Driving a taxi isn't the healthiest profession. The sedentary job and long hours can lead to joint and back pain as well as heart issues.

But in at least one area, taxi drivers do quite well. A new study, released today in The BMJ, shows that taxi drivers die at lower rates from Alzheimer's disease than people in other professions -- potentially because the job involves exercising the parts of the brain that are responsible for navigation day in and day out.

Understanding the reasons behind this association could have important implications for everyone else, too, said Anupam B. Jena, a physician and economist at Harvard who worked on the new study. "Are there things that you could do over your lifetime that might reduce the risk of dementia?"

Taxi drivers have been teaching neuroscientists about the brain for years. Over 20 years ago, a landmark paper showed that compared to other people, London cabbies have a bigger hippocampus, a small, seahorse-shaped part of the brain responsible for learning, memory, and navigating. London cabbies have to take an intensive test called "The Knowledge," which requires them to memorize the thousands of streets in the city.

The new study used recently released data from the Centers for Disease Control and Prevention that connects cause of death to people's occupations. The data showed that while taxi drivers had lower life expectancies (around age 67) compared to the overall life expectancy in the dataset of 74, they died at lower rates from Alzheimer's compared to other people who died at a similar age. Around 3.9% of everyone in the dataset died of Alzheimer's disease, but among taxi drivers, the number was 1%.

"This paper for me was one of these studies [where] I thought, 'I wish I'd been able to do that,'" said Hugo Spiers, a cognitive neuroscientist at the University College London who has studied the brains of London cab drivers. "So I was really pleased to see this, and it fits what I suspected."

The hippocampus is one of the first areas of the brain to break down in Alzheimer's disease. That's why one of the earliest signs of the disease in many patients is subtle issues with memory or navigation, said Scott Small, director of the Alzheimer's Disease Research Center at Columbia University who studies Alzheimer's and the hippocampus but was not part of the new study.

An interesting next step for researchers could be to "image drivers as they age, or with and without early stages of Alzheimer's," he added. Small also noted that he was riding in the back of a taxi during the call: "If you want, I can ask my cab driver" for their thoughts.

The popularity of GPS could affect these kinds of results over time as cabbies rely less on their own internal navigation and more on their phones, experts said. In the BMJ study, people in jobs that involve navigating a predetermined route, like bus drivers, airplane pilots, and ship captains, did not have lower rates of Alzheimer's.

There are other possible explanations for the study's findings: People who have better spatial awareness, and bigger hippocampi, might be more likely to become taxi drivers, or people might take up other work when they begin to experience Alzheimer's-related symptoms.

But a paper by Spiers, which has yet to be peer-reviewed, adds to the theory that the constant navigation required to be a taxi driver could strengthen the hippocampus over time. The paper found that older taxi drivers in London were better able to complete a memory-based task than younger drivers.

Spiers adds that the entorhinal cortex -- the area that is first degraded in Alzheimer's and is responsible for navigation -- is one of the few areas of the brain that can grow cells later in life. But neuroscientists have yet to test whether this is responsible for the larger hippocampi in taxi drivers.

Researchers agreed that one end goal of this line of research is to find ways to mimic the stimulation of the brain that could help protect people from Alzheimer's.

"Prevention is a really important thing when it comes to Alzheimer's," said Vishal Patel, a surgical resident at Brigham and Women's Hospital who was a part of the new study, "just because treatments haven't really worked."