Introduction

Our daily life is becoming increasingly complex and unpredictable in which we face different challenges from avoiding crashes in morning traffic, over memorizing different information at work up to switching between different tasks at our late exercise training.

In order to meet these challenges, we rely on executive functions (EF) as inhibition, working memory, and cognitive flexibility (Diamond, 2013), which are responsible for planning, implementation, coordination, and monitoring of goal-directed behavior (Kiely, 2014). Previous empirical evidence indicates that executive functions seem to have an impact on peak performances in areas such as work (Pluck et al., 2020) and sports (Kalén et al., 2021; Scharfen & Memmert, 2019). In addition, improvements in executive function correlate with a higher quality of life such as general well-being, and performance or daily functioning at work, home, and school (Brown & Landgraf, 2010).

Importantly, executive functions have been found to decline with aging (Fjell et al., 2017) and decline earlier compared to abilities in other cognitive domains (De Luca et al., 2003). In general, the assumption that cognitive decline begins after midlife, as previously assumed, and mainly starting at higher ages as 55, 60, or 70 years (Ronnlund et al., 2005; Plassman et al., 1995; Aartsen et al., 2002) is not supported by evidence. For instance, in cross-sectional age comparisons, evidence suggests cognitive decline relatively early in adulthood and certainly before the age of 60 in healthy adults (Salthouse, 2009). Further, Ferguson and colleagues (2021) modeled curvilinear age relationships describing the development of EFs from adolescence through to older adulthood and found that inhibitory control declined in participants from around 35 years-old, working memory capacity declined in participants from around 30 years old and cognitive flexibility, measured as switch costs, declined across the lifespan. Nevertheless, Salthouse (2009) assumes an acceleration of age-related decline at older ages. This is in line with the inverted U-shaped function of visual search performance (Madden, 2007), which might be a predictor for the accelerated decline in executive functions reducing cognitive flexibility at an older age (Cepeda, et al., 2001). Such a continuous decline starting in early years does not support a binary separation between young- and middle-aged people. Instead, it argues for special consideration of the elderly due to the accelerated decline in executive functions over time. Explanations for the age-dependent decline of executive function performance are provided by several other theories (for an overview see Reuter-Lorenz et al., 2021). However, the consequences of such age-related decline in executive functioning remain the same, being associated with a reduced ability to meet our daily challenges with practical consequences such as a higher frequency of driving errors (Sakai et al., 2012) and lower completion rates of daily activities (Tomaszewski Farias et al., 2009).

A relevant framework for this scoping review is provided by Dresler and colleagues (2019) as it provides a good overview of cognitive enhancement strategies considering biochemical, physical, and behavioral interventions. Thus, the framework addresses how to intervene with age-related decline and potentially even sustain or improve executive functioning. One biochemical aspect that has gained attention over the past years is the micronutrient vitamin D. This is because both major circulating forms of vitamin D, namely 25(OH)D3 and its hormonally active form 1,25(OH)2D3 (Mayne & Burne, 2019), can cross the blood–brain barrier and seem to impact changes of the cerebral microstructure and macrostructure that relate to cognitive functioning (Rodrigues et al., 2020). One direction of evidence for the association of vitamin D and executive function is the presence of vitamin D receptors in neurons and glial cells in most regions of the brain including the prefrontal cortex (DeLuca et al., 2013), which is well understood in its role for executive function (Menon & D’Esposito, 2022; Yuan & Raz, 2014).

In the last decade, the importance of vitamin D for executive functioning has been either supported (Annweiler et al., 2013; Silva et al., 2022) or questioned (Beauchet et al., 2021) by several reviews. Taking a look at individual executive functions, the meta-analysis by Annweiler and colleagues (2013) presented four studies with a total of 802 participants supporting the association of higher serum 25OHD and better cognitive flexibility performance measured via the trail-making-task-B. In contrast, other studies could not support a significant association between serum 25OHD and inhibition scores (Dean et al., 2011; Jorde et al., 2006). Similarly, no effects have been found for supratherapeutic vitamin D levels on working memory capacity (Pettersen, 2016). These mixed findings of vitamin D on executive functions might be explained by the specific sample selection in experimental studies: previous reviews often included the elderly (Silva et al., 2022), or people with mental illnesses (Beauchet et al., 2021). Young- or middle-aged and healthy people have been so far neglected. One potential reason might be the already elevated level of executive functions within this age and healthy groups, which is related to peak performances in different areas of work such as shift work tolerance (Ritonja et al., 2019), sports (Longo et al., 2016; Ruiz & Merritt, 2021), or science (Jones et al., 2014).

