Glucose under most significant circumstances is the most available energy source for the brain. The only times glucose is not sufficient is under times of extreme starvation, which is when glucose has limited amounts for the brain (6). Glucose, in general, is the most highly sought after nutrient for the brain, and while this may be for basic brain functions, the brain usually does not have enough glucose for optimal cognitive performance for complex memory functions (3). Glucose is highly important to our health as a whole, since it is the primary energy source of the body. This energy is derived from glucose by breaking down carbohydrates in the diet and releasing them to the circulatory system, providing substantial energy throughout the body.
According to Owen (15), because neurons or nerve cells within the brain cannot store glucose, they depend on the bloodstream to deliver a constant supply of glucose. Muscles also benefit from glucose as they are used for energy during strenuous exercise (11). All human life ultimately depends on maintaining substantial levels of this nutrient. However, excess amounts of glucose as with any nutrient can cause harmful effects such as diabetes and obesity. Thus far, there are limited case studies in the past 10 years which have not reintroduced the substantial effects of diet and memory function. This presents an obstacle in finding relevant modern data which could eventually advance the understanding of the role of modern glucose regulation in the brain (4).
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Determining the independent role of glucose ingestion on brain functioning will provide more insight to the physiology of blood glucose transport to the brain, potentially modify guidelines for cognitive enhancement, and give direction for further research in this field. Particularly, if glucose has a negative impact on memory cognition, this will have significant implications in the ability to absorb material and further increase memory functions to be more effective (6). The primary purpose is to determine whether limited ingestion of glucose is at risk for reduced cognitive performance. Secondly, an attempt to describe the relationship of other dietary nutrients such as protein, fat, and other vitamins which may be a leading benefactor along with glucose in further improving memory and understanding the strengths and weaknesses of each nutrient in various studies. Research suggests energy intake from carbohydrate, protein, or fat can enhance memory independently of elevations in blood glucose. Each macronutrient may also express unique effects on cognitive performance.
SIGNIFICANCE OF THE PROBLEM:
Because the proportion of people with cognitive impairments is constantly on the rise as the population ages (11), it is imperative in understanding the various factors such as nutrition and environmental factors that could potentially lead to the prevention or reduction of such deficits. Glucose normally serves as the primary, if not the sole, oxidative substrate for brain energy metabolism in humans (3). Because the brain neither stores nor produces sizable quantities of glucose, it must obtain it continuously via the process of facilitative diffusion across the blood-brain barrier (6). If hypoglycemia is not treated and blood glucose levels continue to drop, the resulting changes in neuronal metabolism will trigger warning symptoms in the brain. As blood glucose levels drop further, mental efficiency may deteriorate substantially and clinically significant neurological disorders or coma may eventually occur.
Hypoglycemia in most significant cases cause brain fuel deprivation, which in severe circumstances results in functional brain failure. However, this can normally be altered by raising plasma glucose concentrations (9). The brain under most cases cannot breakdown glucose or store significant amounts as glycogen; this is in part because the brain requires a constant supply of glucose from the blood (9). The rate of blood to brain glucose transport in most cases exceeds the constant rate of brain glucose metabolism at normal glucose levels, but this effect eventually falls and becomes limited to the brain glucose metabolism capability, especially when blood glucose concentrations fall to substantially lower levels (9). Thus, hypoglycemia is the main cause for brain fuel deprivation and, as a result, brain failure will persist (4). Based from various studies, it is evident in order to enhance the cognitive functionality of the brain; glucose ingestion would substantially reverse the negative effects of hypoglycemia (3).
