Igbinigie PO, Chen R, Tang J, Dregan A, Yin J, Acharya D, Nadim R, Chen A, Bai Z, Amirabdollahian F.Nutrients. 2024 Oct 1;16(19):3340. doi: 10.3390/nu16193340.
Discussion
Our population-based case-control study examined the association between egg consumption and dementia. This study demonstrated that the odds of dementia increased with decreased consumption of eggs from Daily to Weekly to Monthly, while the odds of dementia in those consuming eggs ≥ Twice a day or those who were Non-consuming/<monthly were not significantly increased or reduced. Such an inverse association between egg consumption and dementia was independent of important confounding factors, including age, educational level, smoking, dietary intake of red meats, poultry, fish, vegetables and fruits and co-morbidities.
Despite the current literature lacking research on egg consumption associated with dementia, several studies have investigated the association with cognitive decline or impairment (CI) in older people [15,16,17,18,19,20]. An et al. [20] analysed data from 2816 older adults aged 60 years or older from the National Health and Nutrition Examination Survey (NHANES) 2011–2012 and 2013–2014 waves. Cognitive assessments included the Consortium to Establish a Registry for Alzheimer’s Disease Word List Learning Test (CERAD-WL), Word List Recall Test (CERAD-DR), Animal Fluency Test (AF) and Digit Symbol Substitution Test (DSST). Their findings indicated that neither the consumption status of whole eggs nor the quantity consumed daily was associated with cognitive test scores in older adults. However, other studies [16,17,29] have shown that the consumption of eggs could be beneficial for cognitive functioning. Sukik et al. [17] examined data from 4852 participants aged 55 years or older from the China Health and Nutrition Survey (CHNS). The CHES study collected data on dietary egg intake for each participant through 24-h dietary recalls over 3 consecutive days during home visits between 1991 and 2006 and assessed the cognitive function of the participants in 1997, 2000, 2004 and 2006. The authors found that egg intake was positively associated with global cognitive function, and in fully adjusted models, the regression coefficients across the quartiles of egg intake were 0, 0.11 (95% CI −0.28–0.51), 0.79 (95% CI 0.36–1.22) and 0.92 (95% CI 0.43–1.41), respectively. Compared to non-consumers, those with higher egg consumption (4th quartile) had an adjusted OR of 0.93 (0.74–1.19) for self-reported poor memory and 0.84 (0.69–1.02) for self-reported memory decline, both being not statistically significant. Kritz-Silverstein and Bettencourt [16] analysed data from a cohort of 617 men and 898 women aged 60 and older from the Rancho Bernardo Cohort, who were followed for 16.3 years. They found that egg intake at baseline was significantly associated with better cognitive function measured at follow-up in men (performance on Buschke total (p = 0.04), long-term (p = 0.02) and short-term (p = 0.05) recall but not in women (p-values > 0.05). Li et al. [29] examined data from a cohort of 9028 participants aged ≥ 60 years from the Zhejiang Ageing and Health Cohort Study without cognitive impairment at baseline and followed up for 6 years with three waves of measurements using the Mini-Mental State Examination (MMSE) for CI. The authors used log-binomial regression models for repeated measures with the Generalized Estimating Equations (GEE) method to assess the longitudinal effect of egg consumption on the risk of cognitive impairment. The authors found that participants who increased egg consumption had a reduced relative risk (RR) of CI compared to non-consumers or those who consumed eggs less than Weekly; the multiple adjusted RR of CI was 0.82 (0.76–0.89) for participants consuming 0.1–2.9 eggs/week, 0.91 (0.84–0.99) for those consuming 3.0–5.9 eggs/week, and 0.95 (0.86–1.04) for those consuming ≥ 6.0 eggs/week.
