Maintaining cognitive performance throughout the school day is crucial for children, particularly in all-day schools. A nutritious lunch plays a vital role in this. Glucose, the brain's primary energy source, needs continuous supply for optimal cognitive function. While glucose has short-term cognitive benefits, high carbohydrate consumption leading to rapid blood glucose spikes might hinder cognitive performance. Conversely, slower and sustained glucose rises from low-GI carbohydrates have shown to improve attention and memory. Studies on breakfast GI and cognition in children have yielded mixed results. Some studies demonstrated better cognitive function with low-GI breakfasts, while others found no effect. This inconsistency might be due to methodological differences and confounding factors. To date, research on the effects of lunch GI on children's short-term cognition is scarce. Previous Cognition Intervention Study Dortmund (CogniDo) studies revealed no negative short-term effects of lunch on cognition, even suggesting potential benefits for working memory. This study aimed to investigate the short-term impact of lunch dietary GI on schoolchildren's cognitive performance, specifically considering the composition of the lunch itself.

Existing research on the relationship between dietary glycemic index (GI), especially at breakfast, and cognitive function in children is mixed. Some studies suggest that low-GI breakfasts improve cognitive performance compared to high-GI breakfasts or skipping breakfast altogether, attributing the benefits to a more sustained glucose supply to the brain. However, other studies have failed to replicate these findings, possibly due to variations in methodologies, participant characteristics, and confounding factors. The existing literature highlights the need for further research, especially focusing on the effects of lunch GI on cognitive function in children, an area largely unexplored.

A randomized, single-blind, 2x2 crossover study was conducted at a comprehensive school in Germany. 193 fifth and sixth-grade students (188 included in the final analysis) participated. Students were randomly assigned to one of two sequences: medium-high GI (m-hGI) or high-medium GI (h-mGI). In the first period, one group received ad libitum high-GI rice (GI: 86) at lunch, and the other group received medium-GI rice (GI: 62). One week later, the groups switched treatments (period 2). A standardized breakfast was provided each morning, followed by the lunch intervention at 12:25 p.m. Cognitive assessment using a computerized test battery (tonic alertness, task switching, working memory updating) began 45 minutes after the start of the lunch break. The test battery included three tasks: a switch task (visual attention and task switching), a two-back task (working memory updating), and an alertness task (tonic alertness). Data were analyzed using t-tests (for normally distributed data) or Wilcoxon rank-sum tests (for non-normally distributed data). Carryover effects were assessed by comparing the sum of outcome variables from both periods between groups. Linear mixed models adjusted for glycemic load (GL) were also employed to determine associations between GI, GL, and cognitive parameters.

The crossover analysis initially showed no significant difference between the high-GI and medium-GI lunch groups across most cognitive parameters. However, due to carryover effects observed for reaction time in the two-back task and the number of commission errors in the alertness task, the researchers analyzed only the data from the first period. This analysis revealed a significant difference: the high-GI rice group exhibited faster reaction times in the two-back task (p=0.001) and fewer commission errors in the alertness task (p=0.04) compared to the medium-GI rice group. Further analysis revealed period effects for several parameters, suggesting improvements in switch costs in period 2 and decreases in reaction times and false alarms in the two-back and alertness tasks. Adjusted analysis incorporating glycemic load as a covariate did not reveal significant associations between GI or GL and cognitive parameters.

This study initially found no support for the hypothesis that lunch GI directly impacts children's cognitive performance. However, the analysis restricted to period 1 suggested potential benefits of a high-GI lunch for specific aspects of working memory updating and tonic alertness. These results contrast with some studies showing cognitive benefits of low-GI breakfasts. Possible explanations for these discrepancies include differences in the timing of the meal (lunch versus breakfast), the age group studied, and the specific cognitive functions measured. The observed period effects highlight the importance of considering temporal factors in studying dietary interventions and cognitive function. Future research should address these inconsistencies by employing larger samples, longer study durations, and exploring various meal compositions and timing.

This study did not find evidence of short-term effects of lunch dietary GI on the overall cognitive performance of schoolchildren. However, the findings suggest potential positive effects of high-GI rice on specific aspects of working memory and alertness, warranting further investigation. Future research should validate these findings using larger sample sizes and longitudinal studies to examine the long-term effects of varying dietary GI on children's cognitive function. The influence of other dietary factors and individual differences also needs to be considered.

The study's relatively small sample size and the short-term nature of the intervention might limit the generalizability of the findings. The reliance on a single type of high-GI and medium-GI carbohydrate source (rice) limits the extent to which the results can be generalized to other food types. The study only measured short-term effects; a longitudinal study is needed to assess long-term impacts. Furthermore, the potential influence of factors like individual variations in glucose metabolism and overall dietary patterns was not assessed in detail.