Acute Effects of Glucose and Fructose Administration on the Neural Correlates of Cognitive Functioning in Healthy Subjects: A Pilot Study

The Implication of Physiological Ketosis on The Cognitive Brain: A Narrative Review

Optimal cognitive functions are necessary for activities of daily living and self-independence. Cognitive abilities are acquired during early childhood as part of progressive neurodevelopmental milestones; unfortunately, regressive changes can occur as part of physiological aging, or more ominously, pathological diseases, such as Alzheimer’s disease (AD). Cases of AD and its milder subset, mild cognitive impairment (MCI), are rising and would impose a burdensome impact beyond the individual level. Various dietary and nutritional approaches have potential for promising results in managing cognitive deterioration. Glucose is the core source of bioenergy in the body; however, glucose brain metabolism could be affected in aging cells or due to disease development. Ketone bodies are an efficient alternate fuel source that could compensate for the deficient glycolytic metabolism upon their supra-physiologic availability in the blood (ketosis), which, in turn, could promote cognitive benefits and tackle disease progression. In this review, we describe the potential of ketogenic approaches to produce cognitive benefits in healthy individuals, as well as those with MCI and AD. Neurophysiological changes of the cognitive brain in response to ketosis through neuroimaging modalities are also described in this review to provide insight into the ketogenic effect on the brain outside the framework of purely molecular explanations.

Acute hyperglycaemia leads to altered frontal lobe brain activity and reduced working memory in type 2 diabetes

How acute hyperglycaemia affects memory functions and functional brain responses in individuals with and without type 2 diabetes is unclear. Our aim was to study the association between acute hyperglycaemia and working, semantic, and episodic memory in participants with type 2 diabetes compared to a sex- and age-matched control group. We also assessed the effect of hyperglycaemia on working memory–related brain activity. A total of 36 participants with type 2 diabetes and 34 controls (mean age, 66 years) underwent hyperglycaemic clamp or placebo clamp in a blinded and randomised order. Working, episodic, and semantic memory were tested. Overall, the control group had higher working memory (mean z-score 33.15 ± 0.45) than the group with type 2 diabetes (mean z-score 31.8 ± 0.44, p = 0.042) considering both the placebo and hyperglycaemic clamps. Acute hyperglycaemia did not influence episodic, semantic, or working memory performance in either group. Twenty-two of the participants (10 cases, 12 controls, mean age 69 years) were randomly invited to undergo the same clamp procedures to challenge working memory, using 1-, 2-, and 3-back, while monitoring brain activity by blood oxygen level–dependent functional magnetic resonance imaging (fMRI). The participants with type 2 diabetes had reduced working memory during the 1- and 2-back tests. fMRI during placebo clamp revealed increased BOLD signal in the left lateral frontal cortex and the anterior cingulate cortex as a function of working memory load in both groups (3>2>1). During hyperglycaemia, controls showed a similar load-dependent fMRI response, whereas the type 2 diabetes group showed decreased BOLD response from 2- to 3-back. These results suggest that impaired glucose metabolism in the brain affects working memory, possibly by reducing activity in important frontal brain areas in persons with type 2 diabetes.

The value of research on sexual dimorphism in neuroimaging

Spets and Slotnick (in Press) applied activation likelihood estimation meta-analysis to examine sex differences in regional brain activation during long-term memory retrieval. Based on their analyses, each of the eight studies they analyzed showed greater activity for males than females during long-term memory retrieval. These results suggest that analyses of magnetic resonance imaging data should not be collapsed by sex and expand this discussion to the challenge of meta-analysis on studies with small to modest sample sizes in favor of larger imaging studies with more rigorous thresholds.

Cognitive performance, mood and satiety following ingestion of beverages imparting different glycaemic responses: a randomised double-blind crossover trial

BACKGROUND/OBJECTIVE

The relationship between postprandial glycaemic responses and cognitive performance, mood and satiety are inconsistent. The objective of this study is to compare the effects of different glycaemic responses, induced by beverages with different glycaemic index (GI) (sucrose and isomaltulose), and a non-glycaemic control (sucralose), on cognition, mood and satiety.

SUBJECTS/METHODS

In this double-blinded, randomised crossover trial, healthy adults (n = 55) received sucrose (GI 65), isomaltulose (GI 32) and sucralose (non-caloric negative control) drinks on separate occasions. The Complex Figure test, the Word Recall test, Trail Making Test Part B and the Stroop test were administered 60 min after beverages ingestion. Mood and satiety were tested along with cognitive performance.

RESULTS

Comparing between isomaltulose and sucrose, there were no significant differences in the mean (95% CI) for the following: Complex Figure: immediate recall -0.6 (-1.7, 0.5), delayed recall -0.8 (-1.9, 0.3); Word recall: immediate recall 0.2 (-0.7, 1.1), delayed recall 0.5 (-0.4, 1.4); Trail Making: completing time -2.4 (-7.5, 2.7) s; Stroop: time used for correct congruent responses -9 (-31, 14) ms and correct incongruent responses -18 (-42, 6) ms. No differences among beverages were found in the mood and satiety scores with exception that participants felt more energetic 60 min after isomaltulose ingestion (p = 0.028 for difference with sucrose) and hungrier 30 min after isomaltulose ingestion (p = 0.036 for difference with sucrose; p = 0.022 for difference with sucralose).

CONCLUSION

Under these study conditions there is no convincing evidence for an effect of glycaemic response on cognitive performance, mood or satiety.

