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The OP wrote: "Keeping mentally and physically active when young and
middle-aged can help prevent the brain degeneration associated with
Alzheimer's disease, research suggests."

Mary responds:I'm always suspicious of claims that wander into the
neighbourhood of the error in logic called "post hoc, ergo, proctor
hoc" - or after this, therefore because of this. Just because one event
follows another, that doesn't mean the first caused the second - most
especially given how poorly and inadequately diagnosed most cases of
dementia (I'm willing to bet money that a huge whack of what gets
called Alzheimer's are actually caused by some other disease process
entirely).

You have to wonder if the physical and mental activity is the cause of
the intact brain into old age, or instead, the result of an intact
brain into old age.

I can certainly see robust physical health and having taking good care
of ones self resulting in fewer cases of dementias due to strokes and
other health complications that impact cognition etc.

However, I'm not convinced the same argument is valid when it comes to
some of the other dementias, such as Alzheimer's. We don't even know
what the trigger is, let alone what we might do to prevent it.

Mary

There are many studies that show that it makes no difference.

There are a number of things involved in the issues of mental and
physical activity.

Taking mental first, the famous nun's study was a study of women
in a convent who had been required as part of their duties at the
convent to keep diaries.  The researchers who studied the nuns
read the diaries and rated each one for intellectual
sophistication.  They found, to their surprise, that the diaries
showing more sophistication belonged to women who exhibited fewer
behavioral symptoms of Alzheimers than women of the same age who
had simpler diaries.

I heard an interview with Trey Sunderland, an Alzheimer's
researcher at NIH in which he discussed this study.  He theorized
that intellectual sophistication has no effect on the development
of beta-amyloid plaques or neurofibrillary tangles - the obvious
physiological changes in the brain associated with Alzheimer's,
but _would_ affect people's behavioral responses to brain damage.
He thought that a person with six ways of thinking about
something could lose one or two and compensate very well.  But a
person with only one or two ways of thinking about something
could not compensate for similar losses.  Both people might have
the same amount of brain damage, but one appears demented and one
does not.

So his theory was that mental activity has no physiological
protective effect, but does affect one's ability to battle the
mental losses caused by the disease.

According to Sunderland, we know from autopsies of the nuns that
some women who appeared mentally normal when they died were found
to have had serious Alzheimer's type brain damage when autopsied
after death.  This is a piece of evidence for his view.

Now as to the theory of physical exercise, as I understand it, it
is not just any physical activity that provides protection, but
cardio-vascular activity.  Gardening, milking cows, maybe even
weight-lifting, wouldn't help, or wouldn't help much.  But
running, bicycling, or other exercise that got the heart rate up
for significant periods might very well help.

This makes a lot of sense if there is any cardio-vascular
component to Alzheimer's Disease.  Whether there is or not is
beyond my competence to say, however we do know (don't we?) that
people who have had small strokes and/or high blood pressure,
both cardio-vascular injuries or diseases, have a higher
incidence of Alzheimer's.

Finally, it is not valid reasoning to look at a person and say,
"He or she was active and intelligent all his life and still got
Alzheimer's, therefore the theory that mental and physical
exercise is protective can't be right."

Theories about the protective effect of Alzheimer's are based on
statistical studies, not on individual cases.  Any individual
might fall anywhere on the exercise spectrum and still get AD.
But it might still very well be the case that _on average_ people
with more activity get less AD.

Even in individual cases, a highly active person who still got AD
might very well have gotten it sooner had he not been active.

As for me, I work out on those damn exercise machines and strain
my brain to learn new things as best I can.  I don't know if it
will help.  Maybe it won't.  But I'm playing the odds.

   Alan