REVIEW OF LITERATURE:
Glucose Ingestion and Cognitive Function
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The effect of glucose on the brain under most significant circumstances has the most available source of energy for the brain. The only times glucose is not sufficient is under times of extreme starvation, which is when glucose has limited amounts for the brain (1). Several highly distinguished researchers in the field of pathophysiology and nutrition in relation to human development, including White (16) have done extensive studies regarding the molecule of sugar, which is a by-product of glucose in relation to its influences on behavior and cognitive performance particularly in children; he states that there is evidence that glucose is involved in memory storage in both rats and humans in the medial septum region of the brain, also known as the hippocampus region (16). This septum White (16) notes is important in memory processing and is one of the only clues in which he and many other researchers before him have noticed blood glucose in the complicated web of interactions within the brain. Another scholar who can very much relate to White (16) also conducted studies on rats is Karol (11). Karol (11) has done further studies with rats and glucose injections and explains how some memory impairments can be reversed simply by administrating glucose indirectly which she was done extensively with rats, as well as humans. Karol (11) states that since glucose is very well the only substance besides oxygen and water that could potentially enter the brain, this may alter neural metabolism, neural activity, as well as neurotransmitter release. This in essence has became her primary focus since she further has done research in Alzheimer's and Down syndrome, in which she explains how glucose can effectively enhance cognition in persons who have these certain diseases. Kaplan (9, 10) along with Karol (11), has also done research regarding the affects of glucose on Alzheimer's and Down syndrome, and they would both agree that patients with Alzheimer's disease could potentially enhance performance by ingesting a glucose drink tailored to the patient's specific needs. This improved cognitive performance could potentially be a breakthrough in further research studies regarding these diseases.
All scholars would agree that glucose and dietary carbohydrates can improve memory, and that it can rapidly increase the level of blood glucose in healthy adults. Further research regarding the rate of glucose delivery has been studied by Kaplan (10), he notes that since it is well known that glucose is the main metabolic fuel for the brain and is therefore necessary for mental performance, there should be increased provision that glucose is associated with enhanced cognition and that all types of carbohydrates, regardless of structure, does not significantly differ in performance compared to placebo ingestion. This further implies that all types of dietary induced carbohydrates provide substantial amounts of required glucose regulation for brain function (9, 10). This breakthrough has been strongly agreed upon in the field of medicine. However, as Karol (11) mentioned previously, it is perhaps important to remember that research on rats have shown a very broad range of glucose effects not only on memory, but also on motor activities, tremors, sleep, and patterns of neurotransmitter release and other behavioral measures. All scholars, in essence, agree in one common ground that the effects of glucose on memory have many effects on brain function which are across the spectrum of various memory performances as well as deficits. However, it is becoming clear between the recent studies, which have shown glucose as an important cognitive enhancer and is far broader than previously thought. Glucose enhancement of cognitive function extends to a variety of populations and socioeconomic backgrounds including healthy adults, children, and subjects with severe cognitive deficits. Gold (6) concludes that glucose enhances learning and memory not only in rats but also in many populations of humans, for example with schoolchildren. This research implies that the contents and timing of meals may need to be coordinated to have the most beneficial cognitive effects that enhance learning.
Fat Ingestion and Cognitive Function
High-fat diets may do more than have substantial increases on the waistline according to Granholm (7). Thus far, research has aimed to assess the significant role of diet induced with calories, which may affect an individual's cognitive performance. Reducing substantial amounts of fat may improve the cognitive capabilities and further help reduce its negative effects of stress and the aging process on learning new material. Other work indicates that diets high in fats and cholesterol could worsen cognitive losses due to sleep apnea in those prone to the condition (7, 16). Although there is evidence that obesity is caused by a genetic component, data suggests that the epidemic is primarily caused by an increase in dietary fat, caloric ingestion, and a decrease in physical activity (7, 16). Thus far, there is not enough substantial evidence in the scientific community as to why fat induced consumption effects cognitive function which was noted in Granholm's (7) study. According to Granholm (7), the obesity epidemic is second only to tobacco use for the number of health care dollars; this significance plays a substantial role in the premature mortality among the obese population, importance of this issue is vast across the globe.