Few studies have been conducted to investigate the association between egg consumption and dementia. In Italy, Nicoli et al. [30] examined data from 1390 participants aged ≥ 80 years from a cross-sectional study in the Varese province and found that the adjusted OR of dementia in the middle tertile of egg consumption compared to the lowest tertile was 0.60 (0.47–0.77), while the OR in the highest tertile of egg consumption was 0.63 (0.38–1.04). The authors further analysed data from 512 participants in the cohort follow-up and found that the corresponding HRs were 0.68 (0.39–1.17) and 0.85 (0.64–1.14), suggesting no significant association. In Finland, Ylilauri et al. [21] carried out a cohort study of 2497 men aged 42–60 y, followed up for 21.9 years, and observed that each additional 0.5 eggs (27 g)/d was associated with a hazard ratio (HR) of 0.89 (95% CI 0.78–1.01) but not significantly. In Spain, Margara-Escudero et al. [31] anlaysed data from 25,015 participants aged 30–70 years from the European Prospective Investigation into Cancer and Nutrition (EPIC)-Spain Dementia Cohort. Cohort participants were recruited between 1992 and 1996 for the baseline survey and followed up for a mean of 21.5 years. The authors found no association between egg consumption and dementia; the adjusted HR for the 4th quartile vs. the 1st quartile of egg consumption was 1.05 (0.85–1.31). After dividing the population by adherence to the relative Mediterranean diet (rMED) score, they observed an inverse association between egg consumption and dementia in participants with low adherence to the rMED score (HR in Q4 vs. Q1: 0.52, 95% CI 0.30–0.90). But there was no association between participants having medium adherence (1.20, 0.89–1.62) and high adherence to the rMED score (0.93, 0.61–1.39). Our study in China revealed a significant and inverse association between egg consumption and dementia among participants who consumed eggs Monthly, Weekly and Daily. In other words, our study demonstrated that the risk of dementia was significantly reduced in these adults who consumed eggs Daily. This protective effect of Daily consumption was consistently observed regardless of whether Monthly or Weekly consumption was taken for comparison (Table 5).
Our study found that participants who consumed eggs at a frequency of ≥Twice a day might have an increased OR of dementia compared to those who consumed eggs at a frequency of Non-consuming/<monthly (Table 5) or participants who consumed eggs Daily (Table 4), approaching statistical significance. The finding suggests a possible link between excessive egg consumption and dementia risk, likely due to the high cholesterol content of eggs, with each egg containing ~200 mg of cholesterol. Previous studies using randomized controlled trials (RCTs) data [12] have revealed that individuals who consumed more than four whole eggs per week experienced greater elevations in blood total cholesterol, HDL cholesterol and LDL cholesterol compared to those who consumed equivalent amounts of egg substitutes. Animal studies involving mice, rats and rabbits have demonstrated an association between high cholesterol intake and AD-type pathologies [32,33,34]. Some human studies have also shown an association between dietary cholesterol intake and lower cognitive performance [35,36]. Furthermore, recent population-based cohort research found a positive association between blood cholesterol levels and incident dementia [37]. Our study observed an increased OR of dementia with the consumption of eggs ≥Twice a day, but it did not reach conventional statistical significance, which was probably due to the small number of participants in this category of egg consumption. Nonetheless, the potential impact of excessive egg consumption on dementia warrants further investigation.
Our study did not find a significant increase in the OR of dementia among participants who reported non-consuming/<monthly over the past two years. There is no research examining this low level of egg consumption in relation to dementia [30]. However, previous research examining cognitive impairment in China found a relation to non-consumers or <weekly consumers; the risk of CI was significantly increased in comparison to those consuming 0.1–2.9 eggs/week or 3.0–5.9 eggs/week [29]. Also, such a group of never/rarely consuming eggs was found to have an increased risk of CVD, particularly among older people [28]. In China, eggs are a popular food due to their nutritional value, affordability and ease of preparation [22]. Despite this, our study found that 13.5% of older adults did not eat eggs or consumed them less than monthly during the two-year period. The reasons for this low consumption could be attributed to poverty. Our data showed that participants with lower family income had reduced egg consumption. These individuals who did not eat eggs over the past two years also had decreased consumption of other nutrient-rich foods such as meats, fish, vegetables and fruits (Table 2). It is worth noting that they may represent a specific subset of the older population, warranting further investigation.
Our study demonstrated that participants who consumed eggs Daily had a reduced odds ratio of dementia compared to those who consumed eggs Monthly or Weekly, regardless of the confounders adjusted for in the analysis. The protective effect of Daily egg consumption against dementia in older adults may be attributed to the presence of bioactive compounds and numerous other nutrients such as high-quality protein, unsaturated fatty acids and vitamins [38]. The nutrient-dense nature of eggs makes them a valuable source of energy, particularly for individuals at risk of malnutrition, such as older adults [39]. Previous research from RCT data [12] indicated that compared to individuals consuming ≤ 4 whole eggs per week, those consuming > 4 whole eggs per week did not experience elevated blood pressure, lipids and lipoproteins. This suggests that the increased egg consumption in our study would not be likely to contribute to cardiovascular risk, which thereby increases the incidence of dementia. Bioactive compounds like lutein, zeaxanthin and choline found in eggs may have beneficial effects on intestinal cholesterol absorption. On the other hand, due to the high levels of protein in eggs, increased egg consumption may delay sarcopenia in older adults [40], while sarcopenia could be associated with an increased risk of dementia [41].
