A study of 1.1 million Swedish men suggests that exercise levels and IQ play a large role in the development of early-onset dementia.

Teens with poor cardiovascular fitness and lower IQ scores are seven times more likely to develop early-onset dementia, according to a new study published in the Oxford University journal Brain.

Alone, mild cognitive impairment quadruples a teen’s risk of early dementia, and poor fitness contributes to a 2.5 times greater risk, the Swedish research team found.

Though dementia encompasses many diseases, Alzheimer’s is the most common form. About 4 percent of the 5 million Americans with the disease are classified as early-onset, meaning that their symptoms begin as early as their 30s or 40s, according to the Alzheimer’s Association.

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Researchers at the Sahlgrenska Academy at the University of Gothenburg examined data from more than 1.1 million men involved in a national cohort study. Beginning at age 18, the men underwent regular exams for 37 years, providing objective data on their fitness levels and intelligence.

After adjusting for outlying factors such as heredity, medical history, and social-economic status, researchers found a connection between age, intelligence, and early-onset dementia.

Previous research makes experts confident that poor cardiovascular health in old age can contribute to the development of dementia.

“Now, for the first time, we can show that the increased risk also applies to early-onset dementia and its precursors,” said lead researcher Jenny Nyberg in a statement.

Early-onset dementia can be particularly hard on people who are working age, often with families to support.

“This makes it important to initiate more research into how physical and mental exercise can affect the prevalence of different types of dementia. Perhaps exercise can be used as both a prophylactic and a treatment for those in the risk zone for early-onset dementia,” Nyberg said.

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While the cause of early-onset dementia has largely eluded experts, the new study adds to the growing body of evidence that links cardiovascular health to protecting the brain.

“We already knew that physical and cognitive exercise reduces the risk of neurological disease. Physical exercise increases nerve cell complexity and function and even generation of new nerve cells in the adult brain, which strengthens our mental and physiological functions,” said senior author Georg Kuhn, a neuroscientist at the University of Gothenburg. “In other words, good cardiovascular fitness makes the brain more resistant to damage and disease.”

Last fall, a study published in the journal Cell Metabolism identified a specific protein released during exercise that promotes brain health. Harvard Medical School researchers say it could be used to develop drugs to guard against neurodegenerative diseases and protect brain function in seniors.

Until those drugs are available, getting regular exercise and challenging your brain are the best ways to stave off mental decline. Eating a diet low in fat and cholesterol has also been shown to reduce a person’s risk of arterial stiffness, which has also been linked to an increased risk of dementia.

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