A study finds that low cardiovascular fitness and cognitive performance at age 18 is a risk factor for early-onset dementia and mild cognitive impairment later in life.
Early-onset dementia is rare: just 6-10 people out of a population of 10,000 will be diagnosed with early-onset dementia. However, over 35 million people worldwide are currently living with some form of dementia. Understanding early-onset dementia can help researchers pinpoint risk factors that lead to dementia in general. Early-onset dementia is defined as dementia diagnosed before the age of 65. Dementia is not one disease, but rather an assortment of diseases that lead to brain and memory deterioration. Although Alzheimer’s Disease is one of the chief causes of dementia, it is responsible for less than 0.1% of early-onset cases of dementia. A massive longitudinal study published earlier this month in the journal Brain finds two predictors of early-onset dementia: low cognitive performance and low cardiovascular fitness (Cardiovascular and cognitive fitness at age 18 and risk of early-onset dementia, 2014. Nyberg J, et al.).
The study utilized over one million Swedish males who were required to register for the military from 1968 to 2005. When the participants registered at age 18, they were required to take a cardiovascular fitness test as well as a cognitive performance test. The cardiovascular fitness test was performed on a cycle ergometer. The cognitive performance test measured ability in logical, verbal, visuospatial and technical cognition. The cognitive and cardiovascular tests were divided into tertiles: low, medium and high. For the purposes of this study, poor or low performance is defined by being in the lower third.
The researchers found that low cardiovascular fitness and poor performance on the cognitive tests at age 18 led to an increase risk of developing mild cognitive impairment and early-onset dementia. The hazard ratio of low cardiovascular fitness was 2.49 for early-onset dementia and 3.57 for mild cognitive impairment. The hazard ratio of low cognitive performance was 4.11 for early-onset dementia and 3.23 for mild cognitive impairment. Poor performance in both the cardiovascular fitness test and the cognitive performance test generated a hazard ratio of 7.34 for early-onset dementia and a hazard ratio of 8.44 for mild cognitive impairment. It should be noted that hazard ratios measure the amount of people who contract a disease over time, not the the total number of people who contract a disease after x years.
While a 7-fold increase in prevalence of early-onset dementia seems alarming for those who score in the lower third of cardiovascular fitness and cognitive performance, the prevalence of these neurodegenerative diseases is still very small. Although the researchers acknowledge that low cognitive performance may play some role in mediating poor cardiovascular fitness, the significance of cardiovascular fitness held up across a number of confounding factors such as socioeconomic status.
The study authors point to specific neurotropic factors such as brain-derived neurotropic factor (BDNF) and insulin-like growth factor 1 (IGF-1) as potential mediators of cardiovascular fitness on early-onset dementia and mild cognitive impairment. These factors are produced with exercise and increase neuroplasticity in the brain.
This study shows a link between poor cardiovascular fitness and cognitive performance and the diseases associated with mild cognitive impairment and early-onset dementia, but more work must be done to determine the biological mechanism for this correlation.