Low cardiovascular fitness in mid- and late life represents a risk factor for several neurological diseases. However, the respective effect of aerobic cardiovascular and muscle fitness in early adulthood is largely unknown. Therefore, we analyzed these in young men with regard to long-term risk serious depression, bipolar disorders, stroke and dementia.
We performed population-based longitudinal cohort studies of Swedish male conscripts registered in 1968-2005 for mandatory military service (approx. 1,800,000 subjects), representing about 95 % of all Swedish males at age 18 during this time period. Data on cardiovascular fitness (determined by the cycle ergometric test), muscle strength (isometric contraction) and cognitive performance (IQ tests) were subdivided into low, medium, high fitness groups. Over a 40-year follow-up, risk of serious depression, bipolar disorder, stroke and dementia were calculated with Cox proportional hazards models controlling for potential confounders. To identify cases, we used the International Classification of Diseases (ICD) 8-10 in the Hospital Discharge Register and the Cause of Death Register.
Depression. Adjusted hazard ratios (HRs) with confidence interval (CI) show significant relationships between fitness and severe depression for low compared to high fitness: HR (CI) 1.80 (1.64-1.99) for cardiovascular fitness and 1.43 (1.32-1.55) for muscle strength. Bipolar disorders exhibited a lower, yet significant association with cardiovascular fitness and muscle strength (adjusted hazard ratios (HRs) with confidence interval (CI): 1.35 (1.12-1.63) and 1.22 (1.05-1.42), respectively.
Stroke. Adjusted HRs show inverse relationships between fitness and stroke for low compared to high fitness HR (CI) 1.70 (1.50-1.93) for cardiovascular fitness and 1.39 (1.27-1.53) for muscle strength. All three stroke types (subarachnoidal hemorrhage, intracerebral hemorrhage and ischemic stroke displayed similar associations. There were stronger associations for fatal stroke.
Dementia. Low compared to high cardiovascular fitness was significantly associated with an increased risk for diagnosis of dementia before age 60: HR (CI) 2.49 (1.87-3.32). Mild cognitive impairment (MCI) showed an even greater association: HR (CI) 3.57 (2.23-5.74). The combination of low cardiovascular fitness and poor cognitive performance in early adulthood was associated with a >8-fold increase in the risk of early-onset MCI and a >7-fold increased risk of early-onset dementia.
In summary, this study sheds light on physical fitness as an early-life risk factor for psychiatric and neurological conditions, and underlines the potential for lifestyle modification to reduce the risk of late-life CNS impairments.