A cup of coffee a day really can stave off Alzheimer's disease, say scientists.
But having two or more cups or never touching the brew puts you at risk of the condition that causes early memory problems that sometimes lead to Alzheimer's.
Researchers found moderate levels of caffeine protected the brain against rogue proteins that destroy neurons leading to memory loss.
A study of almost 1,500 Italians found moderate consumers of the world's most popular beverage were less likely to develop mild cognitive impairment (MCI) - commonly the first sign of dementia.
The findings add to evidence that the right amount of caffeine - a stimulant known to sharpen alertness and boost the brain - protects against mental decline in older age.
Mentally healthy individuals who regularly drank one or two cups of coffee a day had a reduced rate of MCI than those who never or rarely consumed it.
No significant association was detected between those who habitually had more than two cups and the incidence of MCI in comparison with those who never or rarely indulged.
The study published in the Journal of Alzheimer's Disease also showed that increasing the amount of coffee consumption over time by more than one cup doubled the rate of MCI compared to those who reduced it by a cup.
The rate of MCI was also about one-and-a-half times higher than those with a constant habit of one cup-a-day.
Dr Vincenzo Solfrizzi and Dr Francesco Panza, of Bari Aldo Moro University in Italy, said: 'These findings from the Italian Longitudinal Study on Ageing suggested cognitively normal older individuals who never or rarely consumed coffee and those who increased their coffee consumption habits had a higher risk of developing MCI.
'Therefore, moderate and regular coffee consumption may have neuroprotective effects also against MCI confirming previous studies on the long-term protective effects of coffee, tea or caffeine consumption and plasma levels of caffeine against cognitive decline and dementia.'
The researchers looked at the coffee drinking habits of 1,445 individuals recruited from 5,632 subjects aged 65 to 84 year from the Italian Longitudinal Study on Ageing (ILSA) who were followed up for an average of three-and-a-half years.
They said there is no drug to stop MCI so the identification and subsequent management of risk and protective factors is crucial.
Previous research has also suggested coffee, tea or caffeine consumption could prevent mental decline and dementia which affects 850,000 people in the UK alone.
Dr Solfrizzi and Dr Panza say caffeine could limit damage caused by rogue proteins in the brain that destroy neurons.
Indeed caffeine has been shown to reduce these clumps in mice genetically engineered to develop dementia and restore memory.
They said moderate doses of caffeine improved performance in rodents whereas higher doses impaired it.
Biological effects of caffeine also improves insulin sensitivity so reducing the risk of type 2 diabetes - which is a strong risk factor for cognitive decline.
Moreover caffeine could in part compensate the cognitive decline in older individuals because its effects on vigilance and attention - mainly in situations of reduced alertness.
The researchers said: 'More sensitive outcomes such as findings from neuro-imaging studies should become available from experimental data so further explaining the mechanisms underlying the neuroprotective effects of coffee, tea and caffeine consumption.
'Larger studies with longer follow-up periods should be encouraged, addressing other potential bias and confounding sources so hopefully opening new ways for diet-related prevention of dementia and Alzheimer's disease.'
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