In this population-based, case-control study, we observed that having engaged in both moderate physical exercise and computer use in late life (within 1 year of date of interview) was associated with decreased odds of having MCI. The synergistic interaction between the 2 activities occurred on an additive scale but not on multiplicative, after adjustment for caloric intake and other confounders (age, sex, education, depression, and medical comorbidity). This additive interaction suggests that the observed combined presence of moderate physical exercise and computer use was greater than the expected sum of their independent effects. Additive interaction can occur in the absence of multiplicative interaction. Moreover, additive interaction is also considered to be more applicable to biologic events.30
Previous studies have examined the independent associations of either physical exercise or mentally stimulating activities with MCI,13,15,16
but to our knowledge, no study has specifically examined the presence of simultaneous engagement in physical exercise and computer activities on the outcome of MCI, after accounting for caloric intake.29
On the other hand, several studies have examined the association between combined physical and mental activities and dementia, although the studies did not show an added benefit from having engaged in physical and cognitive activities.10,11,31
Other studies have examined the interaction between physical activity and diet, not an interaction with computer use; for example, a prospective cohort study in New York observed a synergistic interaction between physical activity and adherence to Mediterranean-type diet in reducing risk of Alzheimer disease.32,33
Our study has focused primarily on aging and MCI, not dementia; therefore, we compared our findings with a New York study that had an outcome of MCI, that is, a prospective cohort study that examined the association of baseline participation in physical and cognitive activities with the outcome of incident amnestic MCI. The investigators observed that a 1-point increase on a composite cognitive measure was associated with decreased risk of amnestic MCI, but no such decrease was observed for physical activity. However, they did not specifically examine an interaction between physical activity and computer use. In this respect, our study is different from theirs.13
There are various strengths to our study. First, we examined the impact of the presence of 2 lifestyle factors (physical exercise and computer use) after adjusting for a third factor (caloric intake) on aging and MCI. Second, we used a large sample in a population-based setting. Third, we measured MCI using a comprehensive face-to-face evaluation including 3 well-established components: clinical evaluation by a behavioral neurologist or neuropsychiatrist, neuropsychological assessment, and cognitive measures from the participant and a proxy ascertained by nurses.
The study should be interpreted in the context of the following limitations. The assessment of computer activities was very limited. We only asked how many times per week a person engaged in computer activities. We did not ask for the duration of the computer use. Any future study that investigates computer use should assess both duration and frequency because excessive sedentary lifestyle may additionally predispose a person to health problems.34
Another limitation of our study is recall bias, which is an unavoidable drawback of any survey-based study.30
However, one potentially reassuring factor is that, in the past, we conducted analyses in which 80 study participants from the same cohort completed the survey in 2 successive years. We observed that there was no significant difference between participants with MCI and normal participants in recalling midlife activities on the 2 successive surveys.15
Finally, the case-control design limits our ability to determine causal associations, since one cannot tell the direction of causality in a case-control study.
This study did not investigate mechanisms of action. However, on the basis of the existing literature we can speculate about the following possible explanations. The combined presence of having participated in both physical exercise and computer use may be a marker of a healthy and disciplined lifestyle.15
On the other hand, it is possible that the combined activities may have direct beneficial effect on the brain. Physical exercise may target a particular circuit in the brain (eg, increasing the production of brain-derived neurotrophic factor in the hippocampus),35
whereas cognitively stimulating activity (eg, computer use) may enhance functional connections contributing to cognitive reserve.36-38