*This content has been reviewed by Dr. Nick Bellissimo (School of Nutrition, Ryerson University)
Cognition is an overall term used to describe the mental skills people use to acquire, process, store, and transform information to carry out everyday tasks. Cognition is an outcome of brain function, and is composed of multiple elements, including perception, attention, memory, executive functions (i.e. the control of behaviors, such as organization and planning, initiating tasks, and/or regulating emotions), and decision making.
- Factors Contributing to Cognition. Cognition covers a wide range of mental processes and tasks. The factors that affect cognition range from age and education to environmental factors and diet.
- Carbohydrates and Cognition. The human brain uses a large amount of energy, and typically uses glucose as its sole source of energy for functioning.
- Sugars and Cognition. To date, there is some evidence to suggest that sugars consumption might be beneficial for short-term cognitive tasks. There haven’t been any no longer-term intervention studies in humans conducted yet. Overall, there is no strong evidence to suggest any particular association between sugars consumption and cognition.
Factors Contributing to Cognition.
There are several factors that can impact an individual’s cognition, either positively or negatively (1,2,3), such as:
- Diet (e.g. alcohol, caffeine, antioxidants)
- Education and socioeconomic status
- Physical activity
The effects of overall diet and specific dietary components on cognition have been assessed in many animal studies, and in some human studies. Research in both humans and animals has included cognitive outcomes on one hand like performance on tasks, and on the other hand molecular and chemical changes in brain regions. The information reviewed and discussed below are related to human studies.
Carbohydrates and Cognition
The human brain uses a large amount of energy, and typically uses glucose (a component of sucrose (sugar)) as its sole source of energy for functioning. Glucose can be derived from dietary carbohydrate, synthesized in the body from other substrates such as amino acid, or released from glycogen storage in the liver and muscle. The brain can also use ketones as an alternative source of energy when carbohydrate (glucose) supply is low; ketones are generated from metabolizing fatty acids. The human brain is only about 2% of the weight of an adult body, although it requires almost 20% of the total energy required for the body. Interestingly, the adult brain uses glucose at the same rate that skeletal muscles use glucose during exercise (4).
Low-carbohydrate ketogenic diets have become popular over the years, however research has shown conflicting results on cognition, with a majority of data coming from animal models rather than humans, making translation into recommendations more difficult. Convincing evidence associates the ketogenic diet (low carbohydrate, high fat and adequate protein) and the treatment of epilepsy (5), but it is beyond the scope of this article on cognition.
Sugars and Cognition
Glucose is the most common circulating sugar in the bloodstream, is the main fuel source for cells in the body, and is the primary sugar used by the brain for energy (6). Specifically, the human body requires a minimum of 130g of glucose per day (7), which is based on the amount the brain needs to function. The brain uses glucose from stores in the body, or uses glucose from the foods that people eat by breaking down carbohydrates (starches and sugars) into sucrose, which is then further broken down into glucose and fructose (as shown in the diagram below). Fructose is metabolized in the liver, and the metabolic pathways between glucose and fructose differ (6).
Figure 1. The brain relies primarily on glucose to function. Restricting the brain's glucose supply can impair memory and ability to focus.
The body does not require fructose for energy, and there has been considerable speculation about the role of fructose and excess fructose consumption and health. Some studies evaluating fructose and cognition were reviewed (8), and the totality of evidence suggests there is a significant confounding effect of total energy intake, which makes it difficult to isolate the effect of fructose (9). Overall, authors summarize that there is currently a lack of high quality evidence directly assessing the role of fructose and cognition (9).
The consumption of sugars have different short (minutes to hours) and long-term (months and years) cognition-related outcomes. Available evidence from studies with humans suggests that sugars have positive short-term effects for memory and learning processes (10, 11). It has also been shown that the cognitive response in children to glucose is better for more complex mental tasks (i.e. verbal memory, remembering lists of items), compared to simpler tasks (7,12,13). The effects of the source of glucose (coming from sugars or starches) on cognition has yet to be evaluated. The majority of studies that discuss long-term outcomes of sugars consumption and cognition use animals. Long term effects of sugars on cognition in humans are difficult to establish due to the confounding presence of other nutritional and lifestyle variables. A review on the topic of diet and cognition reports that there is a vast range of methodological approaches to study diet and cognition, and there is not consistency in the methods used (14). Given the available data to date, there is no strong evidence to suggest that the consumption of sugars independent of other dietary components impacts cognition. No systematic reviews or meta-analyses have been published on sugars and cognition. Three human studies are summarized below, which investigate carbohydrate or sugars intake and cognitive outcomes in humans.
