Comparison of Vegan and Non-Vegan Diets on Memory and Sleep Quality

It is widely acknowledged that the quality of nutrition affects many aspects of physical and mental performance. A vegan diet is associated with superior cardiovascular and cerebrovascular conditions. Mediterranean diets (high fruit and vegetable content) are linked with reduced risk of neurodegenerative diseases and improved performance on cognitive tests. In the present study, verbal memory and sleep quality were assessed in a cohort of 62 adults aged 40 and above. Using a modied Mediterranean Diet Adherence Screener, the participants were divided into the categories of vegan, vegetarian, pescatarian, omnivores with low meat/sh consumption, and omnivores with high meat/sh consumption. The California Verbal Learning Test was used to assess verbal learning memory, and the Pittsburg Sleep Quality Index was used to assess sleep quality. Diet was found to have a signicant effect on memory but no signicant effect on sleep quality. The sample size may have been insucient to capture the effects of diet on sleep. To denitively establish the relationships between diet patterns and quality of cognitive functioning and sleep, further research is required. The results of this study cast doubt on the hypothesis that the consumption of animal products boosts memory performance.

. Another study, (Null et al., 2017), suggests that a vegan diet is associated with improvements in mood and neurological disorders, such as Alzheimer's disease.
Studies on the effects of a Mediterranean diet, (Wade, 2018), indicate that it is associated with improved mood and cognitive processing speeds.
There are, however, a number of con icting ndings and the levels of statistical signi cance of some of the results referred to above are low due to small participant numbers, short study periods and diverse

Measures
The primary measures used in the study were the California Verbal learning Test -3 rd Edition, the Mediterranean Diet Adherence Screener and the Pittsburg Sleep Quality Index.

CVLT-3
The CVLT-3, standard form, was used to measure memory performance. Participants were given word lists to memorise and recall was tested after a timed delay. Raw scores were standardised for age. For the current study the key outcome measure was total recall.

MEDAS
The Mediterranean Diet Adherence Screener consists of 14 questions on food and drink consumption. For the present study an amended scoring system was used to categorise individuals to one of the ve speci ed dietary groups.

PSQI
The Pittsburg Sleep Quality Questionnaire assesses sleep quality and disturbances over the previous month. It contains a list of 19 questions giving scores for sleep quality, latency, duration and e ciency and also disturbances, use of medications and dysfunctions during daytime activities.

Procedure
Once informed consent had been given by the participant and they have been given instructions to be focused as much as possible, the experimenter started the questionnaire booklet. This consisted of the CVLT test followed by the MEDAS and PSQI questionnaires. The delayed components of the CVLT were completed after the questionnaires. The entire procedure took between 40 and 60 minutes to complete.
The study was approved by the Ethics Committee of Birkbeck, University of London.

Statistical Analysis
Analyses were performed using IBM SPSS Version 25. The main hypothesis concerning the effect of diet on memory was assessed using ANCOVA with dietary group as the independent variable and the memory score as the dependent variable with sleep quality as the covariate.

Results
All participants were adults aged between 40 and 77 years of age. All were uent English speakers, with English as their rst or second language. There were 62 participants, 33 male and 29 female. Using the diet questionnaire, each participant was placed in one of the ve dietary categories as shown in Table 1.  Table 2 shows the mean memory score for each diet group and the overall mean. Figure 1 shows the differences between the individual group scores and the overall mean. The statistical analysis con rmed that there was a marginally signi cant effect of diet on short-delay if and only there was no other independent or confounding variables taken into the statistical analyses. When an ANCOVA test was run with Gender added as the covariate, the signi cance of the diet was lost.  Figure 1 shows the effect of diet on verbal memory. Table 3 shows the mean scores for sleep quality for the individual diet groups together with the overall mean. Higher scores signify lower sleep quality.  Figure 2 shows the differences between the individual group mean scores and the overall mean scorewith positive differences indicating better sleep quality. The statistical analysis con rmed that there was no signi cant effect of diet on sleep quality.
The vegetarian group with the anomalous low sleep quality score was the smallest of the diet groups with 7 members.

Discussion
The rates of cognitive disorders that impact the memory are increasing globally, and it is also increasingly recognized that diet-related pathological processes such as atherosclerosis and in ammation impact cerebrovascular health leading to cognitive de ciencies. There is limited evidence that feeding on a plant-based diet (which is high in dietary bre and vitamins) can lead to functional protection or even improvement in human cognition. In this cross-sectional observational pilot study, the association between diet and verbal learning memory was evaluated. Consistent with most earlier ndings, it was found that there was a signi cant effect on short-delay memory, with plant-based diets showing improved performance relative to the animal-based diets when diet was the only independent variable and no confounding variable is taken into account utilizing ANOVA (Analyses of Variance). However, when confounding variables (i.e. gender, sleep quality and education level) were added to the statistical analyses using ANCOVA (Analyses of Covariance), the signi cance of the effect was lost. A one-way ANOVA have shown no signi cant effect of diet on sleep quality.A chi-squared test showed that females in the sample showed signi cantly better verbal learning memory performances than the males in overall verbal learning memory, which is short-delay, long delay and recognition. The results of this pilot study strongly suggest that a similar study with a much larger sample size would make a major contribution to resolving the scienti c uncertainties regarding the effects of vegan diet on memory and other cognitive functions.

Declarations Data availability
All data generated or analysed during this study are included in the supplementary information les.