Lutein and zeaxanthin: The possible contribution, mechanisms of action and implications of modern dietary intake for cognitive development in children.

Introduction

The dietary carotenoids lutein and zeaxanthin preferentially accumulate in neural tissue including the central region of the retina (macula) where they form macular pigment (MP). MP’s constituent carotenoids have been studied extensively for their role in eye health and in the prevention of retinal disease such as age-related macular degeneration (AMD) and, more recently, glaucoma^1,2. Within the eye, the protective role of MP appears to relate to its light-filtering, antioxidant, anti-inflammatory and neuro-protective properties^3,4. These carotenoids are also found in several specific regions in the brain including the frontal, occipital and temporal cortices, hippocampus and cerebellum from prenatal development into old age^5–8. Retinal MP can be measured non-invasively and used as a surrogate measure of brain lutein and zeaxanthin status⁵. The selective accumulation of these carotenoids in the brain suggests that they may have a specific role within the central nervous system.

So far the beneficial effects of lutein and zeaxanthin have been primarily studied in relation to older age, due to the antioxidant, anti-inflammatory and neuroprotective properties of these carotenoids and the fact that oxidative stress and inflammation have been implicated as pivotal components of age-related visual and cognitive decline^9–11. Indeed, higher levels of MP in the diet, serum, eye and in the brain later in life have been shown to be associated with better cognition^12–17. At the same time, these compounds are present in the eye and brain from an early age, suggesting that they have an important biological role throughout life^8,18,19. The possible importance of these carotenoids for cognitive development is indicated by the selective accumulation of lutein and zeaxanthin within the developing infant brain⁸. Lutein, in particular, is the predominant carotenoid in the brain tissue from infancy⁸. Despite constituting just 12% of overall infant carotenoid intake, lutein has been shown to account for 59% of total brain carotenoids⁸. Additionally, the relative contribution of lutein to total carotenoids is almost two-fold greater in the infant brain than in adults, accounting for 59% vs. 34%, respectively^8,20. Lutein and zeaxanthin’s concentrations in the eye are still 2-3 times higher than in the brain⁵. Nevertheless, such selective concentration of lutein, and the multitude of effects it has on neural cell viability, suggests that it may play a key role in the early processes of neural and hence cognitive development in children^8,20–23.

The aims of this paper are three-fold. Firstly, given that childhood is a critical period for brain development, this evidence synthesis aims to analyse the available evidence regarding the possible role of lutein and zeaxanthin for neural development and cognitive function in children. Additionally, the available evidence is explored to identify plausible mechanistic processes through which lutein and zeaxanthin might influence cognitive development. Lastly, given that the circulating levels of lutein and zeaxanthin available for uptake into neural tissue are dependent on their consumption in the diet, the evidence in relation to dietary intake of these phytonutrients in children is also appraised. Such an analysis may be useful as a means to inform dietary recommendations for the optimisation of cognitive development in children.

Methods

There is insufficient available evidence to support a full systematic review. An evidence synthesis was, therefore, conducted by performing a comprehensive literature search using the following major databases: PubMed, Scopus, the ISRCTN registry and Cochrane Library with the following search items: “lutein” or “zeaxanthin” or “macular pigment” or “carotenoids" AND “cognitive function” or “memory” or “attention” or “reaction time” or “brain” AND “children” or “childhood” or “infants” or “infancy” or “premature” or “prematurity” or “adolescent”.

Inclusion and exclusion criteria

Phenomenon of interest

The phenomenon of interest in this analysis was the relationship between dietary intake (including breast milk), serum and/or macular concentrations of lutein and zeaxanthin, and cognition in children.

The final search was conducted in May 2018. No language or date restrictions were used in the electronic searches for trials. This search resulted in a total of 543 potential eligible articles. Two investigators independently reviewed the titles, abstracts and full text of the resulting articles for inclusion. The reference list of each publication meeting the inclusion criteria was examined and ancestry searches were performed on relevant review articles to identify any further studies that were not found using the electronic search. The overall search resulted in the inclusion of six cross-sectional studies, five of which are specific to the effects of lutein or zeaxanthin, while one study reported findings for lutein in combination with the non-carotenoid compound, choline. Data extraction was conducted manually, using the following categories: author, year, methodology, results, conclusions. Qualitative evidence synthesis was then conducted, including assessment of methodological limitations of articles selected for inclusion. Given the qualitative nature of this analysis, no statistical methods were used. Figure 1 below summarises the studies retrieved.

