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Why we have more “senior moments” as we age

It has been more than four decades since I was twenty and a Trivial Pursuits star.  In those feisty days I could remember every movie I had ever seen, every book read, every TV show seen.  Not now.  Ask me the star of a movie I know quite well, and I will often draw a blank, the name popping into my mind only later when I am doing something else and no longer thinking about the movie at all.  Then I blurt the name out loud, as if to say, “See, I knew I knew it!” 

A “senior moment” is the graceful name for this sort of mental hiccup, a not-too-subtle reference to the observation that the frequency of these mental slips increases as we get older, and that they seem to affect all of us – a sort of geezers rite-of-passage.

As a scientist, what am I to make of this?  My doctors tell me that I exhibit no signs of early Alzheimers.  So what is going on?  Other than slenderness and hair, what have I lost in those four decades?

Studies published a few months ago shed considerable light on this matter.  Dr. Scott Small of Columbia University Medical Center led a research team that looked closely at the brains of aging people to test an interesting idea.  It has been known for some time that glucose regulation – the ability of your body to maintain blood glucose levels within a narrow range – worsens with age, starting to deteriorate by the third or fourth decade of life.  Like the age-related cognitive decline (medical speak for senior moments), spurts of heightened blood sugar occur more often in older people.  Could one aging phenomenon be related to the other?

Yes.  The Columbia University researchers used high-resolution functional magnetic resonance imaging to peer into the brains of their subjects.  One of the most recently developed forms of MRI scan, functional imaging measures changes in blood flow within local regions of the brain, and how much of the blood’s hemoglobin at that place is carrying oxygen. 

Nerve cells do not have internal reserves of energy.  When they begin conducting nerve impulses in the brain, they start to consume oxygen at a furious rate to produce the needed energy metabolically -- oxygen carried to the nerve cells by hemoglobin molecules within red blood cells.

How does functional MRI work? When a particular region of the brain becomes momentarily more active, there is an increase in blood flow to it, a pulse that lasts for 4-5 seconds. This leads to a brief spike in blood volume and % oxygenated hemoglobin in the active brain region, a spike that functional MRI can detect and record. 

Dr. Small’s team used this sort of functional MRI imaging to map brain regions in 240 elderly subjects, quite a large study for such a sophisticated analysis.  Each subject was studied before and after eating, to see the impact of elevated blood glucose levels on various regions of the brain.

Most of the brain showed no consistent response, with one glaring exception. The high glucose levels correlate with reduced blood flow in a region of the brain’s hippocampus called the dentate gyrus.  It seems that heightened blood sugar leads to reduced metabolic activity in that region of the brain.  Sugar is turning something “OFF,” and that is where it is doing it.

The researchers went on to manipulate blood sugar levels directly in the brains of mice and monkeys, and concluded that there was a cause-and-effect relationship: glucose spikes lead to reduced brain function in the dentate gyrus.

So what?  What does the dentate gyrus do?  It helps form and access memories.  Ah!  Now the mystery fades into clarity.  Poorer blood glucose regulation as we age degrades our ability to process memories.

“It’s part of the normal process of aging,” Dr. Small said, “much like wrinkling of skin. It happens to all of us inexorably, and it worsens progressively across the life span.” 

Nice to know that I am not alone in having “senior moments” – but there is a clinker in all this, a consequence of which I am all too aware.  Like a disturbingly large number of people these days, I am a Type II diabetic.  My body has lost the ability to regulate blood glucose levels on its own, and can only do so with the help of a fist full of pills every day. 

Looking into this with some fear and trepidation, I find that my worry is justified: there are other studies confirming a link between Type 2 diabetes and dysfunction of the dentate gyrus.  Great.  Not only does my diabetes increase my risk of heart disease, retinal damage, and limb loss, it also increases my risk of cognitive decline.  Just what I needed to hear. 

Actually, becoming aware of this is good news.  I cannot change the facts: I am older and a diabetic.  But it turns out this particular consequence is to a large degree avoidable, once identified and understood.  Glucose regulation is improved with physical activity.  Physical exercise should in large measure compensate for the effects of aging, and is certainly a key component of dealing with Type 2 diabetes.

So now I bound around a lot, looking a little silly as I hoof it 45 minutes each way to a hour’s physical training session, three days a week.  That’s a lot of time to invest, but I tuck it in at the end of the day, easy once it becomes a habit. 

Dieting helps too.  A study released a few months ago by German researchers led by Dr. Agnes Floel of the University of Munster found that calorie restriction produced an average 20% improvement in memory performance.  Reading this, I was so encouraged that I told my wife about Dr. Floel’s finding.  Big mistake.  Goodbye Big Macs.

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