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 may be driven by similar forms of addiction.

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 type 3 diabetes.

New Scientist carried this story on its cover on 1 September; 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. Though the story is by no means complete, the evidence so far is compelling.

About 35 million people suffer from Alzheimer's disease worldwide; current projections, based on the rate at which the population ages, suggest that this will rise to 100 million by 2050. 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 United States, the percentage of the population with type 2 diabetes, which is strongly linked to obesity, has almost trebled in 30 years. If Alzheimer's, or "type 3 diabetes", goes the same way, the potential for human suffering is incalculable.

Insulin is the hormone that prompts the liver, muscles and fat to absorb sugar from the blood. Type 2 diabetes 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 that would usually take up the glucose.

The association between Alzheimer's and type 2 diabetes is long-established: type 2 sufferers are two to three times more likely to be struck by this form of dementia than the general population. There are also associations between Alzheimer's and obesity and Alzheimer's and metabolic syndrome (a complex of diet-related pathologies).

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. They found that the levels of both insulin and insulin-like growth factors in the brains of Alzheimer's patients were much lower than 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.

Experiments conducted since then seem to support the link between diet and dementia, and researchers have begun to propose potential mechanisms. In common with all brain chemistry, these tend to be fantastically complex, involving, among other impacts, inflammation, stress caused by oxidation, the accumulation of one kind of brain protein and the transformation of another. I would need the next six pages of this paper even to begin to explain them, and would doubtless get it wrong (if you're interested, please follow the links on my website).

Plenty of research still needs to be done. But, if the current indications are correct, Alzheimer's disease could be another catastrophic impact of the junk food industry, and the worst discovered so far. Our governments, as they are in the face of all our major crises, seem to be incapable of responding.

In this country, as in many others, the government's answer to the multiple disasters caused by the consumption of too much sugar and fat is to call on both companies and consumers to regulate themselves. Before he was replaced by someone even worse, the former health secretary, Andrew Lansley, handed much of the responsibility for improving the nation's diet to food and drink companies – a strategy that would work only if they volunteered to abandon much of their business.

A scarcely regulated food industry can engineer its products – loading them with fat, salt, sugar and high-fructose corn syrup – to bypass the neurological signals that would otherwise prompt people to stop eating. It can bombard both adults and children with advertising. It can (as we discovered yesterday) use the freedom granted to academy schools to sell the chocolate, sweets and fizzy drinks now banned from sale in maintained schools. It can kill off the only effective system (the traffic-light label) for informing people how much fat, sugar and salt their food contains. Then it can turn to the government and blame consumers for eating the products it sells. This is class war, a war against the poor fought by the executive class in government and industry.

We cannot yet state unequivocally that poor diet is a leading cause of Alzheimer's disease, though we can say that the evidence is strong and growing. But if ever there was a case for the precautionary principle, here it is. It's not as if we lose anything by eating less rubbish. Averting a possible epidemic of this devastating disease means taking on the bullies – both those who mock people for their pathology and those who spread the pathology by peddling a lethal diet.

Twitter: @GeorgeMonbiot A fully referenced version of this article can be found at www.monbiot.com