The letter, signed by experts from Britain, France, the United States, Italy and Greece, calls for governments to invest more in teaching the public, including children, about the benefits of a healthy diet and lifestyle.
There is currently no effective treatment for dementia, but the protective effect of a healthy diet is being "largely ignored", they said.
Dr Aseem Malhotra, a cardiology registrar at Croydon University Hospital and one of the letter's signatories, said: "We are not going to overcome the increasing burden of chronic diseases by prescribing more pills. The medical profession has itself been guilty of placing too much emphasis on drugs, the benefits of which are often grossly exaggerated and fuelled by a powerful pharmaceutical industry, who naturally wish to expand the use of their drugs for financial gain.
"The evidence base for the Mediterranean diet in preventing all of the chronic diseases that are plaguing the Western world is overwhelming."
Dr Richard Hoffman, a GP and one of the lead authors of the letter, added: "The Mediterranean diet is possibly the most effective way of helping to prevent dementia."
The Prime Minister has pledged to use Britain's presidency of the G8 to lead coordinated international action on the condition, which currently costs the world an estimated £370 billion a year.
In a separate letter published in The Daily Telegraph last week, leading charities warned that increasing lifespans mean that "dementia is spiralling out of control, holding the world to ransom".
Mediterranean-style diets, which are rich in vegetables, fruit and olive oil, with fish eaten twice a week, meat and sugary foods just once a week, and with moderate consumption of wine, have long been thought to help people maintain better health in old age.
In a recent review of evidence, nine out of 12 studies found that people on a Mediterranean diet had better cognitive function, lower rates of cognitive decline and a lower risk of Alzheimer's disease.
Studies have also suggested the diet reduces the risk of coronary heart disease, high blood pressure and diabetes.
Simon Capewell, a professor of clinical epidemiology at the University of Liverpool, said: "Poor diet causes more diseases than physical inactivity plus smoking plus alcohol combined."
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