By Dr. Pablo Barrecheguren (@pjbarrecheguren)
Of the five senses, smelling is perhaps the least valued nowadays, even when we all have strong emotional memories associated with some odor. For example, we all remember our grandparents’ houses and their distinct smell. Japanese culture has its own term for this smell, kareishu or “old people smell,” and science has discovered that the substance 2-nonenal found in human body odor tends to increase with aging.
This is particularly interesting because it means that, regardless of our level of hygiene, our body odor changes as we age, which leads to a thought-provoking clinical question: can specific smells be linked to certain diseases? Several researchers are working on this matter and, among other things, they have found that trained dogs could accurately (91%) distinguish between the breath of people with and without colon cancer. This opens the door to the search for odorous molecules associated with certain illnesses.
Joy Milne can smell Parkinson’s disease
In this regard, one of the most researched fields is that of Parkinson’s disease and all thanks to one woman: Joy Milne. Joy’s husband suffered from Parkinson’s and while attending a research lecture at the University of Edinburgh, she asked the speaker if people with Parkinson’s had a characteristic smell. This anecdote led to two discoveries: first, just like great sommeliers, Joy had an extraordinary sense of smell that was well above average. Second, Joy was able, by scent alone, to identify if study participants had the smell that her husband had developed and therefore to know if they had Parkinson’s disease or not.
This revelation led to a research study where she identified people with Parkinson’s by smelling t-shirts they had worn (six worn by people with Parkinson’s disease and six by healthy individuals). Her sense of smell was such that she was correct in 11 of the 12 people, incorrectly identifying one of the healthy people as having Parkinson’s disease. Few months later, however, that person was diagnosed with Parkinson’s, so Milne turned out to be correct even ahead of the clinical diagnosis; in fact, she stated that her husband began to have that unique odor six years before he was diagnosed with the disease.
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After this first study, Joy continued to collaborate with the researchers. Little by little, they discovered that the origin of the scent was in the sebum, the skin fat, which is interesting because dermatological problems have already been documented in patients with Parkinson’s disease. Current research focuses on isolating the molecules responsible for this almost imperceptible odor: although work is still in progress, it is known that people with the disease have altered levels of certain metabolites in their sebum (e.g., perillaldehyde and eicosane), the smell described by Joy Milne as being very similar to the scent developed by her husband.
Apart from being very curious, these research studies have great potential; currently Parkinson’s is only diagnosed once the symptoms appear and these are usually detected when approximately 60% of the neurodegeneration causing the disease has already occurred. In other words, doctors rarely diagnose Parkinson’s disease in the early stage, so treatment starts late, and it is difficult to stop the neurodegeneration in time. Current treatments can relieve symptoms for much of the patient’s remaining life (15-year average) but the situation would surely be better if doctors could detect the disease earlier.
And this is where Joy and scents come in: if it is confirmed that patients have different levels of certain molecules in the skin even before Parkinson’s symptoms manifest themselves—even though the average person cannot detect the smell (almost no one has Joy’s sense of smell)—it would then be possible to take a sample of this skin fat, analyze it and see if a person has the disease or not. Thus, even when it is still under study, it is possible that something as subtle as odor could have a huge impact on Parkinson’s research.
- Abellán, A. “¿Sabías que a los treinta años comienzas a oler a viejo?”. Principia Magazine.
- Morgan, J. (2016). Joy of super smeller: sebum clues for PD diagnostics. The Lancet Neurology, 15(2), 138–139.
- Sonoda, H., Kohnoe, S., Yamazato, T., Satoh, Y., Morizono, G., Shikata, K., … Maehara, Y. (2011). Colorectal cancer screening with odour material by canine scent detection. Gut, 60(6), 814–819.
- “’Super-smeller’ helps develop swab test for Parkinson’s disease”. The Guardian.
- Trivedi, D. K., Sinclair, E., Xu, Y., Sarkar, D., Walton-Doyle, C., Liscio, C., … Barran, P. (2019). Discovery of Volatile Biomarkers of Parkinson’s Disease from Sebum. ACS Central Science.
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