Exercise may be good for you, but it doesn’t boost your memory

“Moderate exercise such as walking boosts memory power” claims the BBC.

“Exercise in middle age can improve your memory” says the Daily Mail.

“Older adults improve memory through exercise” reports CNN.

“Want to improve your memory? Take a walk” invites Time.

These are just some of the many headlines today resulting from the publication of a paper in Proceedings of the National Academy of Sciences (PNAS) entitled “Exercise training increases size of hippocampus and improves memory”, by Kirk Erickson from the University of Pittsburgh and colleagues from the University of Illinois. You can access the abstract of the article here, although you may need to pay to read the whole paper.

The authors of the article have provided helpfully excited quotes in many of the news stories.  For example, senior author Art Kramer, from the University of Illinois, was quoted as saying “even modest amounts of exercise ... can lead to substantial improvements in memory.”  Exciting findings indeed!

In the study, 120 older adult volunteers were randomly assigned to an aerobic exercise group or a stretching control group. Participants in the exercise group undertook supervised sessions of moderate intensity walking for 40 min each day, three days per week for one year.  Those in the stretching control group were trained in various stretching and muscle toning exercises over a similar period.  At the start, middle and end of the intervention, structural MRI scans were obtained, and participants undertook a computerized spatial memory task.

The MRI scans revealed that volume of the hippocampus, a brain area known to be important for spatial memory, increased by around 2% in the exercise group participants over the one-year period.  Hippocampal volumes of participants in the stretching control group diminished by around 1.4% over the same period, as would be expected through age-related decline.

However, as illustrated by the headlines above, minor increases in volume of a brain structure with exercise appear to be less newsworthy than the suggestion that such exercise might “boost memory power”.  A common worry among older adults is that forgetfulness, which may seem to be increasingly pervasive as years go by, will eventually result in the loss of their precious store of memories.  If exercise really could “boost memory”, and potentially alleviate age-related memory decline, this would indeed be big news.  Thus, it is this claim that is worth looking at more closely.

Unfortunately, to find the data on this issue, one has to study the paper very closely indeed, because the authors have not made it particularly easy to locate.  However, buried in the text of the results section and in one row of a complex data table, one can read that the exercise group did indeed show increases in memory performance over the year period, going from an average score of 85.9% at the start of the intervention to 88.2% after 12 months, a 2.3% mean difference.  However, the suggestion that the exercise regime was responsible for this “memory boost” is rather undermined by the observation that the stretching control group showed a 3.7% mean increase in their memory performance over the same period.

In other words, the exercise group did not show any increase in memory performance relative to the control group.  Both groups showed similar small increases in spatial memory scores over the three testing sessions, which may be attributable to the well-known beneficial effects of practice when performing the same task repeatedly.

The fact that memory performance in the exercise group was no different to that achieved by the control group is a critical flaw in this study, and severely undermines the claims made throughout the paper, from the title onwards, that exercise training boosts memory.  For example, in the abstract the authors state that “exercise training ... is accompanied by improved memory function.”  Notably, in the abstract, discussion and main figures, no mention is made of the statistically identical memory boost in the control group.  Indeed the main figure only displays memory data from the exercise group, “because it was the only group that showed an increase in volume across the intervention”, according to the figure legend.  Why not show the data from both groups?  Presumably because the control group’s data would reveal a negative correlation between hippocampal volume and memory scores, weakening the authors’ claims considerably.

Thus, this appears not to be a story about misrepresentation of research by journalists, although there is much evidence that such errors do occur.  Rather, it seems to be an example of the scientists involved in the research “talking up” their findings for the press and even, perhaps, obscuring the true nature of the results in the journal article.

One can also question the quality of the peer review and editorial control process in a journal that published such obviously flawed research.  It is worth noting that the article states that it “had a prearranged editor”, in this case Fred Gage from the Salk Institute.  There have been a number of previous discussions about the quality of the research published in PNAS (see e.g., here), mostly relating to a former article submission method, in which members of the National Academy of Science (NAS) could “arrange” publication of papers from non-members each year.  Although this submission track no longer exists, the journal maintains the option for authors to “prearrange” for an NAS member to edit their article.  It may well be the case that the editorial process in this instance was conducted with total care and probity.  As a general point, though, it is difficult to believe that the “prearranged editor” option can be as impartial and rigorous as one might wish to be the case.

To sum up, the findings presented in this article do not support the notion that exercise will boost your memory, or will stop age-related memory decline.  Whereas, of course, it won't hurt anyone to do more exercise, it is unfortunate that potentially vulnerable older people may be misled by this article and its attendant news coverage into thinking that the exercise will cure their memory problems.