Roll over Rex…humans aren’t such bad sniffers after all: Smell part II

by Nicola Temple | May 15, 2014

Last week I was inspired by stinky cheese to consider the science of smell in a little more detail. It has captured my interest these past few months as I research food fraud and our ability to detect it. Looking at all these incredibly sophisticated tests for detecting adulterants in food items…I wondered why we don’t rely more on this amazing piece of equipment situated in the middle of our face – our nose. It has made me question whether our sense of smell is sensitive enough to do more than the simple sour milk sniff! This is what I know so far….

Humans don’t really have a poor sense of smell

It is a common perception that humans have a relatively poor sense of smell compared with our fellow mammals. Looking back through our ancestry, the evolutionary trend has been a reduction in the size of our snouts as well as the number of genes that code for olfactory receptors. Rodents, for example, have 1,100 functional genes that code for olfactory receptors, whereas humans have a mere 350 functional genes [1]. It would seem, therefore that compared to the common sewer rat, we humans have less capacity to smell our environment.

Art depicting the human use of smell

‘The Sense of Smell’ by Philippe Mercier. Philip Mercier (circa 1689-1760) [Public domain or Public domain], via Wikimedia Commons

Yet, one should not judge ability to smell by number of receptors alone. Behavioural studies that test smell perception in humans and other primates suggest that we do as well or better than other mammals [2]. In fact, we can even outperform our canine friends and the most sensitive measuring instruments when sniffing out certain odours [1]. Our sniffers are not as subpar as we may think. Our 350 smell receptors are capable of detecting thousands of different odours some with such sensitivity that we could detect less than a drop in an Olympic sized swimming pool. One of the aromatics in citrus oil for example, (Z)-8-tetradecenal, which (not surprisingly) has a fruity, citrus-like odour, can be detected by humans at a threshold of 0.009 parts per billion when in water [3].

Roll over Rex, we humans don’t have such bad sniffers after all! It would seem that although we have fewer receptors, our post-processing of this information in the brain compensates giving us a diverse and sensitive understanding of odours. So, what do we us this amazing sense of smell for?

Sniffing out mates and the olfactory equivalent of beer goggles

We are likely aware of the daily input our nose provides – its contribution to perceiving flavours and our understanding of the environment around us. However, our noses are also at work at levels that we may not be as conscious of, such as sniffing out potential mates.

The major histocompatibility complex (MHC) is a group of molecules on the surface of vertebrate cells that interact with our immune cells. MHC – also known as HLA in humans – is what determines whether organs are compatible when doing transplants. Our genetic make-up determines our MHC and is therefore very individual. In 1995, researchers showed that women preferred the body odour of men who had an MHC that was least similar to their own [4]. Essentially, women were most attracted to men that had a different genetic composition providing them different immunity to certain parasites and diseases.

This, of course, makes sense as it firstly helps one avoid the potential of mating with a close relative and it secondly gives any potential children from the union more genetic diversity. No doubt you have at some point experienced instances where you find certain body odours quite offensive while others are tolerable, if not attractive. If not, perhaps you need to start sniffing around locker rooms and fitness centres to test it out – humans do have the ability to detect and evaluate MHC peptides in body odour [5].

Though our sniffers are playing a role in our mate selection (and there is a large body of work on this), there are times when our noses can let us down. Many of us are familiar with the beer goggle effect – several beers into the night, people who you didn’t initially find visually appealing suddenly become more so. Well, it turns out that alcohol also clouds our smelling judgement when it comes to selecting partners – so there may be a number of characteristics that are suddenly less appealing in the morning. In fact, heavy drinking can lead to serious olfactory deficits [6]. Smoking also diminishes our ability to smell.

Birth control pills also change a woman’s perception of MHC – they tend to prefer MHC-similar men when on the pill [7]. So ladies if you’re on the pill when you meet a man, it might be a good idea to get off the pill for a while and have a good sniff before you make any long term commitments!