Figure 1. Search strategy and results for clinical studies exploring the relationship between lutein and zeaxanthin and cognition in children.

In order to provide mechanistic explanations for any relationships observed between lutein and/or zeaxanthin and cognition in children, additional sources of evidence were explored in the part II of the evidence synthesis including systematic and non-systematic reviews, in-vitro studies, and studies involving adult participants.

Results

Lutein and zeaxanthin in the eye and brain

Anatomically and developmentally, the retina is a part of the central nervous system, and both retina and optic nerve originate as outgrowths of the developing brain. The retina and the brain share a number of similarities, including anatomy, functionality, immunology and response to insult³⁷. The retina contains a layer of specialised neurons, the retinal ganglion cells (RGCs), which exhibit properties typical of CNS neurons.

Relative concentrations

It is well established that lutein and zeaxanthin are present in the eye and in the brain. These two carotenoids have been found in ocular tissues from 18 weeks of pre-natal development, while in the brain they have also been found in preterm infants^8,18. The study in preterm infants showed that lutein was a predominant carotenoid in all 5 brain regions analysed (mean concentrations ranging from 40.7 ± 7 pmol/g in hippocampus to 55.52 ± 18.33 pmol/g in the auditory cortex). The concentrations of zeaxanthin ranged from 10.03 ± 2.46 pmol/g in prefrontal cortex to 17.8 ± 3.94 pmol/g in the auditory cortex, considerably lower than lutein.

Lutein was shown to be the predominant carotenoid in the brain of older adults too, although in older adults is accounted for 30% of total carotenoids, compared to 59% of total carotenoids in the infant brain^5,8. In older adults, the ratio of brain to ocular lutein (~ 0.4) was slightly higher than brain to retinal zeaxanthin (~ 0.27), but broadly comparable. The ratio in infants has not yet been studied. However, the studies directly relevant to this review focused on either MPOD or breast milk or serum lutein (and not zeaxanthin), so the subsequent analysis will focus on lutein as there is not currently enough evidence to define the possible role of zeaxanthin specifically.

Carotenoid role in brain tissue

In the infant brain, lutein and zeaxanthin have been identified in prefrontal, frontal, auditory and occipital cortices, and hippocampus^8,19. As described earlier, the infant brain appears to be exquisitely sensitive to lutein, capable of accumulating circulating dietary lutein so effectively that it comprises the dominant carotenoid in brain tissue despite limited intake⁸. That lutein concentration is so much higher in infant relative to adult brain suggests that the effects of lutein are increasingly important in early life. The presence of lutein and zeaxanthin in visual and auditory areas of the brain, as well as in areas associated with executive function and memory, is consistent with associations reported in older children, including possible influences on visual recognition memory in infants²⁷, relational memory³⁴, executive processes and academic performance^30,32.

Mechanisms by which lutein and zeaxanthin are believed to protect the retina, such as their antioxidant, anti-inflammatory and neuroprotective action, are likely to apply to the brain as well³⁸. Such neuroprotection is particularly important during infancy when the CNS is developing at a very fast pace. The membranes of the brain and retinal cells contain high quantities of the long-chain omega-3 PUFAs, including docosahexaenoic acid (DHA) - the main n-3 PUFA in the brain, which is essential for maturation of the CNS, but also is easily susceptible to oxidation³⁹. DHA is known to regulate immune function and microglia activation with its derivatives exerting anti-inflammatory and proresolving effects³⁹. Alterations of PUFA metabolism in the brain can be detrimental and lead to neuro-inflammatory events. Lutein, as a potent lipophilic antioxidant that is differentially localised in the membrane domains rich in PUFAs, is well positioned to protect these important lipids⁴⁰.

