Conclusion That Egg Consumption Reduces Alzheimer's Disease Not Very Significant

A study in the peer reviewed Journal of Nutrition claiming a 47% reduction in clinical diagnosis of Alzheimer's during life due to eating at least one egg a week, is much weaker than it appears at face value. 

The results (based upon a clinical diagnosis during life) were consistent with benefits as low as 17% within the margin of error.

 

And, the clinical diagnosis rates weren't very accurate. Post-mortem examination of the brains of more than half of the sample (578 cases) found a 56% false negative rate for clinical diagnosis during life and an 18% false positive rate for clinical diagnosis during life. This casts doubt on whether the lifetime clinical diagnosis data is meaningful at all, given its immense inaccuracy. 

The results when based on autopsies were consistent with benefits as low as 10% for two or more eggs a week.

Both these results are only modestly better than the p=0.05 statistical significance threshold, and the fact that the cases where Alzheimer's disease was accurately diagnosed showed less of a benefit, also casts doubt on the significance of this study. None of the evidence supporting the author's hypothesis that dietary choline intake was the protective factor was statistically significant, and none of the results were significant at anything close to the p=0.01 level. This significance, moreover, is overstated, because the study fails to adjust for the inaccuracy of the clinical diagnosis process or for the uncertainty in the result caused by self-reporting of what people ate. 

Dietary self-reporting is particularly problematic in a population where a large percentage of the reporters will ultimately be diagnosed with Alzheimer's disease in less than ten years. It could be that many people who don't report weekly egg consumption did so because their memory was not good enough to remember everything that they ate in the last month due to having undiagnosed or subclinical levels of Alzheimer's disease, rather than being due to actually eating fewer eggs.

The sample size is small, so the statistical significance of the result is modest. The total sample was 1024 people, and only 370 of them did not eat at least one egg a week.

Also casting doubt on the result is the fact that there was no dose-response effect (i.e. eating more than one egg a week is no better than eating one egg a week, and eating no eggs is not significantly different from eating 1-3 eggs a month) and indeed, eating more than one egg a week reduced the benefit in individuals who had post-mortem exams done, relative to those who had one egg a week.

The average diagnosis at 6.7 ± 4.8 years from the start of the study is also soon enough after the study began, that given strong evidence in the literature (which is not mentioned in the literature review in the study) that Alzheimer's disease starts at sub-clinical levels decades before it is diagnosed in many cases. So, the diet of the people in the study during the study period probably didn't influence the outcomes.

Finally, while the study controls for 13 potential confounding factors (of which some, like education, didn't vary significantly between the subgroups), it could easily have omitted an important factor. And, it doesn't prove cause and effect either. For example, it could be that having undiagnosed Alzheimer's disease influences your body in a way that you don't like eating eggs as much.

Thus, while the study might justify further research, its conclusion the weekly egg consumption greatly reduces Alzheimer's disease onset risk is too weak to make a life choice based upon, or to recommend clinically, despite the fact that their best fit benefit purports to be as much as a 47% reduction in the risk of getting Alzheimers. Indeed, if anything, the details of the data tend to favor some uncontrolled for confounding factor that is merely mildly correlated with egg consumption.

The failure of the study to engage with many of these issues in a forthright manner also casts doubt on the quality of the peer review at the Journal of Nutrition for this study.

The study and its abstract are as follows:

Background: Alzheimer’s disease (AD) is a neurodegenerative disorder with increasing prevalence due to population aging. Eggs provide many nutrients important for brain health, including choline, omega-3 fatty acids, and lutein. Emerging evidence suggests that frequent egg consumption may improve cognitive performance on verbal tests, but whether consumption influences the risk of Alzheimer’s dementia and AD is unknown.

Objectives: To examine the association of egg consumption with Alzheimer’s dementia risk among the Rush Memory and Aging Project cohort.

Methods: Dietary assessment was collected using a modified Harvard semiquantitative food frequency questionnaire. Participants’ first food frequency questionnaire was used as the baseline measure of egg consumption. Multivariable adjusted Cox proportional hazards regression models were used to investigate the associations of baseline egg consumption amount with Alzheimer’s dementia risk, adjusting for potential confounding factors. Subgroup analyses using Cox and logistic regression models were performed to investigate the associations with AD pathology in the brain. Mediation analysis was conducted to examine the mediation effect of dietary choline in the relationship between egg intake and incident Alzheimer’s dementia.

Results: This study included 1024 older adults {mean [±standard deviation (SD)] age = 81.38 ± 7.20 y}. Over a mean (±SD) follow-up of 6.7 ± 4.8 y, 280 participants (27.3%) were clinically diagnosed with Alzheimer’s dementia. Weekly consumption of >1 egg/wk (hazard ratio [HR]: 0.53; 95% confidence interval [CI]: 0.34, 0.83) and ≥2 eggs/wk (HR: 0.53; 95% CI: 0.35, 0.81) was associated with a decreased risk of Alzheimer’s dementia. Subgroup analysis of brain autopsies from 578 deceased participants showed that intakes of >1 egg/wk (HR: 0.51; 95% CI: 0.35, 0.76) and ≥2 eggs/wk (HR: 0.62; 95% CI: 0.44, 0.90) were associated with a lower risk of AD pathology in the brain. Mediation analysis showed that 39% of the total effect of egg intake on incident Alzheimer’s dementia was mediated through dietary choline.

Conclusions: These findings suggest that frequent egg consumption is associated with a lower risk of Alzheimer’s dementia and AD pathology, and the association with Alzheimer’s dementia is partially mediated through dietary choline.

Yongyi Pan, et al., "Association of Egg Intake With Alzheimer’s Dementia Risk in Older Adults: The Rush Memory and Aging Project" 154(7) The Journal of Nutrition 2236-2243 (2024). https://doi.org/10.1016/j.tjnut.2024.05.012.