Thus, it is important to define the age limits of this scoping review in order to better differentiate the findings from previous reviews on different populations. In general definitions of age group classification vary, but people above the age of 65 are, conventionally and by governmental organizations (Canada’s National Statistical Agency, 2023, October 15; National Institutes of Health, 2024, January 17) defined as older adults or seniors (but see Orimo et al., 2006). However, research on people below the age of 65 is missing, especially as these people termed as young- or middle-aged people seem to be responsible for the peak performances in different life areas. The fact that in Western societies people qualify for Medicare at the age of 65 (Economic Policy Committee, 2009; Monk & Munnell, 2009) supports the assumption that especially young- or middle-aged adults are responsible for peak performance. Similarly, athletes can reach peak performance when they are young or middle aged, as elite athletes participating in ultra-endurance competitions (Allen & Hopkins, 2015) or at the Olympic Games in endurance sports peak by their 40 s (Chomik & Jacinto, 2021), while athletes in tactical and precision sports can still compete at an elite level in their 50 s (Longo et al., 2016). Thus, the separation between older and young- or middle-aged populations can be supported by their different capabilities for peak performance. As argued above, the acceleration of age-related cognitive decline at older ages (Salthouse, 2009) may favor the focus on those age groups in earlier studies (Fjell et al., 2017). Given, the introduced U-shaped pattern of executive functioning and age (Cepeda et al., 2001; Madden, 2007) and different age-dependent capabilities for peak performance support also the differentiation between very young and young- or middle-aged populations.

Previous reviews have examined the relationship between Vitamin D and cognitive functioning rather in general and considering executive functions as a holistic construct with the limitation to consider a more detailed examination of executive functions and their components (Beauchet et al., 2021; Goodwill & Szoeke, 2017; van der Schaft et al., 2013). Furthermore, when executive functions were addressed (e.g., Annweiler et al., 2013; Silva et al., 2022), the populations studied were too old to draw conclusions applicable to young- and middle-aged individuals. We argue that young- and middle-aged samples might be of special interest, as they are on the one hand capable of peak performances at work (Pluck et al., 2020) and in sports (Kalén et al., 2021; Scharfen & Memmert, 2019), and on the other hand already affected by age-based cognitive decline (Ferguson et al., 2021). Thus, one might question if they benefit from vitamin D, due to its role for executive function (Menon & D’Esposito, 2022; Yuan & Raz, 2014).

Based on previous evidence in older populations and mechanism that indicate effects of Vitamin D on neuronal processes in the prefrontal areas, we assume that vitamin D enhances executive functions, in healthy young- or middle-aged adults. Closing this research gap requires a sufficient overview of the existing studies, empirical evidence for the presence of vitamin D and executive functions associations as the majority of studies and reviews focused on older or unhealthy populations and did not separate between different executive functions.

Objectives

We conducted the scoping review to systematically map the existing literature on the association between vitamin D levels and executive functions in healthy young- or middle-aged subjects. The current empirical evidence is suitable for a scoping review (Munn et al., 2018) to prepare this field in the future for systematic empirical investigations that can be systematized in meta-analyses. This scoping review aims to purposing new hypotheses pertaining to the effects of different vitamin D levels and vitamin D supplementation on the performance of executive functions in healthy young- or middle-aged subjects, as well as identify research gaps in this field. We were particularly interested in the association between vitamin D and those executive functions relevant for our daily life such as, inhibition, working memory, and cognitive flexibility (Diamond, 2013).

Materials and methods

This scoping review was guided by the following research question “Do higher levels of Vitamin D enhance executive functions in healthy young- or middle-aged participants?”.

