevelopmental outcomes?
Some preliminary evidence derives from animal models:
– In one study, mouse pups receiving gut microbes from mothers with an autism-like condition developed more autistic behaviors than pups colonized by bacteria from healthy mothers.
– Another mouse experiment found offspring receiving a probiotic bacterium via breastmilk had reduced autistic symptoms.
But direct human data linking together changes in the infant gut microbiome, breastfeeding, and autism risk is still sparse. Identifying specific microbes that might be protective could open doors for translation into microbiome-targeted therapies.
Role of epigenetic processes
Breastfeeding could hypothetically affect autism risk through epigenetic processes – chemical modifications that alter gene expression without changing the DNA code itself.
For example, infants receiving breastmilk have different patterns of DNA methylation compared to formula-fed peers. If these methylation differences influence genetic pathways important for neurodevelopment, this could be one mechanism mediating breastfeeding’s impact. However measuring epigenetic markers in human studies remains relatively new.
Future research in this area may help elucidate breastfeeding’s physiological effects on developmental trajectories in areas of the brain related to autism.
Breastfeeding recommendations
Based on the totality of evidence, health organizations unanimously recommend breastfeeding as the optimal early infant feeding method:
World Health Organization (WHO)
– Exclusive breastfeeding for 6 months
– Continued breastfeeding to 2 years or longer alongside solid foods
American Academy of Pediatrics (AAP)
– Exclusive breastfeeding for about 6 months
– Continued breastfeeding alongside solids for at least 1 year, and longer if desired
Centers for Disease Control and Prevention (CDC)
– Exclusive breastfeeding for about 6 months
– Continued breastfeeding with complementary foods through 1 year or longer
These recommendations are endorsed due to the wide-ranging nutritional, immunologic and developmental benefits of breastfeeding. While links to autism remain uncertain, the many proven general advantages still make breastfeeding the mode of infant feeding most aligned with public health goals.
Conclusion
Some but not all observational studies suggest breastfeeding may offer modest protection against development of autism spectrum disorder. However, current evidence overall remains inconclusive and inconsistent. Significant research gaps also exist regarding the mechanisms that could mediate any potential protective effects. While breastfeeding is clearly beneficial based on other infant health outcomes, mothers should not feel that choosing not to or being unable to breastfeed will directly impact their child’s autism risk. Moving forward, higher-quality longitudinal studies controlling for confounders, along with translational studies in model systems, will help clarify whetherbreastfeeding causally affects neurodevelopment relevant to autism.