A strong relationship has been shown to exist between brain lutein concentration and StARD3, an integral membrane protein which is proposed to be involved in intracellular lipid trafficking. Interestingly, the relationship is strongest during infancy (r = 0.75, P < 0.001), is weaker among older adults (r = 0.51, P < 0.05) and becomes insignificant in centenarians (r = 0.08, P > 0.05), perhaps again suggesting a specific role for lutein in early neural development¹⁹. Lutein’s potential role in brain growth and maturation is further supported by findings that it’s content in brain tissue correlates with levels of brain amino acid neurotransmitters (gamma-aminobutyrate, aspartate), neurobiomarkers (activin A) and metabolites of energy pathways (1-octadecanol, phosphate and NADH)⁴¹.

Ocular influences

In early life, the retina is particularly vulnerable to oxidative stress and inflammation due to immature autoregulation of blood flow in the choroid, increased metabolic activity and increased exposure to short wavelength light due to a highly transparent crystalline lens⁷³. The rationale for the proposed role of lutein and zeaxanthin in visual system development is based on findings that these carotenoids are present in ocular tissue from 18 weeks of pre-natal development¹⁸. Additionally, supplementation studies of lutein in infants, particularly those with ROP, have shown possible benefits. Supplementation with lutein in the first hours of life can reduce neonatal oxidative stress⁷⁴, which is particularly important for new-borns whose antioxidant system has not yet developed. Oxidative stress and resulting inflammation are inextricably linked to the pathogenesis of common diseases of prematurity, such as ROP⁷⁵. Additional benefits have been shown with lutein supplementation in infants, including reduced inflammation⁷⁶, and even decreased ROP severity⁵⁰. An RCT in preterm, very low-birth-weight neonates showed that supplementation with lutein and zeaxanthin resulted in 50% decreased progression rate from early ROP stages to threshold ROP⁵⁰. MP’s antioxidant, anti-inflammatory and immuno-modulatory properties may, therefore, provide protection to the vulnerable developing retina and potentially brain^77–80, and thereby influence visual and possibly cognitive development⁸¹.

It is well established that preterm infants more often develop neurological problems than term infants, with the level of impairment ranging from subtle cognitive abnormalities to severe neurological handicap^82,83. Even low risk premature infants have lower scores for cognitive development at 3-4 years of age⁸⁴. Preterm children without measurable neurological damage perform worse on visual perception and visual motor integration tasks, as well as memory, sustained attention and picture vocabulary tests⁸⁴. Interestingly, MP levels have been shown to relate to such tasks in adults, including visual motor response⁸⁵ and visual-spatial functioning⁷². A number of factors could contribute to the development of cognitive deficits in preterm infants, including the same mechanisms of oxidative stress and inflammation involved in ROP pathogenesis. Preterm infants also have brain lutein and zeaxanthin levels that are lower than in the full-term infants⁸. Lutein and zeaxanthin may, therefore, be uniquely suited to protect the premature brain, as well as retina, from the sudden increase in oxygen levels experienced after birth, compared to in utero.

Vision’s contribution to cognitive development

Although normal vision is not a prerequisite for normal cognitive development, in sighted individuals, vision is one of the main drivers of development through observation and facilitating exploration of the environment. In sighted individuals, vision is generally the dominant sense^86,87. Indeed, in sighted individuals, vision-led functions such as distinguishing shapes are important for development of shape bias and associated language learning⁸⁸. Motion perception is key to categorisation of objects⁸⁹, which in turn, is linked to semantic memory. Additionally, imitation, which is a fundamental aspect of learning, is grounded in action and movement recognition^90,91. In later life, vision appears to support the maintenance of cognitive health, with loss of vision associated with a decline in memory⁹².

There is some indirect evidence from various sources to support the view that optimised vision (which may be influenced by MP levels) may be a refining factor for optimal neurocognitive development. Firstly, the cortical areas corresponding to the fovea are greatly magnified, with about half of the primary visual cortex devoted to processing information from the macula, reflecting the importance of this area for later information processing by the more anterior portions of the brain⁹³. Moreover, the development of sensory networks, and particularly visual ones, is prioritised over motor networks, suggesting the special role of vision for overall development⁹⁴.