Search strategy

We used the PICO approach (population, intervention, comparison, and outcome; Tricco et al., 2018) to develop our research question and the Preferred Reporting Items for Systematic Reviews and Meta-Analyses extension for Scoping Reviews was used as a guideline in this scoping review (Tricco et al., 2018). To foster transparency and openness, we provide the PICO description and the study protocol, including search terms and search strategy in the supplementary material (https://osf.io/q6yxm/?view_only=3a34987490ed445f98a1873b3b83c679).Footnote 1

Based on our study protocol, a systematic literature search was conducted in May 2024 in three electronic databases: Web of Science, PubMed, and PsycINFO. These databases were searched in title or abstract for different terms of vitamin D in combination with executive functions. Furthermore, the reference lists of all the included articles and previous reviews and meta-analysis were screened for eligible articles. We focused especially on those executive functions that are of interest for our daily life as inhibition, working memory, and cognitive flexibility (e.g., Diamond, 2013; Lehto et al., 2003; Logan et al., 2023).

Eligible criteria and assessment

The search results were imported in the reference management software Citavi 7 (Swiss Academy Software, Switzerland) and duplicates were deleted.

All articles were reviewed on titles and abstracts by two authors of this manuscript (DR & LS). Prior to this first reviewing phase, both authors screened 200 randomly chosen articles each and reached very good interrater reliability of ĸ = 0.87 (Landis & Koch, 1977). Full text was retrieved if a decision could not be made based on the abstract. In case of disagreement, the respective article was discussed, and the screening criteria have been specified. Furthermore, eligible criteria for full-text screening were specified during the title/abstract screening process. We selected peer-reviewed, published studies examining the association between vitamin D and behavioral executive function outcomes in healthy young- or middle-aged participants.

We divided the eligible criteria into the following categories: Design, Population, and Content. The category Design excluded studies in which (i) executive functions were not assessed via specific tests and therefore not captured on a behavioral level. Furthermore, to address studies using primary data, (ii) case reports, position papers, review articles and meta-analyses were excluded. The category Population excluded studies that met the following criteria: (iii) animal studies, (iv) study samples above the age of 65 years or below the age of 14 years,Footnote 2 as well as (v) study samples with brain-related health restrictions (e.g., Alzheimer disease, dementia). The category Content excluded studies that met the following criteria: (vi) studies on cognitive skills or functions without an association to vitamin D, (vii) studies about vitamin D, but not associated to cognitive skills or functions, and (vii) studies not about cognitive skills or functions, nor about vitamin D. Only studies published in English and meeting all criteria were included.

Data extraction

The following information was extracted from each included article: (i) study characteristics (authors, year of publication, journal, study design, type of intervention, measure of vitamin D, examined executive functions, the respective behavioral tests, a short description of the main results and its statistical key figures), and (ii) population characteristics including potential experimental and control groups (mean age, sex, number of participants, vitamin D level of participants).

Results

The results of the detailed database search and selection process, including reasons for exclusion are presented in Fig. 1. The initial database search took place on the 22nd of November 2023 and yielded 6034 references after excluding duplicates, with six full texts satisfying inclusion criteria. One additional article was identified through citation screening of all included articles and previous reviews and meta-analysis. A total of seven articles including 2933 participants, and eleven behavioral tests of executive functions were included in this scoping review and search was updated during revisions of the paper. Table 1 provides an overview of the extracted data from the retained records in this scoping review. We grouped the studies by the core executive functions and summarized the effect of different levels of vitamin D, and study designs.

Fig. 1
figure 1

PRISMA flow diagram for the identification, screening, and selection of articles included in this scoping review

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Table 1 Main characteristics of the studies included in this scoping review
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Of the summarized seven studies, six studies were cross-sectional studies and one was an interventional study. Sample sizes ranged from 25 participants in a preliminary report (Hansen et al., 2011) up to 1395 for a longitudinal study on US urban adults (Beydoun et al., 2018). Most studies used a mixed gender sample (4) while three used only male participants (Hansen et al., 2011; Lašaite et al., 2011) or female participants (Raszewski et al., 2019).

Discussion

Despite the extensive reviews on the association between vitamin D and executive functions in older (e.g., Annweiler et al., 2013; Beauchet et al., 2021; Silva et al., 2022) or non-healthy populations (e.g., Aghajafari et al., 2018; Balion et al., 2012; Beauchet et al., 2021), the inclusion of only seven articles in our scoping review indicates that the area of research in healthy young- or middle-aged populations is understudied. This seems surprising, as we argued that those aiming for peak performance in sport and work might particularly benefit from sufficient higher cognitive functions.

