In 2014 Ferrán Catalá-López and his colleagues from the University of Valencia in Spain reviewed the inverse association between cancer and neurological diseases including dementia. What they reported is that numerous studies have been showing that if you had one of these two diseases you are less likely to get the other. The first anecdotal evidence came more than fifty years ago when patients with Parkinson’s disease were reported to have a lower rate of cancers. More recently, this inverse relationship has also been documented for Alzheimer’s disease. In fact, this inverse relationships is most pronounced with Alzheimer’s disease and Huntington’s disease. While for cancer it is more pronounced for colorectal cancer and lung cancer.
If you have had cancer you are 50% less likely to get Alzheimer’s disease. While, if you have Alzheimer’s disease you are 60-70% less likely to get cancer. The same results do not exist for vascular dementia or Lou Gehrig's disease (ALS) and for some cancers such a melanoma, non-melanoma skin cancer and breast cancer.
There could be a number of reasons for this, and all could be working at the same time. It could be that once you are diagnosed with cancer or Alzheimer’s disease the focus of clinical care is on treatment and there might be less active interest in searching for additional diseases. However, this does not explain why it does not work with other diseases. It could be that the therapy for both diseases protects you from getting the other disease. Although plausible, it is unlikely. It could also be that the two diseases are separated by vulnerability in age and therefore if cancer kills you first you will not have the opportunity to get dementia. While being spared cancer you are then more likely get dementia. Although there are studies that dispel these arguments—some more conclusively than others—there is however a more subtle and persuasive argument.
There is a growing understanding of the chemical balance that is played in the body especially the process of generating energy for cells. The imbalance in this process—known as Glycolysis—of how the body converts sugar into fuel (pyruvate) for cells could be the balance that determines which of these two diseases you are likely to get. Too little fuel for cells—since neurons have such energy demands—and you get Alzheimer’s disease. Too much fuel, which feeds the erratic cells, and you get cancer.
Although this is an interesting avenue for biological and chemical research, there is an additional offshoot of this way of thinking…and that is the rejuvenation of the concept of homeostasis. That along a continuum of cancer or Alzheimer’s disease there is a balance. First described by Claude Bernard in 1865 and later coined by Walter Bradford Cannon in 1926, homeostasis requires three basis mechanisms. A sensor to detect changes, a mechanism that can modify that change, and a feedback connection between the sensor and a mechanism. The concept that homeostasis can determine Alzheimer’s disease has radical repercussions for psychologists because both the sensor and the mechanism can have psychological components. As an example, for the sensor being happy and content, tells the body that the system is in homeostasis, in balance while being stressed tells a different story. For the mechanism, being active, engaging and having tactile and sensory stimulation moderates and modulates our internal chemistry.