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Based on a meta-analysis of 20 studies, mental performance may be improved with even modest weight loss among overweight individuals, though that may be due not to the weight loss per se, but rather the prescribed exercise. For example, one study randomized obese elders to one of four groups for a year: a weight-loss diet alone, exercise without weight loss, both, or neither. Compared to the control group that did neither, global cognition improved in all the three other groups, but while weight loss and exercise beat out weight loss alone, weight loss and exercise didn’t beat the exercise alone, suggesting that exercise is the most potent component.
The American College of Sports Medicine cites meta-analyses of dozens of prospective studies following tens of thousands of people for years that suggest that those who are active have a 38 percent lower risk of cognitive decline, and a 39 percent lower risk of developing Alzheimer’s disease. And, apparently, the more the better. Every extra 500 calories expended a week appears to correlate with an additional 10 percent drop in dementia risk. However, even light physical activity, such as low-intensity walking, or how yoga and tai chi are often practiced by older adults, has been associated with at least some sort of cognitive gain in about half of the studies done to date.
If the relationship between exercise and dementia prevention is cause-and-effect, as much as 20 percent of all cases of Alzheimer’s disease in Western countries may be attributable to physical inactivity. That would mean more than one million cases of Alzheimer’s just in the U.S. alone. As with all observational data, though, there’s the potential for confounding or reverse causation. Maybe there’s a shared factor causing both, such as atherosclerosis, contributing to both inactivity and dementia, or maybe those genetically predisposed to exercise a lot also have a more robust cognitive reserve (though a Mendelian randomization study failed to find this). Instead of inactivity leading to dementia, dementia could lead to inactivity. Prospective studies following people over time can show the inactivity does precede the dementia, but both mental and physical activity can start declining years before a diagnosis. When researchers looked closely at the data, only inactivity less than 10 years before the diagnosis was associated with inactivity, suggesting the causality may indeed be backward. When put to the test, interventional trials show physical activity can be increased in dementia patients, which can improve physical fitness, but disappointingly did not slow cognitive impairment.
Perhaps we’re just not catching people early enough? Adults with either normal cognition or mild cognitive impairment randomized to exercise training tend to improve their cognition, compared to those ending up in the less active control groups. A slight majority of studies that concurrently tracked brain imaging found those randomized to exercise experienced a beneficial effect on overall brain volume, as well as specifically the hippocampus, a part of the brain that plays a critical role in memory formation and for which atrophy predicts memory decline and dementia.
The seminal study, “Exercise training increases size of hippocampus and improves memory,” randomized older adults to either a year of supervised moderate aerobic exercise (walking), or a control group, which instead involved a year of stretching and toning classes. On average, the hippocampus volume of those in the control group shrank 1.4 percent, which is consistent with the 1 to 2 percent annual shrinkage typical of old age. In contrast, over that same year in the walking group, their hippocampus volume increased 2 percent, which is like adding a year or two of brain volume (though may subsequently shrink back to baseline in as few as six weeks of inactivity; so, you got to keep it up).
A similar study that added a third group—a resistance training arm—found a similar aerobic training reversal of age-related shrinkage in the memory centers, compared to the balance and toning control group, but unfortunately there was no benefit seen in the nonaerobic strength-training group. It’s unclear exactly which type of exercise is best, but the cognitive benefit of aerobic training appears to be larger than resistance training. In most of the randomized controlled trials of older adults, walking was the most frequently chosen mode of exercise. The best volume, duration, frequency, or intensity is also unknown. Based on a meta-analysis of nearly a hundred randomized controlled trials, more important than session duration, weekly frequency, program duration, or intensity, may be total training time. However studies spliced it up, it appeared to take exercising for a total of at least 52 hours before a cognitive benefit can be established.
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