The studies identified within this scoping review provided mixed findings regarding the association between vitamin D and executive functions. Recent observational studies showed that higher levels of vitamin D have not been associated with a change in working memory capacity (Beydoun et al., 2018; Falasca et al., 2019; Palacios et al., 2020). In contrast, only Hansen and colleagues (2011) showed that participants with a high vitamin D level (69 nmol/L) performed better at a 2-n-back task, a measure of working memory capacity, than participants with a low vitamin D level (38 nmol/L). One reason for the exceptional findings by Hansen and colleagues (2011) might be the low power (0.52), due to the small sample size. Moreover, the effect of vitamin D on working memory performance has not been supported as evident in the null findings of the only intervention study (Dean et al., 2011). A similar picture can be seen for the two other core executive functions cognitive flexibility and inhibition control (Diamond, 2013). Cognitive flexibility performance differed considerably among high and low 25(OH)D groups of postmenopausal women (Raszewski et al., 2019). Similarly, the work by Palacios and colleagues (2020) indicated an association between baseline serum 25(OH)D and response inhibition if the analysis is adjusted for multiple variables.Footnote 3 In contrast, such findings could not be supported by a double-blind 6-week vitamin D intervention trial (Dean et al., 2011). In addition to the aforementioned core executive functions, some studies dealt with executive function as a rather general construct (Beydoun et al., 2018; Falasca et al., 2019; Lašaite et al., 2011; Palacios et al., 2020; Raszewski et al., 2019). However, similar to the individual core executive functions, the findings in these studies were inconclusive. One more general measure of executive function is the version B of the trail-making test (TMT) (Battery, 1944; Partington & Letter, 1949), which requires participants to connect an alternating sequence of numbers and letters in ascending order. Studies using this test reported mixed findings either in favor of (Falasca et al., 2019) or against (Beydoun et al., 2018; Lašaite et al., 2011) an association between Vitamin D concentration and executive function. Beyond the TMT, Falasca and colleagues (2019) used Stroop and Digit-Span tests as additional and general measures of executive function, even if both might be specified for inhibition control and working memory capacity, respectively. In line with the abovementioned TMT, worse performance in both tests were associated with insufficient 25(OH)D levels. In contrast, no effects of vitamin D on executive function were found if measured with other tests such as shifting attention tests (Raszewski et al., 2019), figure copying (Palacios et al., 2020), or drawings (Falasca et al., 2019).

Oral vitamin D supplementation has been found to be more effective to increase one’s 25(OH)D concentration than the exposure to summer sun for elite athletes in central Europe (Krzywanski et al., 2016). However, only one interventional study investigated the direct supplementation of vitamin D on executive function and found no effect of daily 600 IU Vitamin D for 6 weeks (Dean et al., 2011). One explanation for the null results might be the already sufficient level of serum 25(OH)D (Dean et al., 2011). Another argument is based on the general effect of vitamin D supplementation independent of the supplementation level or duration. The remaining six studies were cross-sectional in nature, rendering their mixed findings less conducive to robust causal inferences. Similarly, the beneficial effect of vitamin D supplementation on executive functions might be questioned, as mixed findings in a recent systematic review on more general cognitive functions led the authors claim that they failed “to provide evidence for the cognitive benefits of vitamin D supplementation or for a causal association between vitamin D and cognitive functions” (Beauchet et al., 2021, p. 1252).

The potential limitations of the only interventional study in this scoping review and the mixed findings in thematically related reviews do not provide a clear picture. Since vitamin D supplementation seems to be the most promising form to compensate for vitamin D deficiency (Cho et al., 2021; Krzywanski et al., 2016; Wicherts et al., 2011) and for instance only 6% and 1% of females and males respectively in the German population use Vitamin D supplements (Rabenberg et al., 2015), future research using longitudinal and interventional approaches would be useful to better understand the association between vitamin D supplementation and executive functions. We argue that a more complete picture is warranted that allows for alternative approaches related to one’s individual nutrition or sun exposure.