Fructose vs glucose decreased liking/wanting and subsequent intake of high-energy foods in young women

Recent research on the health impacts of added sugar has prompted the comparison of the effects of its 2 major components: glucose and fructose. Fructose was identified as a risk factor for obesity and metabolic syndrome. However, because of the differences in metabolic responses and responsivity of reward circuitry to palatable food, it is unknown if glucose and fructose induce similar appetite-related responses in humans with varying weights. This study compared the behavioral responses to food in young women of a healthy weight (n = 31) and with excess weight (n = 28). We hypothesized that (1) the inhibitory effect of glucose (vs fructose) on food-related responses would be greater in subjects of a healthy weight than in those with overweight/obesity and (2) subjects with overweight/obesity would exhibit a stronger preference for food than subjects with a healthy weight. After an overnight fast, the subjects ingested a glucose or equienergetic fructose beverage on 2 separate days, respectively. Then, they completed liking and wanting ratings and 2 decision-making tasks followed by ad libitum food intake. The results revealed that fructose reduced both liking and wanting for food in subjects with overweight/obesity and also decreased energy intake in all subjects. Relative to the healthy-weight group, subjects with overweight/obesity preferred the immediate reward. Moreover, only in the healthy-weight group were liking and wanting scores for food positively associated with actual food consumption. Overall, fructose (vs glucose) showed an acute inhibitory effect on appetite-related responses in subjects with excess weight.

Fuel for Thought? A Systematic Review of Neuroimaging Studies into Glucose Enhancement of Cognitive Performance

A transient improvement in cognitive performance can be observed following the ingestion of a glucose drink, a phenomenon known as the ‘glucose facilitation effect’. The effect has been studied thoroughly in the last three decades, but its neural underpinnings remain a matter of speculation. A systematic review was conducted to evaluate the current evidence from studies applying neuroimaging or neurophysiological methods to investigate the glucose enhancement effect. Eleven studies met the inclusion criteria of using neuroimaging in conjunction with cognitive outcomes. Six studies employed electroencephalography (EEG), four used functional magnetic resonance imaging (fMRI) and one employed functional near-infrared spectroscopy (fNIRS). All but one study reported modulation of neurophysiology or neuroimaging markers following glucose, while only five studies reported significant changes in cognitive outcomes. The evidence suggests that glucose administration enhances neurocognitive markers of episodic memory and attentional processes underpinned by medial temporal and frontal activation, sometimes in the absence of measurable behavioural effects. Further exploration of glucose facilitation using neuroimaging measures with increased sample sizes is warranted to replicate these findings.

Chronic Stress Combined with a Fructose Diet Reduces Hypothalamic Insulin Signaling and Antioxidative Defense in Female Rats

BACKGROUND

Increased fructose consumption and chronic exposure to stress have been associated with the development of obesity and insulin resistance. In the hypothalamus, a crossroad of stress responses and energy balance, insulin and glucocorticoids regulate the expression of orexigenic neuropeptides, neuropeptide Y (NPY) and agouti-related protein (AgRP), and anorexigenic neuropeptides, proopio-melanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART).

OBJECTIVES

We investigated whether chronic stress and fructose diet disrupt these hormonal signaling pathways and appetite control in the hypothalamus, contributing to the development of insulin resistance and obesity. Potential roles of hypothalamic inflammation and oxidative stress in the development of insulin resistance were also analyzed.

METHODS

Insulin, glucocorticoid, and leptin signaling, expression of orexigenic and anorexigenic neuropeptides, and antioxidative and inflammatory statuses in the whole hypothalamus of fructose-fed female rats exposed to unpredictable stress for 9 weeks were analyzed using quantitative PCR and Western blotting.

RESULTS

Chronic stress combined with a fructose-enriched diet reduced protein content and stimulatory phosphorylation of Akt kinase, and elevated 11β-hydroxysteroid dehydrogenase 1 and glucocorticoid receptor expression, while alterations in appetite regulation (NPY, AgRP, POMC, CART, leptin receptor, and SOCS3 expression) were not observed. The expression of antioxidative defense enzymes (mitochondrial manganese superoxide dismutase 2, glutathione reductase, and catalase) and proinflammatory cytokines (IL-1β, IL-6, and TNFα) was reduced.

CONCLUSIONS

Our results underline the combination of long-term stress exposure and fructose overconsumption as more detrimental for hypothalamic function than for either of the factors separately, as it enhanced glucocorticoid and impaired insulin signaling, antioxidative -defense, and inflammatory responses of this homeostasis- regulating center.

Neural correlates of executive functions in patients with obesity

Obesity is one of the most challenging problems in human health and is recognized as an important risk factor for many chronic diseases. It remains unclear how the neural systems (e.g., the mesolimbic "reward" and the prefrontal "control" neural systems) are correlated with patients' executive function (EF), conceptualized as the integration of "cool" EF and "hot" EF. "Cool" EF refers to relatively abstract, non-affective operations such as inhibitory control and mental flexibility. "Hot" EF refers to motivationally significant affective operations such as affective decision-making. We tried to find the correlation between structural and functional neuroimaging indices and EF in obese patients. The study population comprised seventeen patients with obesity (seven males and 10 females, BMI = 37.99 ± 5.40, age = 31.82 ± 8.75 year-old) preparing to undergo bariatric surgery. We used noninvasive diffusion tensor imaging, generalized q-sampling imaging, and resting-state functional magnetic resonance imaging to examine the neural correlations between structural and functional neuroimaging indices and EF performances in patients with obesity. We reported that many brain areas are correlated to the patients' EF performances. More interestingly, some correlations may implicate the possible associations of EF and the incentive motivational effects of food. The neural correlation between the left precuneus and middle occipital gyrus and inhibitory control may suggest that patients with a better ability to detect appetitive food may have worse inhibitory control. Also, the neural correlation between the superior frontal blade and affective decision-making may suggest that patients' affective decision-making may be associated with the incentive motivational effects of food. Our results provide evidence suggesting neural correlates of EF in patients with obesity.