The effects of caffeine (and by extension coffee) on the risk of developing dementia has been studied many times.

These studies might only be quite small or only apply to a specific group of people, but the media attention they receive can often overstate their impact.

One key study in Florida followed people with mild cognitive impairment – thinking and memory problems beyond normal ageing – and monitored their caffeine levels and their cognitive ability over the next two to four years. The researchers found that people who did not develop dementia had twice as much caffeine in their blood as those who did.

No definitive answer

However, this type of study cannot be relied on for a definitive answer. We have no way of determining if the caffeine levels affected dementia or if it is the other way round; sleeping problems brought on by dementia might cause someone to give up caffeine, for example.

Other observational studies have been conducted with healthy volunteers and the results have been mixed. Some have shown a protective effect against Alzheimer's disease, some have shown a protective effect only in women, while others have shown no effect at all. All of these studies suffer from the same complications as the Florida study – the results cannot distinguish between cause and effect.

The gold standard for this type of research is a randomised controlled trial, where people are randomly split into two groups – one with caffeine and one without – and they are monitored over time. However, there have been no studies of this kind to confirm the link.

Caffeine and hypoxia

One suggestion for how caffeine might counter dementia has come from studying the effects in mice. Researchers looked at what happens when the brain is starved of oxygen, known as hypoxia, and how caffeine affects the response. Hypoxia sends the brain cells into panic mode, similar to what we see in neurodegenerative disorders like Alzheimer's disease. It triggers the release of a chemical called adenosine, which in turn causes a chain reaction of enzymes leading to inflammation. Caffeine interferes with this by blocking the cells' ability to recognise adenosine, reducing the extent of inflammation.

This idea has also been backed up by observations of other drugs which block these receptors in a similar way to caffeine, but more specifically and more strongly. One example improves the brain performance of mice that are genetically modified to have a form of tau protein that is associated with Alzheimer's disease.

There have also been other suggestions as to how coffee can help against dementia. Research has shown that caffeinated coffee increases production of granulocyte-colony stimulating factor, which helps the brain in several ways . Intriguingly, this effect is not seen with caffeine alone or decaffeinated coffee, suggesting that there could be a combination effect between caffeine and an unknown compound in coffee.