Photo courtesy of Herbert Wertheim School of Public Health and Human Longevity Science
A Q&A with Public Health Researcher Donna Kritz-Silverstein, PhD
Plentiful and relatively inexpensive, eggs are a popular staple food worldwide. The most commonly consumed eggs come from fowl. In fact, during the COVID-19 pandemic backyard homesteading increased as many people turned to raising chickens for hobby and eggs.
Eggs are also nutritious. They contain protein, healthy fats, and many nutrients like choline and carotenoids which previous studies report are associated with protective effects for cognitive function.
In a recent study published in the journal Nutrients, Donna Kritz-Silverstein, PhD, professor at the Herbert Wertheim School of Public Health and Human Longevity Science at UC San Diego, reports on the prospective association of egg consumption with multiple domains of cognitive function. The data, gathered from participants in the Rancho Bernardo Study of Healthy Aging, a longitudinal cohort study of residents in the Rancho Bernardo suburb in San Diego, included information from 1,515 community-dwelling men and women aged 60 years and older upon follow up, an average of 16 years later.
Q: Why are some people wary of eating eggs?
A: Eggs are high in protein, but they also contain a large amount of dietary cholesterol. However, unlike meat, which is also high in protein and cholesterol, eggs are low in saturated fats. For almost 50 years, people were advised to limit the number of eggs eaten per week because it was thought that dietary cholesterol would raise levels of plasma cholesterol, leading to cardiovascular disease. However, for the most part, research has not borne out the idea that dietary cholesterol can raise plasma cholesterol, and guidelines have been relaxed in recent years.
Q: What factors led to studying the impact of egg consumption on cognition?
A: Given the aging of the population, the prevalence of Alzheimer’s Disease and cognitive impairment is expected to rise, making the identification of modifiable factors associated with maintenance of cognitive function an important public health priority.
Previous studies report that choline and carotenoids such as lutein and zeaxanthin have beneficial effects for cognitive function. Eggs contain high levels of these nutrients, but only two previous studies examined the longitudinal association of egg consumption with cognitive function. These studies were limited by short durations of follow-up (only two or four years) and either did not include women or did not analyze data separately by sex. Thus, we decided to examine the longitudinal association of egg consumption on different domains of cognitive function in men and women who participated in the Rancho Bernardo Study and who had cognitive function assessed 16 years after assessment of egg intake. A secondary purpose was to examine whether egg intake in middle age was associated with better cognitive function at older ages.
Q: What were the main findings of this research?
A: Overall, the analyses showed that for men, greater egg consumption was associated with better verbal episodic memory as indicated by small but statistically significantly better performance on tests of total recall, and short-term and long-term memory. These associations remained even after considering other factors such as age, obesity, cigarette smoking, cholesterol level, use of cholesterol lowering medication, and histories of heart attacks and hypertension. No other associations with cognitive function were found for men, and no associations were found in women.
Although differences were small, analyses restricted to individuals younger than 60 years old (or middle aged) when egg intake was assessed suggested that egg consumption in middle age was associated with better performance on some cognitive function tests later in life, especially in men. Results of this study were reassuring in we also found that for both men and women, egg consumption was not associated with the likelihood of impaired cognitive function.
Q: While you state that further studies are needed to address sex-specific associations observed in this research, why might there be an association of egg intake with later cognitive function in men but not in women?
A: We were trying to determine if egg consumption was related to cognitive function separately within each sex, and whether the patterns of association were similar in men and women. For both men and women, the number of eggs consumed per week ranged from 0 to 24. However, the average number of eggs consumed per week was significantly higher in men than women (4.2 versus 3.5, respectively). Furthermore, greater proportions of men consumed eggs at the higher levels, whereas greater proportions of women consumed eggs at the lower levels. For example, in men, 5.5 percent consumed no eggs per week, and 18 percent consumed seven or more eggs per week, whereas in women, 9.9 percent consumed no eggs per week, and 13 percent consumed seven or more eggs per week.
It is possible that the smaller variability of egg consumption in women may have led to attenuated, non-significant associations. To our knowledge, this is the first prospective study to examine the association of egg intake with cognitive function separately for both men and women. It would be interesting to see if others find similar results.
Q: Does this study provide insight into how often, and in what quantity, people should eat eggs?
A: This was not what this study aimed to do. However, we also found that egg intake in middle-age was associated with somewhat better cognitive function in later life. This is a novel finding and suggests a potential long-term impact of egg consumption on cognitive health.
— Yadira Galindo