Breastfeeding is directly related to enhanced cognitive development (Anderson et al. 1999). A meta-analysis related exclusive breastfeeding to better intelligence tests results (about 3 IQ points) than being never or shortly breastfed (Horta et al. 2015). We know from observational studies that each additional month of breastfeeding increased language skills at 3 years by 0.21 points in the language skill test (Peabody Picture vocabulary test). Intelligence at 7 years of age increased by 0.35 points per month breastfeeding (Kaufman Brief Intelligence Test) (Belfort et al. 2013). These advantages over formula feeding have beenattributed in part to human milk oligosaccharides (HMO). All three HMO categories – 1 ) sialylated HMO, 2) fucosylated HMO, and 3) non-fucosylated neutral HMO – could potentially contribute to this effect (Wang 2012; Cerdó et al. 2019).

Brain development refers to brain growth including the formation, differentiation, maturation, and connection of brain cells (Kolb und Gibb 2011). Cognitive development refers to the development of cognitive abilities. These describe mental abilities including language skills, learning, thinking, memory, problem solving, and attention. The brain is the neural base for cognitive abilities and therefore cognitive development proceeds coherently with brain development. Consequently, adequate brain development is the foundation for appropriate cognitive development (Casey et al. 2000).

A debate is ongoing about the approach and tools to be used for accurate assessment of cognitive development in neonates and children below three years of age. One of the best known tools is the Bayley Scale of Infant and Toddler Development III (BSITD III). Although this tool was designed to capture delayed cognitive development ("impaired versus normal") – predominantly in preterm infants – it is often used in clinical practice and research to assess "normal versus better" cognitive performance presuming a sensitivity that this tool does not possess (Brito et al. 2019). Other tests may only reflect single cognitive aspects (Hughes und Bryan 2003) and therefore, other approaches and tools for the assessment of the various domains of cognitive function and their combination have been suggested for more accurate assessment of cognitive development than BSITD III alone (Brito et al. 2019).

Evidence about the involvement of HMO in brain and cognitive development is sparse. Data of ongoing clinical trials have not yet been published and data from animal models (Oliveros et al. 2018; Oliveros et al. 2016; Vázquez et al. 2015) or in vitro studies prevail (Hauser et al. 2020d; Hauser et al. 2020b; Hauser et al. 2020a). Although human data are pending, animal and in vitro data suggest that:

1) sialylated or acidic HMO seem to have direct effects on brain development. They provide sialic acid as substrates for brain cells (Wang 2009, 2012). Infants have high needs of sialic acid, which is abundant in neuron structures (e.g. gangliosides) making it a possible essential nutrient (Wang 2009).

2)  all HMO categories may have indirect effects on cognitive development. Sialylated HMO, fucosylated HMO, and neutral non-fucosylated HMO may be prebiotics and are likely to serve as substrate for gut microbiota. Thereby, they could influence the microbiota-gut-brain axis (Carlson et al. 2018). The microbial HMO fermentation products are seen as beneficial substances for the brain (Yu et al. 2013; Wall et al. 2014).

The microbiota-gut-brain-axis is essential in understanding how HMO may affect brain and cognitive development. It describes a bidirectional communication between the central nervous system (brain) and the gut. The microbiota composition affects this communication and different mechanistic pathways have been hypothesised. Gut bacteria produce neuroactive metabolites such as short-chain fatty acids (SCFA) or neurotransmitters, which are absorbed and may cross the blood brain barrier. A second mechanism of the gut-microbiota-brain axis is the interaction of microbiota species with cells of the enteric and autonomic nervous system and thereby affecting signalling to the brain. Other and diverse mechanisms have been suggested and are discussed elsewhere (Cryan et al. 2019).

First insights relate the specific HMO isoforms 3'‑sialyllactose (3'‑SL) and 2'‑fucosyllactose (2'‑FL) to improved cognitive development in humans. Here it is noteworthy that despite data paucity and drawbacks within some studies strong effects were attributed to individual HMO isoforms in some publications. Although results from observational and other studies on the effect of HMO isoforms are in line with animal and in vitro research, data should be considered as "first insights only".

In their cohort, investigators performed Bayley-III scale of infant development tests (BSITD III) with 50 breastfed infants aged 24 months. In parallel, they analysed HMO concentrations in their cohort's milk samples at one and six months postpartum. The BSITD III scores were statistically modelled for associations with HMO concentrations. With this approach, 2'‑FL concentrations in human milk at one month postpartum correlated with better BSITD III scores later. From this, authors concluded that 2'‑FL positively influenced cognitive development during the first month of life. The 2'‑FL concentrations in six months milk did not correlate with cognitive outcomes. From this, the investigators concluded that 2'‑FL had no effect on cognition at six months. The authors suggestedthat the first month of life was a sensitive period for positive effects of 2'‑FL on cognitive development (Berger et al. 2020). The effect of other HMO isoforms in the milk samples was not further considered.

Cho and colleagues quantified HMO in milk samples of healthy breastfeeding mothers from two cohort studies. Additionally, they assessed the cognitive development of n=99 infants, aged between three and 24 months via Mullen Scales of Early Learning score. The investigators related the test scores to HMO concentrations in the corresponding milk samples. They reported a positive association between better cognitive score results and high 3'‑SL concentrations in these cohort's milk samples (Cho et al. 2020).

Different HMO isoforms are allowed to be added to infant and follow-on formulas  in the European Union: 2'‑FL and lacto‑N‑neotetraose (LNnT), difucosyllactose (DiFL) and lacto-N-tetraose (LNT) are allowed as ingredients for infant and follow-on formulas, respectively. The permission for 3'‑SL and 6'‑sialyllactose (6'‑SL) are pending (EFSA NDA 2020b, 2020a). Some researchers examined potential effects of HMO blends rather than those of isolated isoforms (Hauser et al. 2020a; Hauser et al. 2020d). This approach reflects that human milk contains a cluster of 150-200 HMO isoforms (Bode 2012). Since 2020, two clinical trials are ongoing with infant formulas containing blends of five HMO (2'‑FL, DiFL, LNT, 3’‑SL, 6'‑SL) that examine effects on cognitive development and hope to shed light into the topic in a few years (Clinicaltrials.gov 2020a, 2020b).

Animal studies support the potential role of 2'‑FL and sialylated HMO for cognitive development. Mice receiving milk enriched with 3'‑SL and 6’‑SL showed better memory and attention behaviour than mice fed with milk not enriched with 3'‑SL and 6’‑SL (Hauser et al. 2020c). Feeding rats with 6'‑SL enhanced cognitive abilities compared to rats fed sialic acid free foods (Oliveros et al. 2018). Likewise, feeding 2'‑FL to rodents improved their learning, memory, and attention behaviour (Oliveros et al. 2016; Vázquez et al. 2015). In minipigs, the blend of 2'-FL, DiFL, LNnT, and LNT resulted in improved cognitive outcomes. Blends of 3'‑SL and 6'‑SL and a mixture of 2'-FL, 3'-SL, 6'-SL similarly improved cognitive results in this animal model (Hauser et al. 2020a).

In conclusion, HMO intake seems to enhance cognitive development in animals and possibly in humans. Results from observational cohorts and preclinical studies sound promising but require confirmation in more studies and clinical trials.