Gut Check is a periodic look at health claims made by studies, newsmakers, or conventional wisdom. We ask: Should you believe this?

The Claim:

Physical exercise benefits the aging brain, slowing the decline in memory and “executive function” that occurs as people get older.

The Background:

If the return of warmer weather isn’t enough to put brisk walks and Ultimate Frisbee back on your schedule, then the regular reminders that exercise is good for the aging brain might be. In fact, this year brings the 50th anniversary of a study reporting that lab rats remember and learn better after they start using a running wheel. That unleashed what has become a torrent of research on whether physical activity improves brain function in older adults by countering the harmful effects of aging.

It’s easy to get the impression that this science is settled. Hundreds of studies have reported encouraging findings: physically active older adults have a larger hippocampus (the structure that encodes memories) and prefrontal cortex (responsible for executive functions such as task switching, ignoring distractions, and planning). Scientists have also reported that active older people have more and younger-looking white matter (the “wires” that connect neurons), more efficient brain activity, superior executive function, and better memory. Research reviews regularly make statements such as “extensive research confirms the benefits of physical exercise for brain health.”

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First Take:

Usually, Gut Check gives more credence to randomized controlled studies than to observational ones, since the latter can’t reliably tell whether the outcome you’re interested in (better brain function) is due to the factor you suspect (physical activity), or one you don’t (higher socioeconomic status or higher health awareness, for example). But on this question, the randomized controlled trials might be giving misleading results.

The observational studies have been huge: they commonly include more than 1,000 people. They’ve also been consistent, finding that more physical activity goes along with less cognitive decline in older adults, especially those 65 and older. Women seem to benefit more than men, and aerobics plus strength conditioning seems better than aerobics alone.

“The credibility” of the observational studies “comes from their replicability,” said psychologist Kirk Erickson of the University of Pittsburgh, who has studied exercise and brain health for more than a decade. That credibility comes, too, from their agreement with studies of lab animals, which have identified molecular and cellular mechanisms by which physical activity can benefit the aging brain. For instance, it raises levels of BDNF, a protein that acts like fertilizer for neurons (as some but not all human studies have found).

Randomized controlled trials of physical activity and age-related brain function — assigning some volunteers to exercise and others not to, or to some non-exercise activity — have been more equivocal.

Many of those studies have found functional improvements, mostly in executive functions controlled by the prefrontal cortex, and in memory, controlled by the hippocampus. Some have found increases in the volume of gray matter, as a 2006 study did. But others have not found previously reported benefits, leaving things unsettled.

Assessing the plethora of results from randomized trials, a 2014 analysis called them “largely inconsistent.” A 2015 report from the Institute of Medicine cautioned that “studies with the highest quality [and] longest follow-ups”— meaning randomized trials — have yielded the most modest findings about the value of exercise, a downer of a conclusion that seriously rattled the field.

Second Take:

Randomized trials sit at the apex of the “hierarchy of evidence” in biomedicine, considered more rigorous and reliable than observational studies and animal research. But on this question, that might not be so.

For starters, most randomized controlled studies have few participants. That makes it hard for them to show an exercise effect; as statisticians say, the studies are “underpowered.”

In addition, many include volunteers who are already physically active. The assigned exercise might not have additional benefits if those benefits plateau at volunteers’ preexisting level of physical activity.

Also, in many of the randomized studies, only some — even as few as half — of the volunteers assigned to the exercise group actually exercise as requested. Any benefit from exercise in the “exercise group” would, therefore, be diluted by the fact that only some volunteers are actually exercising. Yet they’re counted as doing so.

“It shouldn’t surprise us that the human randomized trials are the weakest of the three” sources of evidence, said psychologist Arthur Kramer of the University of Illinois at Urbana-Champaign, a pioneer in this field.

That said, exercise is not a cognitive fountain of youth. For one thing, a lot doesn’t seem to be much better than a little. Fifteen observational studies, taken together, found a 38 percent reduction in risk of cognitive decline in healthy participants who were very active, but nearly as great a reduction — 35 percent — in those with low to moderate activity, a 2011 analysis found.

Kramer and his long-time collaborator, the University of Pittsburgh’s Erickson, said there is “modest support” for the claim that moderate to intensive exercise, such as vigorous walking, can “preserve and even enhance cognitive vitality” in older adults, and bring about “a modest reduction” in the risk of cognitive decline as a result of such exercise.

What’s “modest”? After a year of a vigorous walking regimen, healthy older adults had a 2 percent increase in the size of their hippocampus, compared with a 1.4 percent decline in the non-walkers, their 2011 study reported. That works out to turning back the calendar by one to two years.

The Takeaway:

STAT wishes the studies on exercise and the aging brain were cleaner, but the weight of the evidence says there is a modest cognitive benefit from physical exercise.

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