The incidence of Alzheimer’s Disease varies widely around the world. There are far more cases diagnosed in Europe and North America than in Africa, Asia or India. The question is why. Is it genetics? Some difference in lifestyle? Or is the disease just underreported in developing countries? India has a low rate of Alzheimer’s but with less access to physicians and fewer tests available it is possible that many cases of Alzheimer’s are not being recognized as such.
Genetics is not likely to be an important contributor given that Asians who have grown up in North America have the same rate of Alzheimer’s as other Americans. Ditto for Nigerians in the U.S. who experience a higher rate than Nigerians in Africa. In Japan, a country that has traditionally had a low rate of dementia, Alzheimer’s has been increasing significantly. Since this cannot be attribute to a change in genetics, it seems that some sort of lifestyle factor that distinguishes poorer and wealthier countries is at play.
Diet would seem to be a prime candidate because of significant differences in what people consume around the world. Rice is a staple in most developing countries and is far less widely consumed in the West. Meat, on the other hand, is a western institution. Could it be that the fat content, or the cholesterol, or the iron in meat somehow predispose to Alzheimer’s? In Japan, the incidence of the disease appears to have increased in step with a increase in meat consumption and a decrease in rice consumption. That could mean that rice has some protective factor, or that something in meat is a problem or it could mean nothing.
Epidemiological studies cannot distinguish between associations and cause and effect relationships. For example, there is a highly significant correlation between the divorce rate in the state of Maine and the per capita consumption of margarine. Interesting, but in all likelihood meaningless. There is also a correlation between strict hygiene and sanitation methods as practiced in wealthy countries and the prevalence of Alzheimer’s Disease. Could that be meaningful? In countries with access to clean drinking water the incidence of Alzheimer’s is increased and in countries with a low rate of infectious disease such as Switzerland or Iceland the rates of Alzheimer’s are higher by some 12% than in China and Ghana, both countries with high rates of infectious disease.
Is this some spurious correlation or is it meaningful? The “hygiene hypothesis” is gaining traction when it comes to allergies and asthma with the theory being that exposure to bacteria, viruses and worms early in life primes the development of a healthy immune system. In the absence of exposure to organisms that can actually cause disease, the immune system targets innocent bystanders such as certain food components. Maybe, some researchers suggest, the deposition of proteins in the nervous system, one of the hallmarks of Alzheimer’s, is a result of an immune system that has gone astray. Sounds pretty far fetched, but the absence of any proven cause of Alzheimer’s makes for all sorts of half-baked theories being hatched.