Research has shown that giving healthy older people vitamin supplements to reduce high blood levels of an amino acid linked to dementia does not help their thinking. 

An Otago University clinical study published in the New England Journal of Medicine showed the vitamins did not boost the cognitive performance of 250 healthy older people over a two-year clinical trial. 

The Otago researchers investigated whether lowering high blood levels of the amino acid homocysteine boosted the cognitive function. 

"Doctors who are currently advising older patients to take supplements as a preventive measure against cognitive decline might need to reconsider that advice," said study co-author Dr Jennifer McMahon, who ran the trial as part of her PhD in human nutrition. 

A lot of researchers had hoped that homocysteine was a key risk factor that could be tackled to prevent or treat illness. 

Previous international studies had shown an inverse relationship between homocysteine levels and cognitive function in older people, leading to hopes that lowering homocysteine levels might be the key to combating cognitive decline, said Associate Professor Murray Skeaff, another co-author. 

A high blood concentration of homocysteine had been seen as a significant risk factor for developing Alzheimer's, and was also linked to other forms of dementia and cardiovascular disease, Professor Skeaff said.