In order to better understand and classify the abovementioned findings, it is useful to consider the individual study characteristics and potential covariates. We found only one interventional study (Dean et al., 2011), that indicated no beneficial effect of vitamin D supplementation or increasing serum concentration of 25(OH)D3 on selected core executive functions. One argument might be that the baseline 25(OH)D3 serum concentration of 76.2 and 77.2 nmol/L for the intervention and control group respectively, were already sufficient and provide less space for improvements (Amrein et al., 2020). Thus, one might argue that benefits of vitamin D supplementation apply only to individuals with deficient vitamin D concentrations but provide nonspecific pharmacological effects in those with adequate vitamin D concentrations. This line of reasoning is supported by a recent systematic review that indicates no strong evidence for a beneficial effect of vitamin D supplementation on mental health in healthy adults (Guzek et al., 2021). Although the deficient (< 20 ng/mL or < 50 nmol/L) or insufficient (20–29 ng/mL or 50–75 nmol/L) levels of serum vitamin D are well defined (Amrein et al., 2020), different populations and thresholds have been used, in the six observational studies, in order to compare different groups (Fig. 2). Considering serum vitamin D concentration, it seems that in populations providing a strong variability in serum vitamin D concentration it might be more likely to detect associations between vitamin D and executive functions. Such wide ranges in serum vitamin D concentration also enable reasonable group separations based on different vitamin D ranges (Hansen et al., 2011; Raszewski et al., 2019), or provide enough variability for potential effects in regression analyses (Falasca et al., 2019; Palacios et al., 2020). In contrast, it might be less likely to detect such associations, if the range of vitamin D concentration is too narrow (Beydoun et al., 2018) or too broad as found in vitamin D-deficient populations (Lašaite et al., 2011).

Fig. 2
figure 2

An overview of the groups studied based on the vitamin D serum concentration and the respective definitions and labels of distinct levels for each study included in this scoping review. The common classification of different vitamin D levels on the left side by () provides a reference point for better contextualize the respective groups and findings. The investigated executive functions are visualized by different shapes (circle with vertical fill = executive function as general construct, black down-pointing triangle = working memory, black diamond = cognitive flexibility, black square = response inhibition) and marked in green for significant positive and in orange for significant negative associations between vitamin D and the respective executive function

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Although we focused on young or middle aged and healthy populations another potential confounding factor might still be the participants’ age within the range between 14 and 65. Associations between vitamin D and executive functions have been primarily found in populations above the age of 50 years (Falasca et al., 2019; Palacios et al., 2020; Raszewski et al., 2019), whereas no associations have been reported in younger populations below 30 years (Dean et al., 2011; Lašaite et al., 2011). This is quite plausible, especially as the skin’s ability to synthesize vitamin D from sunlight decreases in older people; kidney function, which is important for the conversion of vitamin D into its active form, declines with age and less estrogen and testosterone^1e are synthesized in the body, both of which play a role in vitamin D metabolism (Gallagher, 2013; MacLaughlin & Holick, 1985). The neuroprotective, anti-inflammatory, and antioxidant effect of vitamin D on neurons promotes brain health, and thus, might already apply to middle aged populations, as age-related cognitive decline seems to begin already at a younger age (Salthouse, 2009). Our findings seem to support such early but slow cognitive decline, as positive effects of Vitamin D have been reported in middle aged (Mage = 35 years, Hansen et al., 2011), but not young populations (Mage = 22 years, Lašaite et al., 2011; Mage = 21 years, Dean et al., 2011). This would support the argument, that low vitamin D concentrations operate over several years, so that brain-related outcomes are rather long-term disorders (Heaney, 2003). The associations between Vitamin D and executive functions reported in studies examining middle-aged populations might, thus, be based on the effects of long-term vitamin D deficiencies. This is in line with the work by McGrath and colleagues (2007), reporting no association of 25(OH)D levels with impaired performance on various psychometric measures in a large sample of young- and middle-aged subjects.

As estrogen can modulate vitamin D metabolism, it could also affect cognitive function differently in men and women. This interaction may have an impact on how vitamin D affects the cognitive functions of the different sexes (Lee et al. 2011; van der Schaft et al., 2013). However, the findings provide no reasons to assume that gender influences the association between vitamin D and executive functions in healthy young- or middle-aged participants, as mixed findings have been reported in studies investigating both genders (Beydoun et al., 2018).

