Introduction
Epidemiological data suggest that frequent consumption of citrus fruits is associated with various health benefits such as a reduced risk of cardiovascular disease, cerebral infarction and ischemic stroke [
1‐
3]. Recently, there has also been interest in the relationship between fruit consumption and cognitive function [
4], particularly given that fruits and juices provide a rich, easily available source of flavonoids such as hesperidin and narirutin. Human research of this field is in its infancy; however, recent reviews indicate that increased flavonoid consumption over the lifespan may attenuate age-associated cognitive decline and the onset of neurodegenerative disease [
5,
6]. For example, increased consumption of citrus fruits and orange juice was associated with better cognitive outcomes in a cross-sectional analysis of 2031 females aged 70–74 [
7].
In support of the epidemiological evidence, a handful of intervention studies in older adults with mild cognitive impairment indicate that daily intake of flavonoid-rich (FR) juices over 12–16 weeks can benefit memory function [
8‐
10]. Cognitive benefits following chronic fruit juice consumption are not exclusive to adults with mild cognitive impairment; improvements in global cognitive function were observed in healthy older adults (mean age 67) following 8-week daily consumption of flavanone-rich orange juice (305 mg/day) relative to a low-flavanone control (37 mg) [
11]. It has been hypothesized that increased cerebral blood flow (CBF) and increased neural activity following consumption of flavonoid-rich drinks could provide an underlying mechanism for the aforementioned cognitive benefits [
9,
12]. In support of this, daily dietary supplementation of cocoa flavanols (900 mg/day) over 8 weeks was associated with increased cerebral blood volume in the dentate gyrus of the hippocampus [
13]. Crucially, concomitant improvements in pattern recognition were also observed, such that response times were 630 ms quicker following high-flavanol relative to low-flavanol supplementation. This provides evidence of possible causation between flavonoid intake, increased CBF and behavioral effects.
It remains to be seen whether acute cognitive benefits occur within the immediate postprandial phase following juice intake. However, acute improvements in memory and executive function have been observed in healthy young adults following consumption of flavonoid-rich cocoa-based drinks and solids [
14,
15]. The aim of the present research was to investigate whether consumption of FR orange juice is associated with acute cognitive benefits in healthy middle-aged adults. Orange juice is one of the most commonly consumed sources of flavonoids, and the potential for fruits and fruit-based drinks to enhance cognitive function or attenuate cognitive decline deserves investigation, particularly in healthy populations for whom very limited published data exist. The flavonoid concentration of whole oranges can be higher than the concentration found in orange juice [
16], principally because the fibers in orange act as an entrapping matrix [
17]; thus, some of the flavonoids are lost during the juicing process. Therefore, in the present study, the intervention drink is composed of whole processed juicing orange which retains the fiber, and thus delivers a high dose of flavonoids.
Discussion
Consumption of FR orange juice was associated with improvements in cognitive function and subjective alertness relative to an energy-matched placebo in healthy middle-aged adults free of disease or mild cognitive impairment. Specifically, scores for the CPT (a measure of attention and more broadly executive function) were significantly better 6 h post orange juice consumption relative to the placebo, while finger tapping (a measure of psychomotor speed) was significantly better 2 and 6 h post consumption relative to the placebo. These findings were reflected by a nonsignificant trend showing improvement in global performance (all tests combined) throughout the day following the orange juice relative to baseline, in contrast to a subtle decline in global performance at both time points relative to baseline following the placebo. These effects on objective measures of cognitive function were mirrored by subjective alertness ratings whereby orange juice consumption attenuated a decline in alertness at both time points following the placebo. This indicates that FR orange juice can enhance objective and subjective cognitive function acutely overly 6 h.
It is important to acknowledge that orange juice consumption was not associated with a significant improvement on every individual cognitive test; this would not be expected given that the effects of nutritional interventions on cognitive performance are small and difficult to detect in healthy adults [
25]. However, consistently higher means were observed following orange juice relative to the placebo in the vast majority of outcomes (see Table
2). This is consistent with the findings from an 8-week chronic orange juice intervention in healthy older adults which showed improved global cognitive function relative to a low-flavonoid control [
11]. This emphasizes the importance of considering cognitive function as a whole rather than focusing exclusively on individual cognitive tests and domains. Nevertheless, it is interesting to note that the strongest benefits of orange juice were observed for the CPT and Simple Finger Tapping, tasks which required strong elements of executive function and psychomotor speed, respectively. Kean et al. [
11] also reported subtle improvements for executive function following dietary supplementation of flavanone-rich orange juice; however, other chronic studies have focused almost exclusively on memory. For example, 12–16 weeks of grape or blueberry juice consumption has been shown to benefit memory in adults with mild cognitive impairment, but executive function was not assessed [
8‐
10].
