Human Limits

Last week in the Opinion section of the New York Times, the food writer Mark Bittman made the case in layman’s terms that Alzheimer’s disease might be considered “type 3” diabetes.   There are two main lines of evidence in support of this idea.  The first is that glucose and insulin in large amounts do bad things to the brain.   In type 2 diabetes blood levels of both glucose and insulin can be high.   The second is that diabetes is a risk factor for Alzheimer’s disease, and the increased rates of diabetes and Alzheimer’s in the population tend track each other.  However, the story is a bit more complex than simple “type 3” diabetes.

The current thinking is that Alzheimer’s is caused by the buildup of one or more proteins (one of the major ones is called beta-amyloid) in the brain that damage nerve cells and ultimately cause brain function to decline.  However, there are holes in the protein theories and drug therapies designed to lower the levels of amyloid have not improved brain function in clinical trials.   It is also interesting to note that in addition to diabetes, several behavioral risk factors seem to put people at increased risk for Alzheimer’s disease.

When I look at these risk factors I am struck by how similar they are to traditional risk factors for heart disease.   For both heart disease and Alzheimer’s, diabetes, hypertension, obesity, inactivity, and smoking all damage blood vessels and their negative effects tend to multiply in people with more than one risk factor.   Also, the only genetic risk factor with a big effect in Alzheimer’s some something called ApoE4 which affects cholesterol metabolism and can also damage blood vessels.  To me this suggests that there is an interaction between the buildup of “bad” proteins in the brain and poor blood flow to the brain.   There is also some evidence that exercise causes an increase in the levels of something called BDNF (brain derived neurotrophic factor), which promotes the growth of new brain cells even in middle aged and older people.

If you take the evidence outlined above a step further and ask what effect improvements in “life style” might have on Alzheimer’s the data is pretty startling.  Here is a quote from the summary of the key paper on behavioral risk factors mentioned above.

“At present, about 33.9 million people worldwide have Alzheimer’s disease (AD), and the prevalence is expected to triple over the next 40 years. The aim of this Review was to summarize the evidence regarding seven potentially modifiable risk factors for AD: diabetes, midlife hypertension, midlife obesity, smoking, depression, cognitive inactivity or low educational attainment, and physical inactivity…….Together, up to half of AD cases worldwide (17.2 million) and in the USA (2.9 million) are potentially attributable to these factors. “

Who knows if there will ever be a simple explanation for Alzheimer’s disease that will lead to effective drug therapy.  However, it is pretty clear that what is good for your heart is good for your brain.

This entry was posted on Monday, October 1st, 2012 at 6:04 am and is filed under Research and Health. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.