According to a study people who carry the gene variant associated with the strongest risk for Alzheimer’s disease may lose their ability to detect odours earlier than people who do not carry the gene variant, which may be an early sign of future memory and thinking problems.

The study published in the online issue of Neurology showed that the gene variant associated with this increased risk of Alzheimer’s is called APOE e4.

"Testing a person’s ability to detect odours may be a useful way to predict future problems with cognition," said Matthew S. GoodSmith, from the University of Chicago.

"While more research is needed to confirm these findings and determine what level of smell loss would predict future risk, these results could be promising, especially in studies aiming to identify people at risk for dementia early in the disease," he added.

The study involved an at-home survey that included testing the sense of smell of over 865 people -- both their ability to detect an odour at all and their ability to identify what odour they were smelling.

Tests were given at five-year intervals. People’s thinking and memory skills were also tested twice, five years apart. DNA samples gave researchers information about who carried the gene associated with an increased risk of Alzheimer’s.

People who carried the gene variant were 37 per cent less likely to have good odour detection than people without the gene at a single time point. The gene carriers started experiencing reduced smell detection from age 65 to 69.

The people carrying the gene variant did not show a difference in their ability to identify what odour they were smelling until they reached age 75 to 79.

Once they started to lose their ability to identify odours, the gene carriers’ ability declined more quickly than those who did not carry the gene.

Thinking and memory skills were similar among the two groups at the start of the study. But as expected, those carrying the gene variant experienced more rapid declines in their thinking skills over time than those without the gene.

"Identifying the mechanisms underlying these relationships will help us understand the role of smell in neurodegeneration," GoodSmith said. A limitation of the study is that people with severe dementia were not included.