My patients are surprised when I ask them if they exercise regularly. "What does that have to do with memory?" they ask. A lot, it turns out.

A landmark study conducted in the 1980s and 1990s—the MacArthur Foundation Study of Aging in America—found that sedentary people exhibited greater memory decline than people who engaged in regular physical activity. The active people didn't necessarily work out at a gym, but they did build more activity into their daily schedule. They went for walks almost every day or climbed stairs at home.

We don't know exactly why physical fitness influences brain fitness, but there are a few possible explanations. The most intriguing possibility is that physical activity increases levels of beneficial brain chemicals. In a MacArthur laboratory study, rats that were most active had the highest levels of nerve growth factor, a substance also found in the human brain that helps maintain neurons and repair them after injury. If exercise has the same beneficial effect for us, then it could be one of the best methods we have to keep our neurons in good shape. 87

Exercise might also benefit memory indirectly by keeping the lungs and the cardiovascular system healthy and delivering a steady supply of oxygen to the brain. The MacArthur researchers found that good lung function was one characteristic of elderly individuals who had the strongest memory and overall cognitive function. As for cardiovascular health, it's well established that regular exercise reduces the risk of hypertension, high cholesterol, and heart disease and that, in people who have these conditions, exercising helps keep them under control. Once again, what's good for your heart is also good for your brain.

A 2004 University of Illinois study found that people with high levels of aerobic fitness exhibited greater activation in key brain regions during performance of cognitive tasks as compared with their less fit counterparts. The researchers speculated that cardiovascular fitness leads to increased density of synapses and increased blood flow to active brain regions. All of these effects would be beneficial for brain function and the mitigation of typical aging effects.

So if you don't currently exercise regularly, find ways to be more physically active. Aim for at least thirty minutes of vigorous activity each day, by walking, biking, or doing whatever kind of exercise you enjoy. I describe specific strategies for getting started and finding time to exercise in Chapter 9.

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