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Can Our Sense of Smell Predict Alzheimer’s or Parkinson’s?

New studies are beginning to take advantage of a common symptom that may serve as a potential earlier indicator for both Alzheimer's and Parkinson's diseases: the deterioration of the sense of smell.

By
Sabrina Stierwalt, PhD,
April 24, 2018
Episode #282

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According to the National Institute of Health, an estimated 50,000 people are diagnosed with Parkinson’s disease each year in the United States, and around half of a million people are living with the disease. Worldwide, that estimate is as high as 10 million people. Parkinson’s disease, a neurodegenerative disorder, is characterized by the loss of nerve cells in the brain that are responsible for releasing dopamine, a neurotransmitter that helps monitor the signals related to pleasure, emotional responses, and movement regulation.

As many as 47 million people worldwide are living with Alzheimer’s disease, a more general degeneration of the brain, characterized by a decline in memory and reasoning skills. An estimated 5.4 million Americans have Alzheimer’s and that number is expected to reach 13.8 million by 2050.

Despite their ubiquity, the causes for both diseases are still mostly unknown making it a challenge to catch the disease early. The symptoms generally develop slowly over many years and are more commonly found in older patients. Early signs of Alzheimer’s cannot always be distinguished from normal memory loss with age. Parkinson’s is usually not diagnosed until there are clear outward symptoms like the tremors, rigidity, or muscle weakness that characterize the disease, which means the nerve cell loss is already taking place.

For both diseases, both genetic and environmental risk factors have been identified. For example, consumption of Vitamin D and caffeine can be associated with a lower risk of Parkinson’s disease. And now, new studies are beginning to take advantage of a less obvious but still common symptom that may serve as a potential earlier indicator for both diseases: the deterioration of the sense of smell.

Sense of Smell and Parkinson’s Disease

In a recent study published in the journal of the American Academy of Neurology, scientists monitored a group of more than 2,400 people over an average of 10 years to look for a possible link between sense of smell and the onset of Parkinson’s disease. First, the study participants were tested on their ability to properly identify common odors like gasoline and citrus, and they were placed into groups of good, intermediate, and poor smellers. Then, after several years had passed, the participants were tested for signs of Parkinson’s.

Even when controlling for other factors that may influence the onset of the disease, like caffeine consumption or smoking, scientists found that on average those with a poor sense of smell were more likely to be diagnosed with Parkinson’s disease up to six years later.

Although the link was clear when averaging over all of the participants, the link was found to be stronger in men than in women. There was also no definitive link found between sense of smell and Parkinson’s in the black participants studied. The efficacy of smell as a potential biomarker may thus depend on race or, as noted by the authors, the study may have suffered from a small sample size. (Only 12 of the approximately 950 black participants developed the disease.) More research is thus needed, and in particular research that separates participants by race and sex.

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