Does one egg a week help prevent Alzheimer's disease?
People who eat eggs weekly develop Alzheimer's disease less often in research. Whether eggs themselves cause that difference is uncertain, but one to a few eggs a week fits perfectly within a varied diet.
People who regularly eat eggs develop Alzheimer's disease less often than people who never or rarely do. This comes from a large study in which nearly 40,000 people were followed for an average of more than 15 years. Even at one egg a week the risk was measurably lower: the chance of developing Alzheimer's was about 17% lower than in people who ate hardly any eggs. At five or more eggs a week that difference had grown to nearly 27%. These are associations, however, not proof that eggs themselves cause the protection.
A combined look at eleven studies involving more than 38,000 participants together gives a more nuanced picture. Moderate consumption, roughly half to one egg a day, was linked to better memory scores and a lower chance of cognitive decline. At more than one egg a day, one study actually found an increased risk. The studies are difficult to compare with one another because they were set up very differently, so firm conclusions cannot yet be drawn.
What the protective factor in eggs might actually be is unclear. Omega-3 fatty acids are a logical candidate, since higher omega-3 intake was linked to a lower chance of mild cognitive decline. But for Alzheimer's disease itself that link was not found. Saturated fats, cholesterol, and other fats found in eggs also showed no demonstrable role. In other words, the mechanism behind the observed association remains unexplained.
In practical terms, one egg a week or a few eggs a week carries no demonstrated risk and fits well within a varied diet. People who eat more than one egg a day would be wise not to view that as a purely protective habit, given the conflicting signals seen at high intake.
This answer draws on three sources: a large study of nearly 40,000 people followed for more than 15 years; a review of eleven studies involving more than 38,000 participants combined; and a combined analysis of studies with people followed for years, totalling more than 54,000 participants. None of these studies can prove cause and effect: participants were observed over time rather than randomly assigned to groups. The protective association found for egg consumption is large enough to take seriously, but the mechanism is unclear and one study in the review actually found an increased risk at high intake. The researchers behind the eleven-study review themselves point to the large differences between the included studies, which limits how reliable the combined results are.