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In a study conducted by Veerendra (16), researchers mainly studied how a diet high in fat affected memory in mice. Researchers aimed to differentiate the young adult male divided into four groups according to diet which included: normal (control); high saturated fat, high sugar, and high saturated fat and sugar. After approximately four months, in order to test the significance of diet in the mice, the researchers conducted a maze task in which the mice completed from start to finish. Veerendra (16) noted that the mice on the diet high in both fat and sugar had gained weight compared to those on the control and high sugar diets. Upon conclusion of this research study, Veerendra (16) found that the mice on the high fat and high fat, high sugar diets could not learn and remember the maze as well as the others. These results provide substantial evidence that foods high in fat, particularly a diet high in saturated and unsaturated fats can have profound detrimental effects on the basis of learning and memory.
Kaplan (10) conducted a study similar to Veerendra (16), but the significance of this research was to differentiate the cognitive performances of rats on a diet high in fat, similar to what humans consume if they don't eat nutritionally compared to a lower fat meal plan. After three months, researchers indicated that the rats on the high fat diet had severe impairment on a wide spectrum of learning and memory tasks related to those rats that consumed the preferable lower fat diet. The research also proved that glucose treatment significantly improved the memory of rats fed high fat diets. After the study had been completed, it was proven that when high fat rats were injected with glucose, their cognitive function improved in relation to the glucose which worked selectively on one part of the brain, specifically the hippocampus (10). The hippocampus is located near the center of the brain and is critical for normal memory function. This area of the brain enables individuals to register new memories and recall those memories after a period of time (10).
Vitamin Ingestion and Cognitive Function
As people age, vitamin deficiency becomes more prevalent in almost every single geographical location; this is in part due to the inability to release vitamin B-12 from food proteins, inadequate intake, as well as intestinal malabsorption. Eussen (5) conducted a study and discovered that vitamin B-12 deficiency may eventually have an astounding result, triggering irreversible neurologic brain damage and cognitive impairment. Bryan (2) conducted a study similar to Eussen (5), but found similar effects on women, in which she claimed that as we age, the effects of fluid function in the brain decline in relation to the inability to absorb required amounts of vitamins. This finding supports the robust outcomes of age related cognitive decline on cognition reflecting fluid functions which are paramount in keeping up with memory performances in longer durations. Another study conducted by Haskell (8), also found similar findings in relation to cognitive impairments, except she found those effects primarily on healthy children, in which was found that a high proportion of them have deficiencies in one or more vitamins. and since vitamins could modulate cognitive performance and mood in healthy children, this was a paramount finding. Haskell (8) later concludes in the study that mood and cognition go hand in hand in relation to vitamin supplementation and the potential to improve brain function in healthy children should suggest significant increases in vitamins in the daily diet which are essential for the optimal performance. These physiological processes include both direct and indirect effects on brain function, including neurotransmitter synthesis, receptor binding, energy metabolism, and cerebral blood flow. In general, all scholars would agree that there are only small effects of short term B vitamin supplementation on cognitive performance and none on mood and behavior. Since the effects of supplementation were seen mainly for memory performance, the general lack of effects could be due to the short duration of supplementation compared to more crucial nutrient intake, such as dietary carbohydrates and glucose.
CRITICAL ANALYSIS OF LITERATURE:
One of the most significant issues regarding consistency between studies is the selection of cognitive tests. There is currently no consensus regarding a standard neuropsychological test battery, leaving researchers to choose among hundreds of tests to assess cognition. According to this information and other recent reviews of glucose and cognitive function, memory and attention/concentration tests seem to be the most widely used. The measure of cognitive function for each study ranged from a single neuropsychological test to a battery of 8 tests, with an average of 5-6 tests administered among the studies. The method of analyzing the studies in this review using an independent classification system for tests was chosen for several reasons. A number of studies were found to show discrepancies in classifying cognitive tests. For example, both Kaplan (9) and Messier (14) assessed memory based on several tests including the visuomotor task and word list recall. Karol (11) also used the word list recall and the visuomotor task test, except classified it as a test of executive function and used the results based on rodent studies. In this regard, the research on rodents has shown a broad range of glucose effects not only on memory, but also on locomotor activity, sleep, pharmacogically induced tremors as well as other neural measures which had a direct link with human case studies. Several important details regarding the study of glucose and diabetes were inconsistently evaluated among the studies, effecting the interpretation of results. Only two of the studies explicitly stated a valid method of diagnosing hypoglycemia or hyperglycemia by laboratory measurement, one only doing so for the undiagnosed diabetes sample (4, 15).
