The “sweet” effect: Comparative assessments of dietary sugars on cognitive performance
Introduction
Sugar, associated with a positive taste quality – sweetness, has become a major component in the modern human diet. While there is increasing interest in studying effects of sugar consumption on cognitive performance [1], [2], [3], relatively little attention has been given to evaluation of the effect profiles of different sugars.
In the existing literature, the relationship between sugar consumption and cognitive effects is primarily studied through assessing acute effects of glucose ingestion [4], [5], [6], with most studies focusing on glucose effects on memory performance [7]. Glucose ingestion has been found to have a facilitating effect on memory tasks known to depend on the hippocampal region [8] in both young and elderly groups, including individuals with neurological diseases or metabolism conditions [7]. In contrast, studies that assessed glucose effects on other cognitive domains have produced somewhat mixed results. Specifically, previous studies have examined glucose effects on reaction time [10], attention [11], face recognition [12], working memory [3], and related types of tasks. Of these, a few studies observed small facilitative effects on information processing speed and attention, when measured using the trail-making test, letter symbol digit test, or the Stroop test [13], [14]. By contrast, other studies found either no difference [15], [16], [17], [18] or deteriorative effects [10]. These inconsistent findings may be attributed to choices of cognitive tasks and domains of interest [7]. Baumeister and colleagues [19], [20] in particular, stressed the importance of uncovering the link between cerebral glucose availability and prefrontal processes, due to the relevance of glucose on self-regulation. Clearly, more research is required in this area.
In comparison to glucose, cognitive effects of other dietary sugars such as fructose or sucrose have received much less attention [7], [9]. Indeed, the metabolism of fructose and sucrose undergoes a very different pathway as compared to glucose. Fructose does not traverse the blood-brain barrier and is instead metabolised through the liver [21], resulting in a much slower increase in blood glucose levels. Sucrose, as a disaccharide, is first hydrolysed into glucose and fructose, and then metabolised via separate pathways. Such metabolic differences across the key sugar sources might be assumed to lead to potentially different impacts on cognitive performance, yet there exists no study demonstrating whether that is the case. More research in this area is clearly important, particularly given the increasing trend of substituting sugar sources of a high glycaemic index with ones of a lower index (e.g., fructose or artificial sweeteners [22], [23]).
In addition to the issues raised above, the exclusive role of sweetness perception on cognition presents a fascinating research topic in its own right. The question – whether sweetness perception moderates cognitive performances – has received little direct attention. Neuroimaging data have demonstrated that sensory inputs from the gustatory system are initially represented in both the primary taste cortex and the orbitofrontal cortex [24], [25]. In addition, emerging data imply that the brain processing to the physical taste substance and its sensory quality may be dissociated, although the specific neural circuitry has not been defined [24]. It is therefore interesting to understand the relative cognitive effects across different types of sweeteners when their sensory characteristics are the same. An answer to this question will not only add understanding to the neural processing of various sweetness substances, but also will provide pertinent insights into the effectiveness of sugar substitutes from a cognitive perspective.
Overall, the present study aims to test the effects of different sugars on selected cognitive tasks, using a double-blind, cross-over experimental design. Rather than focusing exclusively on a particular cognitive domain, the study is designed to include a variety of tasks that implicate prefrontal lobe functioning. According to Gailliot and Baumeister [19], processes that rely heavily on the prefrontal cortex possibly require more glucose than processes associated with other brain regions, given the critical role of prefrontal function in tasks depending on effortful, controlled and executive processes. Specifically, this study assesses effects of sugar intake on information processing, executive functioning and attention. In light of previous findings regarding the role of fasting [8], the present study incorporates a comparison between fasting and non-fasting participants.
Section snippets
Participants
A total of 49 individuals participated. They were randomly assigned into an overnight 10-h fasting group (N = 26; 15 females; 22.6 ± 4.2 years of age) or a non-fasting group (N = 23; 13 females; 24.3 ± 4.9 years of age). All participants were recruited from the university of Otago community. Prior to their participation, all underwent a screening session to ensure they were free from chronic or major diseases, type 1 or 2 diabetes, or psychiatric disorders. Participants fell within a BMI range of
Blood glucose measurements
Fig. 2 illustrates changes in the averaged glucose measures across three time-points for each of the tested sugar solutions, for the fasting (Panel A) and non-fasting group (Panel B), separately. A mixed-model ANOVA was performed to indicate changes in the glucose measures (in mmol/L) across the time-points within an experimental session, and to detect differences between four sugar conditions and fasting status. Although a three-way interaction was absent, a significant two-way interaction was
Discussion
The present study, based on a repeated-measures design, assessed cognitive effects of three dietary sugars – sucrose, glucose and fructose—against a non-caloric sweetener. Overall, the findings showed that glucose and sucrose had relatively negative effects on the assessed cognitive tasks. In contrast, no apparent effect on task performances was found with fructose ingestion compared to the placebo.
Indirectly, these results imply that the observed differences were not moderated by the
Acknowledgements
The authors are grateful to University of Otago Research Grant (111930.01.R.IC) that supported this work.
The authors' contributions are as following: RG and MP performed the research and wrote the manuscript; MP and EAF designed the experiment and revised manuscript drafts; IO helped with a part of data analyses and manuscript revision. All authors have approved the final manuscript contents.
The authors have no financial or personal conflicts of interest to declare.
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