It does not remain unnoticed, that the selected articles in this scoping review provide limitations and challenges open for discussion when comparing the individual findings. One factor contributing to the mixed findings is that the selected tests focusing on individual executive functions are seldom entirely devoid of interference from other executive functions and additional cognitive processes (Hill, 2004; Miyake & Friedman, 2012). The lack of methodological differentiation between individual executive functions may lead to findings which only appear questionable at second glance. For instance, vitamin D seems to have a negative impact on response inhibition (Palacios et al., 2020) and a positive impact on executive functions in general (Falasca et al., 2019). However, both use Stroop tasks to measure response inhibition and executive functions respectively, so that they might measure the same construct and indicate mixed findings for the association between vitamin D and the Stroop tasks. This example underlines the challenge of disentangling definitions of individual executive functions from one another, as evidenced by the methodological choices in selecting specific tests for executive functions.

Another aspect that can account for the mixed findings are statistical and methodological properties of the selected studies. The effect sizes for some findings seem to be moderate and power is sometimes low (Dean, et al., 2011; Hansen et al., 2011). Sufficient powered studies are, thus, needed to provide a reliable picture on the relationship between Vitamin D and executive functions in young- and middle-aged populations. The absent correction for multiple testing is another line of concern (Dean et al., 2011; Lašaite et al., 2011; Palacios et al., 2020; Raszewski et al., 2019). Some effects might disappear, if such a correction is applied to the analysis, which in turn would strengthen the broader picture of no or small association between vitamin D and executive functions. While Falasca and colleagues (2019) applied a procedure to correct for multiple testing, the findings should be interpreted with caution, as all significant findings were just below 0.5 (0.049, 0.035, and 0.042) and might depend on the unequal distribution of the groups (49 vs. 8). Although we focused on studies including young- and middle-aged individuals, the mixed findings might be less transferable to the general population of this age range, as some samples might be seen as independent populations such as HIV-infected individuals (Falasca et al., 2019), postmenopausal women (Raszewski et al., 2019), or incarcerated individuals (Hansen et al., 2011).

In general, the association of vitamin D and executive functions in older (e.g., Annweiler et al., 2013; Silva et al., 2022) or non-healthy populations (e.g., Aghajafari et al., 2018; Balion et al., 2012), might not provide a clear argument that vitamin D levels can cause changes in executive functions (Dean et al., 2011). One might argue that individuals with depression or impaired cognition might be less likely to engage in outdoor activities (see Roshanaei-Moghaddam et al., 2009). This in turn leads to a reduced vitamin D production via the action of UV radiation from the sun in the skin, and consequently, might explain the association of vitamin D and executive functions in previous reviews (e.g., Annweiler et al., 2013). However, when these findings were put to test in randomized controlled intervention studies, no beneficial effect of vitamin D supplementation in enhancing overall cognition could be supported (Beauchet et al., 2021). Since the articles in previous reviews neglect young- or middle-aged individuals in peak performance populations, contain methodological challenges, and mixed findings, this current scoping review broadens the scope for potential research gaps and provide future directions to target these to gain a better understanding of the association between Vitamin D concentration and executive functions.

Conclusions

This scoping review illustrates the small and selective range of research investigating the association between vitamin D and executive functions and reflecting mixed findings for the three core executive functions cognitive flexibility, response inhibition, and working memory. Such mixed findings are difficult to interpret, due to their statistical and methodological properties. Thus, this scoping review does not give reason to develop specific hypothesis pertaining to the effects of different vitamin D levels or supplementations on executive functions in healthy young- or middle-aged populations. It is evident that further research is needed to identify the existence, as well as nonexistence, of potential effects and their directions of vitamin D levels on executive functions. Currently, null-finding are prominent such as the null-effect of the only intervention study (Dean et al., 2011), the lack to validate long-term of brain-related outcomes (Heaney, 2003), and similar findings in other areas (Guzek et al., 2021; McGrath et al., 2007). However, these findings should not obscure the fact that a sufficient vitamin D level is important for the performance of young and healthy humans, as evident in athletes, due to the benefit of vitamin D for physiological factors as skeletal muscle growth, immune and cardiopulmonary functions, and inflammatory modulation (de la Puente Yagüe et al., 2020).