According to the “effortfulness” hypothesis first proposed by Rabbitt in 1968, an individual’s cognitive processes can be affected by increased effort required when trying to identify degraded sensory input⁹⁵. It is possible, therefore, that degraded (or non-optimised) visual input may put increased demand on cognitive resources and thereby deprive other cognitive activities. It has also been demonstrated that even a modest reduction in image quality can have a marked effect on the speed of response on cognitive tests in both younger and older adults^96,97. In the eye, MP acts as a pre-receptoral optical filter that serves to optimise and refine the visual signal to be delivered to the brain⁹⁸. How much suboptimal vision and any associated “effortfulness” could affect an individual’s cognitive development long-term remains, however, unclear.

There is evidence that vision impairment can affect language and communication development in children, especially during the early stages⁹⁹. Moreover, perceptual processes later in life and normal development of the neural architecture appear to depend to a significant degree upon the early visual input received. The deprivation of patterned visual input due to congenital cataracts in new-borns has been shown to result, for example, in permanent deficits in configural face processing, a function mediated by both vision and cortical mechanisms, even after more than nine years’ recovery (cataracts were removed at 2–6 months of age)¹⁰⁰.

Whether the visual benefits of lutein and zeaxanthin outlined herein translate into cognitive benefits, however, remains entirely speculative at this time. The benefit of sensory stimulation enhancement, if any, is likely to be small relative to its broader effect inside the brain. There may be other, as yet uncharacterised, roles and mechanisms of action through which carotenoids can exert a positive influence in the developing retina and brain³⁸. An overview of the various mechanisms by which lutein and zeaxanthin might influence cognitive development are described in Figure 2.

Figure 2. Schematic of the proposed mechanistic processes through which lutein and zeaxanthin might influence cognitive development.

Gestational Transfer

The transfer of lutein and zeaxanthin across the placenta is believed to be dependent on their respective concentrations in mother’s plasma, indicating a passive diffusion process and emphasising the importance of mother’s diet during pregnancy and lactation¹⁰⁴. While lutein and zeaxanthin are detectable at 18 weeks gestation^18,105, the concentrations of lutein and zeaxanthin within the eye increase with later gestational stages. It appears that most lutein deposition occurs during the last trimester^8,106, which explains why lutein levels are much higher in neural tissue among term infants when compared to pre-term infants.

Lutein and zeaxanthin in human milk

Infancy is a critical period of rapid growth and maturation which places high demands on nutritional intake. Human milk is the sole source of nutrients for breast-fed new-born infants until the introduction of solid food, and it provides for all the dietary requirements during the first few months after birth, including lutein and zeaxanthin^103,107. In a nine-country survey conducted on breast milk carotenoid composition among 471 women, the overall mean ± SD for breast milk lutein plus zeaxanthin was 25 ± 19 µg/l, but individual country means varied three-fold, from a low of 15 ± 5 µg/l in the U.S. to a high of 44 ± 18 µg/l in Japan. The highest individual lutein concentration measured was 232 µg/l in China and the lowest was 3 µg/l in the U.K, a staggering 77-fold variation¹⁰². Interestingly, it appears that breastfeeding and its duration may have effects on cognitive development in children during their first three years of life¹⁰⁸. One study has shown that infants who were breastfed for ≥ 9 months had significantly better cognitive development than those who had not been breastfed, although this finding was not explored in relation to nutrient concentrations in breast versus formula milk¹⁰⁹.

Although formula milk is designed to contain all the major components of breast milk, breast milk remains the best source of nutrients for infants and contains many additional factors¹¹⁰. Lutein and zeaxanthin have been found in several formula products, with lutein concentrations ranging from 0.7 to 9.7 nmol/g fat and zeaxanthin ranging from 0.1 to 1.2 nmol/g fat in those containing the carotenoids, while none was found in other market-available formulations¹¹¹.