The remaining glucose samples were based on self-reported research findings, with the use of various glucose drinks, placebo samples, and standardized questionnaires. Three of the four studies not stating the method of diagnosis were those whose sample was selected from an already established elderly population. Along the same lines, only five of the six studies provided a description of the method of obtaining high glucose levels, with only one specifying the time period in which the laboratory and cognitive testing was performed. As time passes between glucose and cognitive testing, the more inaccurate the result of comparisons become; therefore the results of the studies not providing this piece of pertinent information have reduced validity.
The trend shown throughout various research studies has clearly indicated that the ingestion of a carbohydrate polymer which is converted to glucose can substantially increase the cognitive function in regards to various mental tasks and situations (4, 6, 9, 11). Studies by Gold (6) and Karol (11) also found that each subject throughout their studies of cognition had significant upward trends in fully engaging in these cognitive tests such as: reaction times to cognitive tests, faster results in visuomotor tests, raised awareness in attention tests, as well as increased comprehension abilities in paragraph recall. Blood was the major determinant of the increase in cognitive function and the present study on glucose was first to show that cognitive performance is associated with glucose regulation from various ages, in addition to glucose, common carbohydrate containing foods can also improve cognition (4).
The downfall to these studies related to the fat ingestion and research has found that a diet loaded with calories may affect an individual's memory as well. Reducing the amount of dietary fat may improve memory and help reduce the negative effects of stress and aging on thinking and learning (7). Other work indicates that diets high in fats and cholesterol could worsen cognitive losses due to sleep apnea in those prone to the condition. Although there is a significant genetic component to obesity, data suggests that the epidemic is primarily caused by an increase in dietary fat, caloric ingestion, and a decrease in physical activity which may affect the capabilities of the blood-brain barrier due to thickening of the saturated blood (7).
Consistently, research studies have shown that the main benefactor as to why the brain reacts so differently to various macronutrients is its intrinsic ability to change in relation to the hippocampus stimulus which ultimately drives the homeostasis action toward the body. This is why from data ranging from all disciplines of medicine; we cannot rule out the role of various macronutrients specific effects in the brain. Especially since humans have the ability to reduce its risks by simply staying away from fat induced foods and to counteract that indulgence with a balanced diet (7).
Cognitive battery tests should be the most important criteria in determining the role of various macronutrients in relation to implementing an improved design for future research. This modality in essence, will allow researchers to see the strengths of each nutrient criterion. This finding was seen by Gold (6) and Kaplan (10) as useful tools in determining the strengths and weaknesses of each nutrient. In the future, research surrounding the cognitive phenomena could include having a standardized IQ testing center in which potential subjects are determined by IQ range rather than age or socio economic backgrounds seen in Gold (6) and Kaplan (10) studies. This will further elaborate the significance of subjects potential scores compared to a fair and stable program. The possibilities of this future study will have stronger cases in regards to the significance of cognition to various research institutions. In many instances, programs implementing the three main macronutrients (glucose, protein, fat) are left limited in the possibilities of results, this is why with further IQ testing and experimentation with combined nutrients the effects will be related closely to real world scenarios. In many instances, individuals combine their diet with various nutrients, and so having this test implement will have closer scores to
In the future, research could include the role nutrients have in combinations with the health status of the individual, or the role it has when one does not engage in physical activity towards those who do. It is imperative to note that it is not just the macronutrient that is enabling the deficiency of cognitive function and this could very well be how that macronutrient is stimulated in the blood which correlates to the blood-brain barrier in which it only accepts glucose and oxygen.