Available evidence indicates that brain inflammation is frequently present in the pathology of dementia. This inflammation, caused by misfolded proteins, such as tau proteins, amyloid beta and alpha-synuclein, could activate microglia toll-like receptors (TLRs) and trigger an innate immune response, which in turn produces inflammatory mediators [42]. Furthermore, neuroinflammation harms the blood–brain barrier’s integrity, increasing the risk of dementia. Consumption of foods such as eggs, which contain antioxidants, choline and omega-3 fatty acids, has been linked to a decrease in systemic inflammation [43,44]. The bioactive components in eggs have anti-inflammatory qualities and may, therefore, contribute to the prevention of dementia [9,45]. Consumption of eggs has also been found to lower the risk of cardiovascular disease in Asian populations [28,46]. Qin et al. [28] examined data from a cohort study of half a million participants in China and revealed that daily egg consumption, compared to non-consuming, was associated with a lower risk of CVD (HR 0.89, 0.87–0.92) [28]. Egg’s components, including antioxidants and anti-inflammatory qualities, as well as their potential to lower the CVD risk, could help reduce the risk of dementia. Previous studies [3,47,48] have also found that the consumption of fish, vegetables and fruits can reduce the risk of dementia since these dietary intakes contain antioxidants and omega-3 fatty acids. The data of our study showed that there were positive associations between these dietary intakes and egg consumption (Table 2). The finding that daily egg consumption reduces dementia risk in our study is in line with the results of previous studies examining the impact of fish, vegetables and fruits consumption on dementia [3,47,48].
Our study examining data from this population-based case-control study in China identified that eating one egg per day could reduce the risk of dementia. The study findings contribute to the development of better policies and improve public health aimed at reducing the burden of disease by promoting egg consumption among older adults.
Strengths and Limitations of This Study
To the best of our knowledge, this study is the first to examine the association between egg consumption and dementia in China, a country with the largest number of people with dementia and the highest egg consumption worldwide. It is also the first to report a significant association between daily egg consumption and a reduced risk of all types of dementia in the world. Our study produces new insights suggesting that daily egg consumption may be associated with a reduced risk of dementia. We included important confounders for adjustment, such as educational level, smoking, alcohol consumption, CVD, depression and consumption of red meats, fish, vegetables and fruits apart from age, thereby minimising residual effects. Additionally, we performed sensitivity analyses using different levels of egg consumption as references, ensuring the robustness of our findings. This study has limitations. First, there were significant age differences between the case and control groups due to the lack of individual age matching in the case-control design for data collection. However, in the data analysis, we adjusted for age as a continuous variable, thereby minimising the residual effect of age on the association between egg consumption and dementia risk. Furthermore, we performed separate data analysis by age of <75 years (mean age was 62.3 in people with dementia vs. 65.1 in people without dementia) and ≥75 years (81.8 vs. 75.3) and found that the results were similar to those from the overall data analysis; for example, the data from Monthly, Weekly to Daily egg consumption showed a significantly reduced odds of dementia per average increment in egg consumption from Monthly, Weekly to Daily in the age group of <75 years (Model 3-adjusted OR 0.22, 0.08–0.61) and in the age group of ≥75 years (0.45, 0.22–0.93). Second, our food frequency questionnaire collected data on the frequency of egg consumption but did not measure the quantity of eggs consumed at each level of the frequency. It may have diluted the association we identified. Previous research suggests that the daily consumption of eggs among Chinese adults usually amounts to 0.76 egg/day [28]. We analysed the frequencies of egg consumption to determine the association between egg intake and dementia and found that Daily egg consumption was associated with a lower risk of dementia in Chinese adults. Third, the case-control design of our study limits the ability to refer causality inference in the association between egg consumption and dementia. However, the observed association between increased egg consumption from Monthly to Daily and reduced risk of dementia is consistent with findings from a recently published study in Western populations [49]. Pan et al. examined data from 1024 older Americans with a mean age of 81.4 who were followed up for 6.7 years in the Rush Memory and Aging Project cohort and found that the risk of Alzheimer’s dementia was significantly reduced with Weekly consumption of >1 egg/wk (adjusted hazard ratio 0.53, 95% CI 0.34–0.83) and ≥2 eggs/wk (0.53, 0.35–0.81), compared to those who consumed ≤1 egg/wk [49]. The trend for the protective effect of egg consumption on dementia is consistent with the findings of our study. Nevertheless, cohort studies in China are required to further examine the protective effect of Daily egg consumption on dementia.
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