Human milk samples analysed in this study had comparable levels of lutein and zeaxanthin to the formula milk, with median concentrations of 4.79 nmol/g fat (range 0.42-9.98) and 0.55 nmol/g fat (0.00-1.70) in human milk samples, respectively. However, there is evidence to suggest that lutein in formula milk has lower bioavailability than that in breast milk. One study compared lutein levels in three groups of healthy term formula-fed infants randomised to study formulas containing different concentrations of lutein (from 20 which was considered unfortified to 225 µg /l of lutein) and a breastfed reference group. It appears that the bioavailability difference is such that approximately four times more lutein would be needed for formula-fed infants to achieve similar serum concentrations to breastfed infants¹¹². It is plausible to suggest, therefore, that the relative lack of lutein and zeaxanthin in some formula milk, coupled with the reduced bioavailability of formula derived sources could disadvantage formula-fed infants with respect to carotenoid accumulation in macular and brain tissue.

Given individual and regional differences in breast milk carotenoid concentration, dietary fortification among expectant and lactating mothers merits consideration, and there is some evidence to suggest this can be effective. Supplementing lactating mothers with a formula containing two different concentrations of lutein, in combination with docosahexaenoic acid (DHA) and alpha-tocopherol for 6 weeks resulted in increased total lutein and zeaxanthin in plasma and breast milk compared to placebo¹¹³. The infants of mothers assigned to the low- and high-dose lutein supplement also had a significantly greater concentrations of total plasma lutein and zeaxanthin, which correlated significantly with their mother’s breast milk total lutein and zeaxanthin¹¹³.

Dietary intake

Green leafy vegetables are the major dietary source of lutein and zeaxanthin once a child progresses to solid food. The two carotenoids are found in many of the same foods and most dietary databases include them together¹¹⁴.

The 2015-2020 Dietary Guidelines for Americans recommend specific age-appropriate caloric and vegetable intake levels for children. For children aged 1-3 (caloric intake 1000K), for example, the recommended vegetable intake every week is seven cups of vegetables. This includes 0.5 cup per week of dark green vegetables- the richest sources of lutein and zeaxanthin in the diet¹¹⁵. According to the 2002 Feeding Infants and Toddlers Study (FITS), a national sample of US children 12–24 months old, mean vegetable intake in this age group was just 2.8 cups per week with no documented intake of dark green vegetables¹¹⁶. In another study, only 3.1% of non-Hispanic children consumed dark green vegetables between the ages of 6–11months increasing to 7.5% between the ages of 12–24 months¹¹⁷. This low dietary intake pattern would seem to present the risk of inadequate lutein and zeaxanthin consumption among children.

Some degree of variability has been observed in dietary intake levels among older children. Analyses of a single 24 h diet recall collected in the NHANES in 2003–2004 revealed that the mean (SD) estimated lutein intake among 4–8 year old children was just 311 µg/day (474), with intakes of 335 µg/day (730) for children 9–13 years of age and 432 µg/day (1062) for those 14–18 years of age¹¹⁸. For comparison, in another more recent study of 160 Canadian children (mean age 5.75 years), the mean (SD) lutein intake based on 24 h recall records was somewhat higher, at 1230 (2020) µg/day. This may suggest higher intakes of lutein among children in that study, or possibly increased awareness and availability and hence intake of lutein-rich foods over the decade since the 2003–2004 NHANES survey²⁸. However, the food sources accounting for the higher intake of lutein were not identified. Another study of 13-17 year old Canadian children using one 24-hour recall, reported a mean (SD) lutein intake of 1132 (1888) (5^th to 95^th percentile range 50-1306) µg/day for 84 males and 1331 (2090) (5^th to 95^th percentile range 92-1561) µg/day for 94 females¹¹⁹. Of note, the large standard deviation values indicate wide variation and positive skewing in all these studies. The limitations of all methods to quantify dietary intake of specific nutrients do need to be acknowledged. Most importantly, however, dietary intake analysis only provides intake estimates and does not account for the absorption and distribution of carotenoids to target tissues in the body.

