Statins, anti-inflammatory drugs, alcoholic drinks and coffee have all been linked to a reduced risk of developing Alzheimer's, scientists have said.

But diabetes, depression and high blood pressure can increase it in certain groups, according to a major review of more than 300 studies, which was published in the Journal of Neurology, Neurosurgery & Psychiatry.

Researchers found several other risk factors and a number of protective factors, which highlight the fact that changes to diet, medication and lifestyle could reduce the risk.

Having cancer, heart disease, arthritis or metabolic syndrome (a combination of diabetes, high blood pressure and obesity) were linked to a lower risk of Alzheimer's, and a protective effect was found from taking statins, the female hormone oestrogen, drugs to lower high blood pressure, and anti-inflammatory drugs (NSAIDs).

Folic acid, coffee and vitamins C and E were also associated with helping to stave off the disease, along with current smoking (only amongst Western populations), light-to-moderate drinking (one to three alcoholic drinks a day), stress, and high body mass index (BMI) in late life.

But they found significant evidence that there was a higher risk of Alzheimer's if a person was affected by any of five pre-existing diseases or conditions: frailty, carotid atherosclerosis, high blood pressure, low diastolic blood pressure, and type 2 diabetes (in Asian populations).

They also noted grade one evidence that one biochemical exposure - hyperhomocysteine, an amino acid manufactured in the body - and one psychological condition - depression - "significantly increased the risk of developing Alzheimer's".

Certain lifestyle issues - low education, low BMI or high BMI in mid-life - also led to a greater risk.

High alcohol consumption or alcoholism showed no significant association.

The study, led by Qingdao University in China, said a history of cancer is associated with a 37% decreased risk of developing Alzheimer's, possibly due to the contradictive pathophysiological process of the two conditions - degeneration vs replication.

A high intake of folic acid reduced the risk by roughly 49%, while oestrogen did by around 40%, and taking NSAIDs by about 26%.

Researchers said depression probably showed a slight increase in risk because patients are inclined to carry out less physical and cognitive activity and have less purpose in life, which are correlated with risk of developing the disease.

They said it was an observational study, so no definitive conclusions can be drawn about cause and effect, but they suggested preventive strategies targeting diet, prescription drugs, body chemistry, mental health, underlying disease and lifestyle might help curb the number of new cases.

Eric Karran, director of research at Alzheimer's Research UK, said some of the findings contradict other evidence and should be approached cautiously.

"Alzheimer's disease is likely to be caused by a combination of genetic and lifestyle factors that we don't yet fully understand," he added.

"Studies like this one, which review large amounts of existing evidence, can be useful in determining which factors have the strongest link to Alzheimer's, but they don't give us a clear idea about cause and effect.

"This study should be interpreted cautiously and examined carefully by the academic community, as some of the conclusions appear to be counter to previously well-established findings in the field.

"As there is still no surefire way to prevent Alzheimer's, we need to see more research like this and make sure the public are aware that there are things they can be doing now to reduce their risk of the condition."