Toronto, Aug 17: Inability to distinguish between the smell of a lemon and petrol could point to the risk of developing Alzheimer's disease, new research suggests. It is believed that the damage to brain associated with Alzheimer's disease (AD) occurs up to 20 years before symptoms start showing. Scientists are therefore interested in finding ways to detect the presence of the disease early on. The findings, published in the journal Neurology, suggest that simple odour identification tests may help track the progression of the disease before symptoms actually appear, particularly among those at risk. "Despite all the research in the area, no effective treatment has yet been found for AD," said one of the study authors John Breitner of McGill University in Canada. "But, if we can delay the onset of symptoms by just five years, we should be able to reduce the prevalence and severity of these symptoms by more than 50 per cent," Breitner said. Close to 300 people with an average age of 63 who are at risk of developing AD because they had a parent who had suffered from the disease, were asked to take multiple choice scratch-and-sniff tests to identify scents as varied as bubble gum, gasoline or the smell of a lemon. One hundred of them also volunteered to have regular lumbar punctures to measure the quantities of various AD-related proteins in the cerebrospinal fluid (CSF). The researchers found that those with the most difficulty in identifying odours were those in whom other, purely biological indicators of AD, were most evident. "This is the first time that anyone has been able to show clearly that the loss of the ability to identify smells is correlated with biological markers indicating the advance of the disease," said Marie-Elyse Lafaille-Magnan, a doctoral student at McGill and the first author on the study. "For more than 30 years, scientists have been exploring the connection between memory loss and the difficulty that patients may have in identifying different odours," she said. "This makes sense because it is known that the olfactory bulb (involved with the sense of smell) and the entorhinal cortex (involved with memory and naming of odours) are among the first brain structures first to be affected by the disease," Lafaille-Magnan added.