WASHINGTON, Nov 8 (Reuters) - Exercise fanatics may be right -- getting out and moving increases blood flow in the brain, U.S. researchers said on Saturday.

Tests on monkeys show that exercise helps foster blood vessel development in the brain, making the animals more alert than non-exercisers.

"What we found was a higher brain capillary volume in those monkeys who exercised than in those monkeys who did not," Judy Cameron of the divisions of Reproductive Sciences and Neuroscience at Oregon Health & Science University said in a statement.

"Specifically, changes were most noted in older animals that were less fit at the start of the study," she added in a statement.

"The next step of this research is to determine whether other areas of the brain undergo physical changes. For instance, how are brain cells affected and does that impact cognitive performance."

Cameron, who presented her findings to a meeting of the Society for Neuroscience in New Orleans, said the findings should help explain why exercise also seems to make people more alert.

"While we already know that exercise is good for the heart and reduces the incidence of obesity, this study shows exercise can literally cause physical changes in the brain," she said.

"Furthermore, we believe the study results show exercise causes a person to be more engaged and provides another reason for Americans to make physical activity part of their daily regimen. This is especially true in the case of older Americans with whom decline in mental function over time is a common occurrence."

For their study they separated 24 monkeys into three groups.

One group exercised on treadmills for a set distance five days a week. A second group did not exercise, and a third group exercised for 20 weeks and then remained sedentary.

They measured the volume of small blood vessels, called capillaries, in the motor cortex region of the brain in all three groups of monkeys.

They also ran several tests on the monkeys' mental abilities.

In one, a treat is placed under two toys. After a brief delay, the monkey was allowed to find the treat.

Exercisers were "more aroused, alert and engaged", Cameron said, although they did not find the treats any faster.