A study of more than one million 18−year old Swedish men suggests that higher cardiovascular (CV) fitness leads to higher intelligence, while higher muscular strength does not. This not only suggests that exercise is important in adolescence and early adulthood, it also suggests that aerobic exercise is more useful than strength training.

When tracked later in their lives, those with higher CV fitness at age 18 were found to have fared better, going further in school and generally ending up in better occupations.

Specifically, the Swedish study found that higher CV fitness was associated with higher scores on four separate intelligence tests at age 18. It also found that men whose fitness improved between age 15 and 18 scored much higher on the intelligence tests than those whose fitness decreased between the ages of 15 and 18. When tracked later in their lives, those with higher CV fitness at age 18 were found to have fared better, going further in school and generally ending up in better occupations.

Most previous studies of the relationship between physical activity and cognitive function have either been in the elderly or in children. Like the Swedish study, most have shown cognitive benefits from increased physical activity.

The researchers note that young adulthood is the time in which important behavioral habits and cognitive function are shaped. It's the period when academic performance has the biggest effect on future life. And it's a period when the central nervous system is still growing. Getting sufficient exercise during this period can confer lifelong benefits.

Any number of factors could explain why enhanced CV fitness leads to better brain function. These include improved blood flow to the brain, decreased anxiety, enhanced mood and less fatigue.

The study also looked at how much of the link between CV health and intelligence was due to genetic causes and how much was due to environmental factors. Over 250,000 of the subjects were brothers; more than 3,000 were twins, nearly half of whom were identical twins. A statistical analysis suggests that over 80% of the effect on intelligence was due to environmental factors (lifestyle), while less than 15% was due to genetics.

The role of genes vs. environment, sometimes called nature vs. nurture, in the health and life of an individual has been debated for centuries and probably will continue to be debated till the end of time. In this particular study, how a person lived proved much more important than their inheritance.

Suzanne Steinbaum, a preventive cardiologist at Lenox Hill Hospital in New York City and a spokeswoman for the American Heart Association, sees this result as empowering: "You can't determine that exercise or eating well is not going to help you because of your genetic background. This is showing you that, regardless of genes, what you choose to do and how you live can make a difference."

The study used records of men conscripted into military service between 1968 and 1994. Conscription at age 18 is mandatory in Sweden. Upon conscription, the men were given separate tests in logical, verbal, visuospatial and technical intelligence. These scores were combined to give an overall intelligence score. Cardiovascular fitness at age 18 was measured by a cycle ergonometry test. CV fitness at age 15 was estimated from physical education scores in school. Educational and career achievements were tracked through the year 2004.

Details of the study were published November 30, 2009, in an early, online edition of The Proceedings of the National Academy of Sciences (PNAS). The article is freely available.