The researchers at the National Institute for Health and Medical Research in France carried out a four-year study involving 8,085 men and women all residents of Montpellier, Dijon and Bordeaux.
They were examined for signs of dementia and asked to complete a questionnaire about their eating habits and the psychiatric examination was repeated every two years.
At the start of the study all the participants were over 65 and did not have dementia; over the four-year period, 183 people developed Alzheimer's disease, while another 98 developed a type of dementia.
The researchers then took a closer look at the dietary patterns of the participants.
The team found that a diet rich in omega-3 oils which most of the participants got through salad dressing, were associated with a 60% drop in dementia risk ; there was also a 30% drop in dementia risk among regular fruit and vegetable eaters.
The researchers also found that regular fish consumption reduced the risk of Alzheimer's by 35%, but only if subjects did not have a known genetic risk factor for the disease.
In the UK alone around 500,000 people are believed to suffer from Alzheimer's and women are slightly more at risk than men.
The risk increases markedly with age, before 65 only 1 in 1,000 are affected but around 1 in 20 of over-65s are affected and after 80 nearly half have the disease.
The study appears in the November 13, issue of Neurology, the medical journal of the American Academy of Neurology.
The health benefits associated with taking the drugs varied. For example, orlistat reduced the incidence of diabetes in one trial and all three drugs lowered patients' levels of certain types of cholesterol. Adverse effects were recorded with all three drugs, in particular, rimonabant increased the risk of mood disorders such as depression or anxiety. The authors noted that no trials examined rates of death and disease as a result of taking anti-obesity pills. They recommend that trials looking at this should be carried out in the future.
The authors also noted that there were high drop-out levels in all the trials. On average 30 “ 40% of patients failed to complete the trial. They say this suggests that a failure to properly adhere to the treatment could be a major factor limiting the effectiveness of anti-obesity drug therapy.
In an accompanying editorial, Professor Gareth Williams warns of the potential damage to society if anti-obesity drugs are licensed to be sold without prescription. This already happens in the United States, and as Glaxo Smith Kline (GSK) has applied to sell orlistat over the counter throughout Europe, it could happen here. He warns:
Selling anti-obesity drugs over the counter will perpetuate the myth that obesity can be fixed simply by popping a pill and could further undermine the efforts to promote healthy living, which is the only long term escape from obesity.
bmj