- A within-subjects repeated measures intervention study among Canadian children (15 boys and 7 girls, aged ~12 years) evaluated the effects of different carbohydrate sources on cognitive performance. Six separate eating occasions were included, with treatment meals containing 50 g of carbohydrate from either mashed potatoes, French fries, hash browns, white rice, white beans, or a skipped meal as the control. Cognitive function was assessed by several memory tests and by a test for information processing speed. The results found that cognitive performance didn’t differ between the treatment meals or the meal skipping groups. Specific results found that children recalled more words after consuming French fries compared with after consuming mashed potatoes and white rice. Future studies could consider accounting for the composition of the evening meal prior to testing, as this might have influenced satiety and cognition in the morning. The authors conclude that long-term effects of habitual consumption on cognitive performance in children have yet to be evaluated (15).
- An intervention study with repeated measured design among Canadian children (15 boys, 9-14 years old) evaluated the effect of glucose, sucrose, and high-fructose corn syrup solutions on immediate and delayed word list recall. During the experiment, the participants received either 200 kcal / 250 mL of glucose, sucrose, high fructose corn syrup, or a non-caloric sucralose control (16). The individuals were given a list of 15 words to memorize, and were asked to recall these words at 30, 45, 60, and 90 minutes. The results found that there were no significant differences among the beverages and the resulting word memorization. The authors mention that this effect may also be related to the overall foods and beverages eaten during a given day.
- An intervention study among Canadians elderly adults (10 men and 10 women, aged 60-82 years) evaluated the effects of glucose consumption on cognitive tasks. The participants consumed 50g of carbohydrate from different foods or liquids, on four separate days. The results found that the differences in blood glucose after consuming either glucose, potato, barley, or a placebo didn’t predict performance on the cognitive task. Some data from this study also found that the poorer the person’s memory at baseline measurement, the greater the measured improvement in memory over time (independent of sugars intake) (10).
- An intervention study of American adults (105 young adults) evaluated the effects of sugars consumption on cognitive tasks. The participants consumed either a sugars-containing (sugar sweetened carbonated beverage with high-fructose corn syrup (60% fructose, 39% glucose, 1% maltose)) or no sugars-containing (artificially sweetened) drink. Various cognitive tasks were performed, including answering a set of questions and calculating the amount incorrect (false alarm rate). The results found that accuracy and sensitivity to cognitive tasks were higher and false alarm rate was lower following sugars intake, compared to the results when no sugars were consumed (17).
- A cross sectional survey study in the United States of 1,234 mother-child pairs evaluated the association between the consumption of sugars in the mother and the child. The study examined the reported daily intakes of sucrose, fructose, and servings of sugars-sweetened beverages (SSB), juice, and diet soda. The results found that the maternal and child fructose (avg. 32.7g and 27.8g) and juice (avg. 1.3 and 1.8 servings) consumption were not associated with cognition. Maternal sucrose consumption (avg. 49.8g/day) was inversely associated with mid-childhood intelligence test non-verbal scores. Additionally, maternal SSB consumption (avg. 0.6 servings) was inversely associated with mid-childhood cognition; maternal diet soda consumption (avg. 0.2 servings) was inversely associated with early and mid-childhood cognition scores. From the childhood scores, early childhood consumption of SSB (avg. 0.2 servings) was inversely associated with mid-childhood verbal scores; and early childhood fruit consumption was positively associated with higher cognitive scores in early and mid-childhood (18). The authors include limitations to this research such as small serving sizes, and thus small differences in consumption between the groups. Further, the authors note that as this is an association study only, and causal effects aren’t established.
Overall, there is not a clear and consistent effect of sugars on cognition, and future research on this topic will enhance understanding of the relationship between diet and cognition.
For more information, additional resources include:
Sugar 101: Journey of a Sucrose molecule (video)
- Carbohydrate News - Carbohydrates and the cognitive performance of children
- Clips on Sugars - Breakfast Brain Booster
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