Across the globe, the incidence of diabetes is undergoing exponential growth. According to the WHO, more than 346 million people worldwide have diabetes. In the US alone, the prevalence of type 2 diabetes is expected to more than double in the next few decades from 6% of the population in 2005 (16 million) to 12% in 2050 (48 million) [1].
The consequences of diabetes will be well known to many. The disease carries a heavy burden, which includes a high risk of cardiovascular disease (responsible for between 50-80% of deaths in people with diabetes), as well as complications such as blindness, amputation and kidney failure. But could we add another to that list, Alzheimer's disease?
The connection appears to be strong. People with type 2 diabetes are much more likely to develop Alzheimer's disease than their healthy counterparts (up to 65% [2]). In fact, the association also appears to be strong linking the metabolic syndrome, essentially a pre-diabetic state, and Alzheimer's disease [3]. If Alzheimer's disease goes hand in hand with these metabolic abnormalities, should we be braced for a tsunami of dementia in the wake of the burgeoning epidemics of type 2 diabetes and the metabolic syndrome? It's certainly what the data predicts, with experts forecasting that the cases of dementia will virtually double by 2030, reaching 65.7 million, and could reach an immense 115.4 million by 2050 (4). For those who like mechanisms, the increased risk of dementia seen in people with type 2 diabetes and obesity could be explained by such factors as chronic hyperglycemia, peripheral insulin resistance, oxidative stress, accumulation of advanced glycation end products, increased production of pro-inflammatory cytokines, and/or cerebral microvascular disease [5].
It's an intriguing idea; could Alzheimer's be a metabolic disease? Certainly, some researchers think so, and have dubbed it 'type 3 diabetes'. This is based on research showing that levels of insulin, and its receptors, diminish significantly in the brain in Alzheimer's disease [6]. It is the idea that impaired insulin signaling has such a central role in the neurodegenerative process that has led to the proposal that Alzheimer's is indeed a distinct form of diabetes.
Much remains to be discovered about this whole idea of Alzheimer's as a form of diabetes, and the extent to which having diabetes or being obese are implicated in driving the disease process (probably it is a contributing factor, not the cause). But it does again raise the question as to what extent our 21st Century diet and lifestyle could be fuelling not only the epidemics of obesity and diabetes, but also Alzheimer's disease. It adds yet more strength to the idea that diet could be a powerful modifier of Alzheimer's disease risk, as has already been aptly illustrated by research on such factors as regular fish consumption [7], diets rich in fruits and vegetables, light-to-moderate alcohol intake [8], and adherence to a Mediterranean-style diet [9].
How many more catastrophic consequences to our health do we need to witness before we start taking the whole idea of diet and lifestyle as preventive medicine more seriously?
[1] Mitri et al (2011) Vitamin D and type 2 diabetes: a systematic review. European Journal of Clinical Nutrition 65:1005-1015
[2] Diabetes UK (2012)
[3] Vanhanen M et al (2006) Association of metabolic syndrome with Alzheimer disease: a population-based study. Neurology 67(5):843-7.
[4] Gulland A (2012) Number of people with dementia will reach 65.7 million by 2030, says report. British Medical Journal 344:e2604.
[5] de la Monte SM, Wands JR (2008) Alzheimer's disease is type 3 diabetes-evidence reviewed. J Diabetes Sci Technol 2(6):1101-13.
[6] Steen E et al (2005) Impaired insulin and insulin-like growth factor expression and signaling mechanisms in Alzheimer's disease - is this type 3 diabetes?Journal of Alzheimer's Disease 7:63-80.
[7] Morris MC et al (2003) Consumption of fish and n-3 fatty acids and risk of incident Alzheimer disease.Archives of Neurology 60(7):940-6.
[8] Solfrizzi V et al (2011) Diet and Alzheimer's disease risk factors or prevention: the current evidence. Expert Rev Neurother 11(5):677-708
[9] Sofi et al (2008) Adherence to Mediterranean diet and health status: meta-analysisBritish Medical Journal 337:a1344