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Walking: Wonder Drug for Your Brain

An Emerging Story

The effect of physical activity on brain structure and function is a fast emerging story; each new discovery strengthens the belief that we can substantially influence our mental state, especially as we get older. Greek philosophers believed exercise kept the mind vigorous. Modern technology is proving it with increasing specificity. In 1998, we discovered that humans can grow new brain cells. In 2006, researchers reported that only three months of brisk walking gave people the brain volume of those three years younger. (Article #182, Fitness & Health) Now we have a long-term study that quantifies how much walking is required to significantly increase the gray matter in the brain that supports memory and mental ability, and another study that looks at the effect of exercise on the coordination between different parts on the brain over a one year period. 

The long-term study was co-authored by Kirk I. Erickson, PhD, and Cyrus A. Raji, PhD, (and colleagues) and published online October 13, 2010, in the journal Neurology.

Physical Activity Predicts Gray Matter Volume

In the Erickson-Raji study of 299 cognitively normal Pittsburg seniors (average age 78), those walking at least 72 blocks weekly (about 6 miles) had significantly greater gray matter volume in MRI scans nine years later and better cognitive function 13 years later. The study suggests that a manageable amount of exercise (walking a little less than a mile a day) promotes better brain health and function in later years.

Participants were divided into four groups based on weekly walking distance at the start of the study. The groups averaged 8, 21, 45, and 156 blocks per week. When scanned nine years later, those in the first three groups did not show significant difference in brain volumes, although the trend was toward greater volume with increased activity. But the group averaging 156 blocks weekly had markedly greater volumes in the gray-matter areas of the brain.

The minimum amount of walking required to show an increased gray-matter volume was 72 blocks. Walking more than 72 blocks was not associated with significantly greater gray-matter volume.

Walking amounts ranged from 0 to 300 blocks. The sweet spot seems to be about 72 blocks a week, which would be about a mile and a quarter a day, assuming you walk 5 days a week.

“Gray matter (GM) volume shrinks in late adulthood, often preceding and leading to cognitive impairment,” the researchers wrote. Based on earlier results, they predicted that “participants who were more physically active would have greater GM volume…and that greater GM volume would be associated with a reduced risk of developing cognitive impairment.” As indicated above, that’s what they found 13 years after the earlier physical activity evaluation.  

At the final 13-year followup, 116 participants were found to have some cognitive impairment, while the other 183 subjects still had normal cognition.

“Greater gray matter volume with physical activity reduced the risk of cognitive impairment two-fold,” they reported.

Mechanism and Conclusion

While Erickson and colleagues were not able to identify an underlying mechanism explaining the effect of exercise on brain volume and function, they did offer several educated guesses. “Our results are in line with data that aerobic activity induces a host of cellular cascades that could conceivably increase gray matter volume. For example, running enhances learning and promotes the proliferation and survival of new neurons in the hippocampus. The addition of new cells requires increased nutrients, which are supplied by new vasculature. In mouse models of Alzheimer’s disease, exercising animals show a reduction in B-amyloid deposits…and superior learning rates compared to sedentary animals.”

They also listed several limitations on their findings. First, self-reported activity is not as reliable as objective assessment. Second, it would’ve been helpful to have measures of brain volume at several time points to more accurately determine the rate of gray matter decay. Finally, the observational nature of the study doesn’t allow them to make a cause and effect determination. (Only a controlled intervention can do that.) “Despite controlling for several health factors that could have explained the gray matter-walking relationship, there remains a possibility that reduced amounts of walking is the result of ill health and that ill health leads to both reduced amounts of walking and gray matter volume loss.” 

Importantly, they were able to conclude: “Greater walking distances are associated with greater gray matter volume in a time period of life in which cortical deterioration and risk for dementia is greatest." 

Now, let’s move on to the second study, which adds more bricks to the connection between exercise and brain function.

This enlightening study is a randomized intervention trial reported August 26, 2010, in Frontiers in Aging Neuroscience. Dr. Arthur F. Kramer, Department of Psychology, Beckman Institute, University of Illinois at Urbana-Champaign, is the lead author.

Brisk Walking Improves Brain Connectivity

Dr. Kramer’s one-year controlled trial found that walking coordinates brain activity.

“Research has shown that the human brain is organized into separable functional networks during rest and varied states of cognition, and that aging is associated with specific network dysfunctions,” Kramer and his colleagues wrote. “[This] study provides the first evidence for exercise-induced functional plasticity in large-scale brain systems in the aging brain,” they added. This is important because connectivity between brain circuits affects how well we do everyday tasks, such as driving. 

“Almost nothing in the brain gets done by one area—it’s more of a circuit,” Kramer explained to Science Daily. “These networks can become more or less connected. In general, as we get older, they become less connected, so we were interested in the effects of fitness on connectivity of brain networks that show the most dysfunction with age.”

 Scientists have identified several distinct brain networks. Often studied is the default mode network (DMN), which controls brain activity when a person is passive or mentally at rest. Studies have found that a loss of coordination in the DMN is a common symptom of aging and in extreme cases can be a marker of disease. The aging brain can have trouble turning off the DMN when switching from resting to active mode.

