Is Alzheimers Just Another Form of Diabetes? How Our Diet May Be Destroying Our Brains
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When you raise the subject of over-eating and obesity, you often see people at their worst. The comment threads discussing these issues reveal a legion of bullies, who appear to delight in other people’s problems.
When alcoholism and drug addiction are discussed, the tone tends to be sympathetic. When obesity is discussed, the conversation is dominated by mockery and blame, though the evidence suggests that it can be driven by similar forms of addiction (citations 1,2,3,4). I suspect that much of this mockery is a coded form of snobbery: the strong association between poor diets and poverty allows people to use this issue as a cipher for something else they want to say, which is less socially acceptable.
But this problem belongs to all of us. Even if you can detach yourself from the suffering caused by diseases arising from bad diets, you will carry the cost, as a growing proportion of the health budget will be used to address them. The cost – measured in both human suffering and money – could be far greater than we imagined. A large body of evidence now suggests that Alzheimer’s is primarily a metabolic disease. Some scientists have gone so far as to rename it. They call it diabetes type 3.
New Scientist carried this story on its cover this September(5): since then I’ve been sitting in the library trying to discover whether it stands up. I’ve now read dozens of papers on the subject, testing my cognitive powers to the limit as I’ve tried to get to grips with brain chemistry. While the story is by no means complete, the evidence so far is compelling.
Around 35 million people suffer from Alzheimer’s disease worldwide(6); current projections, based on the rate at which the population ages, suggest that this will rise to 100 million by 2050(7). But if, as many scientists now believe, it is caused largely by the brain’s impaired response to insulin, the numbers could rise much further. In the US, the percentage of the population with diabetes type 2, which is strongly linked to obesity, has almost trebled in 30 years(8). If Alzheimer’s, or “diabetes type 3”, goes the same way, the potential for human suffering is incalculable.
Insulin is the hormone which prompts the liver, muscles and fat to absorb sugar from the blood. Diabetes 2 is caused by excessive blood glucose, resulting either from a deficiency of insulin produced by the pancreas, or resistance to its signals by the organs which would usually take up the glucose.
The association between Alzheimer’s and diabetes 2 is long-established: type 2 sufferers are two to three times more likely to be struck by this dementia than the general population(9). There are also associations between Alzheimer’s and obesity(10) and Alzheimer’s and metabolic syndrome (a complex of diet-related pathologies)(11).
Researchers first proposed that Alzheimer’s was another form of diabetes in 2005. The authors of the original paper investigated the brains of 54 corpses, 28 of which belonged to people who had died of the disease(12). They found that the levels of both insulin and insulin-like growth factors in the brains of Alzheimer’s patients were sharply reduced by comparison to those in the brains of people who had died of other causes. Levels were lowest in the parts of the brain most affected by the disease.
Their work led them to conclude that insulin and insulin-like growth factor are produced not only in the pancreas but also in the brain. Insulin in the brain has a host of functions: as well as glucose metabolism, it helps to regulate the transmission of signals from one nerve cell to another, and affects their growth, plasticity and survival(13,14).