Dementia is a huge global health concern with the number of patients being diagnosed increasing annually. As a result, the number of annual deaths from dementia is also rising with every year. In England and Wales, for example, dementia is now the leading cause of death, replacing heart disease.
Dementia itself is not a single, specific disease but a term that encompasses symptoms that result from degeneration of the brain. Common examples include memory loss and an inability to perform everyday functions. Multiple diseases result in dementia but the most common is Alzheimer’s disease (AD), accounting for up to 80% of all dementia cases.
Early signs of AD are extremely hard to detect and much of the research is focussing on prevention rather than cure. If we can understand the mechanisms that trigger onset of AD, it may be possible to stop it before it progresses.
Many factors are known to increase the risk of developing the disease but one that is gaining significant coverage is obesity. 1 1. Alford, S., Patel, D., Perakakis, N. & Mantzoros, C. Obesity as a risk factor for Alzheimer's disease: weighing the evidence Obesity Reviews, February 2018, DOI: 10.1111/obr.12629 ×
How Do Obesity and AD Link?
A lot of research has been conducted on links between obesity and AD. The exact ways in which the two conditions link is still not fully understood but scientists generally agree they can be narrowed down to 3 factors.
1 – High Blood Pressure
High blood pressure is a common feature of obesity. However, there is evidence to suggest it can have negative effects on more than just the heart. Recent studies have shown that high blood pressure can also increase your risk of developing Alzheimer’s. 2 2. Roussotte, F. F., Siddarth, P., Merrill, D. A., Narr, K. L., Ercoli, L. M., Martinez, J., Emerson, N. D., Barrio, J. R. & Small, G. W. In vivo brain plaque and tangle burden mediates the association between diastolic blood pressure and cognitive functioning in nondemented adults The American Journal of Geriatric Psychiatry, January 2018, DOI: 10.1016/j.jagp.2017.09.001 × , 3 3. Marfany, A., Sierra, C., Camafort, M., Domenech, M. & Coca, A. High blood pressure, Alzheimer disease and antihypertensive treatment Panminerva medica, January 2018, DOI: 10.23736/S0031-0808.18.03360-8 ×
A common hallmark of AD is the build-up of a protein called amyloid in the brain. These build-ups are toxic to brain cells and are regarded as a major driving factor in disease progression.
Interestingly, research has shown that high blood pressure can increase the production of amyloid and impair the brain’s ability to remove toxic amyloid via the bloodstream. This in turn increases the risk of build-up leading to damage to the brain.
2 – Insulin Resistance
Another factor thought to influence brain health is insulin resistance. This is more commonly associated with diabetes as insulin is an important hormone in regulating blood sugar.
However, insulin is also involved in the formation of synapses (junctions between brain cells). Cells within the memory region of the brain interact with insulin, giving it a major role in memory formation. Insulin, therefore, seems to have a part to play in normal brain function.
As a result, insulin resistance in brain cells can lower the ability of the hormone to maintain healthy brain cells, leading to memory loss 4 4. Arnold, S. E., Arvanitakis, Z., Macauley-Rambach, S. L., Koenig, A. M., Wang, H.-Y., Ahima, R. S., Craft, S., Gandy, S., Buettner, C. & Stoeckel, L. E. Brain insulin resistance in type 2 diabetes and Alzheimer disease: concepts and conundrums Nature Reviews Neurology, January 2018, DOI: 10.1038/nrneurol.2017.185 × .
3 – Damaged Blood Vessels
The third major link between obesity and Alzheimer’s is damaged blood vessels. In a healthy person, brain cells are protected from any chemicals that could be damaging by a structure called the blood brain barrier (BBB).
As the name suggests, the BBB is a physical barrier between the brain and its blood supply. The BBB is commonly termed to be ‘selectively permeable’. This means that only chemicals that are good for the brain can leave the blood supply and reach brain cells.
However, obesity can lead to damage to blood vessels. This damage is not exclusive to heart vessels and can spread to the brain. The consequence is breakdown of the BBB, allowing toxic chemicals to reach brain cells, causing damage that can ultimately lead to Alzheimer’s disease 5 5. Strickland, S. Blood will out: vascular contributions to Alzheimer’s disease The Journal of clinical investigation, February 2018, DOI: 10.1172/JCI97509 × .
Decoding the Facts
Looking at the scientific evidence, the link between obesity and Alzheimer’s seems clear but we must consider the facts with caution. Professor Nigel Hooper runs an Alzheimer’s research lab at the University of Manchester and his team look at the links between obesity and dementia but they also investigate multiple other aspects of the disease. When I spoke to him he stated;
“Whenever I give a talk I tell people what is good for the heart is generally good for the brain. There are links between obesity and dementia and we are starting to understand the science behind them.”
We then discussed the broader implications of this. He said;
“What we must also remember is that Alzheimer’s is a very complex disease. Obesity is a risk factor but it is not the only one. Other factors including smoking, education and family history can also play a role. Avoiding obesity doesn’t necessarily mean you can prevent Alzheimer’s disease later in life”.
It is, therefore, important to consider all the factors that can increase the risk of developing Alzheimer’s. Recent findings suggest that, while improving global cardiovascular health could reduce incidence of the disease, the most effective way to fight Alzheimer’s is to address all risk factors. By doing so we could observe as many as 1·1 – 3·0 million cases worldwide 6 6. Barnes, D. E. & Yaffe, K. The projected effect of risk factor reduction on Alzheimer's disease prevalence The Lancet Neurology, September 2011, DOI: 10.1016/S1474-4422(11)70072-2 × .
More Research Needed
Additionally, we should not ignore the need for constant research into the biological pathways that drive the disease. Understanding the risk factors important but there are thousands of patients every year who are diagnosed with Alzheimer’s despite never being overweight, never smoking and being mentally and physically active.
Like cancer, Alzheimer’s disease is complex and varies from patient to patient. Ultimately, the main risk factor of developing the disease is age.
Generally speaking, Alzheimer’s is not a disease of the young. It affects some of the most vulnerable members of society and the main difficulty still lies in finding ways to detect it early enough to prevent disease progression.
A healthy lifestyle can help prevent developing the disease but it is not the ultimate solution to the problem. It is important for us to continue to investigate how lifestyle can have a negative impact on our cognitive health. However, this research should not be at the expense of the thousands of patients who are subjected to this debilitating disease despite a healthy heart.
We need to focus on all factors that drive disease progression. Only by doing this can we gain a full understanding of how Alzheimer’s disease initiates and how we can stop it in its tracks.
Live well, exercise your mind frequently and hopefully scientists can take care of the rest.
Author: Dr Sam Moxon; Biomaterials Research Associate at the University of Manchester. Image: Tasha/Flickr