Recommended daily/dietary allowances (RDA) are recommendations for the minimum amount of a nutrient that is needed for most individuals to stay healthy. Despite their potential benefits for vision and cognition, lutein and zeaxanthin are not currently considered as essential nutrients, and at present there is no RDA guide for either lutein or zeaxanthin. Intakes of approximately 6mg/day have been associated with a decreased risk of age-related macular degeneration, however, the average American adult consumes only 1-3 mg lutein/day and children appear to consume even less¹²⁰. For children, the importance of their consumption relates more to the possibilities for optimisation of visual and cognitive development. This is evidenced by the positive association between the quality of diet (particularly increased intake of fruit and vegetables, rich sources of lutein and zeaxanthin) and academic performance as demonstrated in 5^th graders¹²¹. Although no RDA exists, the evidence cited herein suggests that lutein and zeaxanthin intake levels during the important developmental stages of childhood appears to be highly variable and potentially deficient in many cases throughout all stages of child development.

Of note, nano-encapsulation of carotenoids should be considered as it may enhance the potential of carotenoids for brain function, by increasing stability and bioavailability. The cognitive-enhancing benefits of encapsulated lutein was shown in a study in mice whose performance in a recognition memory task was significantly increased after administration of encapsulated lutein compared to administration of free lutein¹²².

Obesity represents another potentially important consideration, with childhood obesity becoming increasingly common across the globe which raises concerns relating to changing diet patterns with processed, sugar-laden foods replacing fruit and vegetables. In addition to its detrimental metabolic effects, increased adiposity has been shown to be related to brain health and cognitive function¹²³. Although the consequences of childhood obesity for cognitive development are as yet unknown, it is likely that the effects of excessive body fat and metabolic misbalance on cognitive development are pronounced during childhood, when the brain is still developing and experiences a high degree of plasticity¹²⁴. Interestingly, adipose tissue is the major storage organ for carotenoids outside the CNS¹²⁵, and there appears to be an inverse relationship between MPOD and measures of obesity^126,127. It has been suggested that body fat acts as a reservoir for lutein and zeaxanthin¹²⁸, with excessive adiposity and its associated metabolic changes (such as an unfavourable ratio of low- to high-density lipoproteins) potentially contributing to impaired transport and delivery of lutein and zeaxanthin to the eye and the brain¹²⁹. The possible impact of the obesity epidemic on retinal and brain carotenoid levels and associated visual and cognitive function remain to be seen, and have yet to be explored empirically.

Conclusions

Nutrition during the pre- and early postnatal period of life may have a key role for the prevention of neurodegeneration later in life¹³⁰. It is possible that simple dietary modification or supplementation with a dietary antioxidant and anti-inflammatory agent such as lutein might exert a tangible impact on early neural development as well as on the delay of age-related cognitive decline^22,38,131. The beneficial role of lutein and zeaxanthin for the structural integrity and function of the ageing brain has been demonstrated across numerous studies¹¹. Much less is known about their role during infancy, childhood and pre-adolescence, the critical periods of brain development.

The findings of positive associations between MPOD and cognitive abilities in children provide some initial evidence for the role of lutein and zeaxanthin in cognitive development. Indeed, there are numerous potential mechanisms by which lutein and zeaxanthin may protect and enhance retinal and brain function, and thereby support cognitive development in children, including their antioxidant, anti-inflammatory and neuroprotective properties (see Figure 2). Although not yet supported by intervention trials, these findings highlight the importance of habitual intake of lutein and zeaxanthin in children.

However, the specific linkages between the intake of lutein and zeaxanthin, their biological effects and clinical outcomes in children need to be elucidated to provide the robust evidence base that is needed to enlighten any beneficial effects of lutein and zeaxanthin in early life. Although there is promising preliminary evidence for a positive association between lutein and cognitive performance in childhood, the cross-sectional nature of the few studies available and the lack of RCTs, represents an evidence gap that merits further investigation before any firm conclusions can be drawn. As there is currently no RDA for lutein or zeaxanthin, studies demonstrating its influence on cognition would contribute to the evidence base supporting consideration of lutein and zeaxanthin as important phytonutrients.

Data availability