“That exercise is associated with improved resting or task-independent cortical function suggests exercise affects the most basic aspects of the brain’s functional architecture,” the researchers explained.

Kramer’s team randomly divided 70 previously sedentary adults (ages 60 to 80) into two groups. One group did aerobic walking, while the other group served as controls, doing flexibility, toning, and balancing (FTB) exercises.

Brain function was measured using functional magnetic resonance imaging (FMRI) to monitor brain networks and determine whether aerobic exercise increased connectivity in the DMN and two other brain networks known to implement complex cognition. The researchers measured brain connectivity and performance on cognitive tasks at three time points: before training began, at six months, and after a year of either walking or stretching, toning, and balancing (FTB).

As you’d expect, the scanning process was highly technical; it takes a neuroscientist to fully comprehend the procedure. But simply put, they measured and compared brain activity in specific areas during three passive viewing tasks and while engaged in four separate tests of cognitive function (Digit span recall, Spatial working memory, Task-switching, and Card sorting).

The aerobic group walked three times a week. They started with 10 minutes and increased by 5 minutes each week until they reached 40 minutes at week seven. They continued walking 40 minutes per session for the rest of the program. All walkers wore heart monitors and were encouraged to stay in their target heart rate zone: 50-60% of maximum heart rate reserve (difference between resting and maximum) during weeks one to seven and 60-75% thereafter. Participants kept an exercise log, which was reviewed every four weeks.

For comparison purposes, a group of young adults, ages 20 to 30, was also tested for brain function. This allowed the researchers to identify “peak regions of age-related disruption in brain networks.” It also highlighted the effect of exercise on the aging brain. 

The results were quite encouraging. Kramer and his team hypothesized that aerobic training would result in a shift toward the younger state and functional organization more like young adults. That’s what happened. As the older people became more fit, the aerobic exercise improved their memory, attention, and several other cognitive processes. Amazingly, the coherence among regions in the brain networks increased so much that it mimicked that of the 20-something comparison group.

“The study demonstrated that moderate exercise enhances functional connectivity between regions with age-related disruption in cognitively relevant brain networks,” Kramer’s team reported.

Specifically, by the end of the year, DMN connectivity was significantly improved in the brains of the older walkers, but not to the same extent in the older controls (FTB). The walkers also had increased connectivity in parts of another brain circuit called the fronto-executive network, which aids in the performance of complex tasks.

(The (FTB) control group also showed some increase in functional connectivity; see more below.) 

The walkers also did significantly better on cognitive tests, compared to the control group.

 “The higher the connectivity, the better the performance on some cognitive tasks, especially the ones we call executive control tasks—things like planning, scheduling, dealing with ambiguity, working memory, and multitasking,” Kramer explained. These are the skills that tend to decline with aging, he emphasized. 

Importantly, the improvement didn’t happen over night. It took a full year of walking for the results to appear. Testing at six months showed no significant brain changes.

“Overall this study represents an important extension of previous knowledge on the effect of exercise on the aging brain,” Kramer’s and colleagues wrote in summary. “We have shown the aging brain is plastic and responsive to changes in exercise behavior, and that this effect is modulated by length of training.”

*  *  *

The Kramer study shows that aerobic exercise improves brain function, but we also have evidence that other forms of exercise improve mental function.

A new study from Canada found that once-a-week progressive resistance training also improved executive cognitive function. For full details, see http://www.cbass.com/Aerobics&StrengthTraining&Brain.htm 

Another recent study revealed that resistance training induces a robust transient increase in circulating brain-derived neurotrophic factor (BDNF). As you may recall, Harvard psychiatrist John J. Ratey called BDNF “Miracle-Gro for the brain.” See http://www.cbass.com/Strengthtraininggoodforbrain.htm  

Walking is, of course, only one form of aerobic exercise; there are many others, including dancing, biking, rowing, and swimming. In my experience, a variety of aerobic exercise is more appealing and more effective, especially over the long term. In seems logical that a varied program of aerobic exercise and strength training is also best for improving and maintaining overall brain function. As Marco Pahor, MD, professor, Department of Aging  and Geriatrics, University of Florida, wrote recently: “[It may be] that physical activity in any form can improve cognitive function.”

In fact, Dr. Kramer’s study provides support for this concept.

As indicated above, the flexibility, toning, and balancing group (the controls) also increased connectivity, but in a more limited combination of networks. Kramer and his colleagues say this may reflect “experience-dependent plasticity.” Specifically, the controls responded to “activities that sequentially became more difficult over the first 6 months of the program and thereafter…were maintained.” They had to learn new movements and stay focused during the flexibility, toning, and balancing sessions. The FTB program challenged their brains as well as their bodies and both apparently responded.

Brain cells, old and new, must be put to work if they are to be maintained. It’s use it or lose it. So it stands to reason that a varied and challenging exercise program (aerobic, strength, flexibility, balance, etc) is more likely to produce the coveted “sound mind in a sound body.”

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