The human body constantly produces and excretes hundreds of odorous volatile organic compounds (VOCs) via several routes. The complement of compounds excreted reflects the metabolism of an individual and may vary significantly. Evolutionarily, it has been hypothesised that the unconscious detection of disease-associated body odours protected healthy individuals from the sick by altering interpersonal contact behaviour. Recent studies have looked into how these compounds may reflect diseased states, and as such, how the detection of specific compounds may be used to diagnose diseases and also to monitor treatment efficacy.
Routes by which VOCs are excreted from the body include breath, sweat, skin, urine, faeces and vaginal secretions. Specific diseases are characterized by a change in the odour, brought about by a disease-induced alteration in cellular metabolism which produces different VOCs. Countless diseases have been reported to cause such a change in body odour, such as infections; cancer; diabetes and schizophrenia, each having a distinct smell. So, what do diseases smell like?
In type 1 diabetes, insulin is not produced, thereby limiting cells’ access to glucose to use as fuel. Instead, in exacerbations of the disease, cells make use of fatty acids, which are broken down into ketones. An excess of ketones results in the condition known as diabetic ketoacidosis, and is characterised by the patient’s breath and urine having a fruity smell. Moreover, diabetic alert dogs are trained to detect the VOCs excreted when blood glucose levels are too high or low.
Cancer cells have abnormal metabolism when compared to normal cells. Therefore, certain types of cancer may secrete specific VOCs, which may allow for early detection and thereby better treatment outcomes. Lung; breast and colorectal cancers have been found to emit such VOCs that could be detected by dogs. With future research, scent may be utilised as a non-invasive screening tool for cancer.
Several animals have been used to detect disease via VOCs, namely dogs, rodents, weasels and ferrets. These animals have many more olfactory receptor neurons than humans, and a more developed olfactory cortex in their brains. As such, they can detect odorous compounds when we cannot. One study reported that trained dogs were able to detect lung cancer with 99% accuracy, and breast cancer with 88% accuracy. The high accuracy in detection has been backed-up by several studies. Currently, research is being undertaken to discern which compounds are being detected by the animals and on how their training can be standardised. Apart from disease, animals are also used in other situations, such as bomb and drug detection.
In conclusion, diseases may have characteristic odours which we can detect ourselves, or with the help of animals. Although further research is needed, this is a rapidly evolving field in healthcare, and promises innovations in disease detection. In fact, some studies are now attempting to detect COVID-19 using dogs, and report a high degree of accuracy.