A key finding from the present research was the observation that cognitive benefits following a FR drink can occur within the immediate 6-h postprandial time period. Hendrickson and Mattes [
26] failed to find any acute effects when cognitive performance in young adults was assessed in the afternoon following consumption of a grape juice drink during lunch. The younger sample recruited by Hendrickson and Mattes [
26] (mean age 26) could account for the null effects relative to the present study (mean age 51); it is conceivable that healthy young adults typically perform at peak cognitive capability, and therefore, nutritional interventions may have little scope for impact. Having said that, benefits for executive function [
15], spatial working memory and reaction time [
14] have been observed in healthy young adults 2 h post consumption of non-fruit-based flavanol-rich cocoa drinks. Furthermore, general increases in mental fatigue over the morning were attenuated by a 520-mg cocoa flavanol drink [
15], which reflects the attenuation of a decline in subjective alertness following the FR drink in the present study. Collectively, these data indicate that flavonoid-based acute nutritional interventions can improve performance on cognitive tests and enhance subjective mood and alertness in healthy adults. This highlights the possibility that cognitive effects are mediated by subjective mood, and proposed mechanisms of action for flavonoids on the brain should consider this.
Examining the potential underlying mechanisms was not the aim of this research; however, the greatest cognitive benefits were observed at 6 h which is consistent with the anticipated peak in plasma flavanone metabolites [
21,
22]. In support, pilot bioavailability data (
n = 6) in healthy adults following consumption of the same FR drink showed peak concentrations of 0.25 µM hesperetin and 0.12 µM naringenin in the plasma at 6 h. Therefore, it is reasonable to propose that mechanisms relating to the flavanones hesperidin and narirutin may underlie the cognitive benefits at 6 h. This hypothesis requires investigation as plasma flavanone concentration was not measured concomitantly with the behavioral outcomes. Effects at 2 h are more difficult to explain via this mechanism given that the concentration of metabolites would not be expected to be peak at this point according to the timescale for these flavanones to be absorbed into the small intestine, converted into glycosides, hydrolyzed by microflora in the large intestine, and appear as glucuronidated and sulfated metabolites in the plasma [
22,
27,
28]. However, aforementioned research with cocoa shows that behavioral effects can be achieved within the 2-h postprandial period following flavonoid consumption [
14,
15]. It is possible that the flavanone metabolites may be exerting effects in advance of reaching peak concentration in the plasma.
Once ingested and metabolized, the specific mechanisms by which flavonoids may affect cognitive function have been described in detail elsewhere [
29,
30]. Flavanone metabolites are known to cross the blood-brain barrier following oral ingestion [
31], and research on rodents indicates that flavanones may have specific neuroprotective effects such as increasing the expression of brain-derived neurotrophic factor (BDNF) [
32,
33]. However, it is unlikely mechanisms relating to BDNF and synthesis of proteins and enzymes in the brain such as cAMP-response element-binding protein (CREB), which affect neuronal signal transduction [
34], could be effective within a 6-h timescale. One plausible hypothesis is that flavonoid consumption may lead to acute cognitive benefits via increased CBF as a result of enhanced endothelial function and increased bioavailability of nitric oxide [
35,
36]. Human studies show significantly increased CBF several hours following cocoa flavanol consumption [
12,
36]. These are supported by chronic studies which show increased activation in the right middle prefrontal cortex and the right superior parietal cortex following anthocyanin and flavanol-rich grape juice consumption [
9]. Moreover, increased steady-state-evoked potentials in posterior parietal and central–frontal regions [
37] and increased CBF in the hippocampus during a spatial memory task [
13] have been observed following several weeks daily consumption of cocoa flavanols. As yet, there are no published data examining peripheral or CBF in humans following flavanone consumption; therefore, these potential mechanisms are speculative at this time.
It is possible that the observed cognitive effects in this study could be in part accounted for by vitamin C and/or folate which were not matched across the FR and placebo drinks. Cognitive benefits associated with these micronutrients are only likely to occur following chronic consumption over several weeks and months, or in cases where there are deficiencies [
4]. Therefore, it is unlikely that vitamins or folate could account for the cognitive benefits observed 2–6 h post consumption in this healthy sample. A strength of the present study is that the beneficial effects occurred within the everyday context of consuming a standardized breakfast and lunch, indicating that positive effects are not dependent upon a period of fasting and relative energy deprivation. To extrapolate, FR orange juice-based interventions can offer acute cognitive benefits over the course of the day when consumed in conjunction with normal dietary intake. Having said that, participants were instructed to consume a low-flavonoid diet prior to each test day; therefore, it is unclear whether a habitually high-flavonoid diet would mask the effects of similar interventions. With regard to the generalizability of these findings, the orange juice drink was a commercial available product (Tropicana Pure Premium Orange Juice) with the addition of FR pomace, which is an edible part of the whole orange usually leftover during the production of the juice (see method). This FR drink is not currently available to the public although it is important to point out that it consists entirely of edible oranges.
To conclude, consumption of an orange juice containing 272 mg flavonoids was associated with acute improvements in cognitive function and subjective alertness up to 6 h post consumption relative to an energy-matched control drink containing no flavonoids in healthy middle-aged adults. Fruit juices and their respective fruits are an easily available commonly consumed source of flavonoids and polyphenols. These data demonstrate that fruit juice-based flavonoids can acutely enhance cognition in healthy adults. This is consistent with the accumulating evidence from chronic interventions and epidemiological research that increased consumption of fruits, fruit juices and other flavonoid-rich foods over the lifespan is associated with cognitive benefits such as a reduced risk of neuropsychological disease, attenuation of aging-induced cognitive decline and maintenance of optimal cognitive facilities [
4].