We already know that exercise can help make us smarter, happier, and less stressed. But all exercise is not created equal. If you're a time-crunched entrepreneur, might you like to know which kind of exercise is most beneficial for your brain?

A recent study reveals the answer, and it's a surprisingly simple one that doesn't require athletic skill or teamwork. Conducted by researchers at the National Institutes of Health, the study found that distance running produces positive cognitive benefits.

Over the course of four months, subjects ran for three to four days a week for an hour or more each time. Throughout the study, the researchers gave them memory and thinking tests. As the runners got fitter, their test scores also improved.

The study was also conducted on mice and rhesus monkeys. The researchers also conducted tests on the memories of the animal test subjects. After running, the mice could learn faster and form new memories.

Running produces a brain-boosting enzyme

New York Times science fitness writer Gretchen Reynolds unpacks the deeper results of the study. She writes about an enzyme the scientists honed in on at the start of their research. The enzyme is called Cathepsin B, and it's already known that muscles create this protein as a recovery mechanism--and runners' muscles create a lot of it.

In sum, the scientists found the more Cathepsin B that the human runners and mice test subjects produced, the better they performed on the memory and thinking tests. The researchers even bred mice who could not produce this enzyme, then put them on the same running regime as normal mice. Running had no effect on the non-Cathepsin-B-producing mice; it did not improve their learning or memory retention.

All scientific research comes with caveats. Is endurance running the only exercise in which the body produces Cathepsin B, and thus the only exercise that can improve your memory and ability to retain new information? Of course not, it's just the type of exercise that was used in this particular study.

Additionally, the researchers didn't study the brain-boosting effects of running shorter distances. Henriette van Praag oversaw the study and told Reynolds there's reason to believe any type of exercise is great for brain health. It's entirely possible that running shorter distances could be just as effective.