The scent of sickness

Diagnosing sickness by smell

Edmund von Neusser (1852-1912) was known for his ability to smell illness with great accuracy. By anonymous/unknown (Images from the History of Medicine (NLM) [1]) [Public domain], via Wikimedia Commons

Edmund von Neusser was an Austrian internist of the late 19th and early 20th century. He specialised in disorders of the blood, circulatory system, liver and adrenal glands and was considered one of the best diagnosticians of his time. He apparently used olfactory cues as one of his tools and was able to smell illness with great accuracy.

It is not entirely surprising that certain illnesses would carry with them a distinct smell – consider the changes in your breath when you are feeling unwell. Bacteria and viruses or changes to the body’s chemistry – the root causes of many illnesses – no doubt bring about the release of different compounds that have an associated odour.

In the last couple of years, scientists have provided evidence that melanoma cells (cancerous skin cells) produce compounds that are not detected in normal melanocytes. These compounds – dimethyldisulfide and trisulfide – are released as part of the vapour signature associated with the cells, giving them a distinct smell compared with normal cells [8]. There have been a number of anecdotes of people whose dogs have sniffed a mole on their body with great interest, which later turned out to be confirmed melanoma. Dogs have even been trained to detect melanoma cells on patients [9] – offering a new and interesting approach to early detection of melanomas.

From an evolutionary perspective, it is advantageous to healthy individuals to be able to detect sick individuals and avoid the possibility of acquiring the illness themselves. This might be very easy to do when illness has settled in and there are numerous signs of the disease, but often people can be more contagious at an earlier stage where symptoms have not yet developed. It would therefore be most advantageous if healthy people could somehow detect when an immune response has been triggered in an individual – so when infection has happened, but obvious symptoms have not yet developed.

Dogs can smell melanoma

We make use of the keen sense of smell that dogs have, using them to sniff out drugs and explosives, track animals and now, perhaps sniff out disease. By Cpl. Johnny Merkley [Public domain], via Wikimedia Commons

Earlier this year, a group of scientists from Sweden and the US decided to see if this was possible [10]. They infected healthy individuals with an endotoxin – which are large molecules found in some bacteria that trigger a strong immune response in animals. They found that within hours of being injected, individuals had a more unpleasing body odor relative to when they were exposed to a placebo.

This was the first experimental evidence that an activated immune response is smelly, and that this can be detected by other humans so that they might avoid personal contact with affected individuals. Of course our use of deodorants, antiperspirants, perfumes and colognes mask all of these subtle cues.

What happens when we lose our sense of smell?

When I was at the MIT Knight Science Journalism Food Boot Camp, I had the opportunity to listen to Gary Beauchamp, Director of the Monell Chemical Senses Center. He had us plug our noses and then plunk a jelly bean in our mouths and give it a good chew. Aside from the texture of the bean in my mouth and a sense of sweetness, not much else was going on. Then, we were instructed to release our noses. While I knew what was going to happen, I was still not prepared for how dramatic the effect was. I was suddenly overwhelmed by the flavour (albeit not my favourite) – a little insight into what life would be like without a sense of smell.

A friend of mine recently told me that his daughter had asked which sense he would lose if he had to lose one. He said smell and I don’t think this would be an uncommon answer – sight and hearing likely being last on the list. But do we underestimate what losing our sense of smell would be like?

About six million people in the US are unable to smell [11] – a condition known as anosmia. A greater percentage of the population suffers from some aspect of olfactory dysfunction, which includes partial loss of smell and/or distortions to perception of olfactory cues. The most common causes of olfactory dysfunction are sinonasal disease, upper respiratory infection and head trauma.

People whose sense of smell has been compromised have reported a lower quality of life and a list of practical problems that they must contend with. Without a sense of smell, people have a lowered ability to sense hazardous situations, such as a fire, gas leak or other hazardous chemical vapours.

Healthy and safe food choices become more of a challenge as anosmics are unable to sniff the milk on its ‘Best Before’ date to see whether it has gone off. Food in general, in fact, becomes less pleasurable for anosmics. Many people with this condition develop eating disorders as their desire to eat is greatly diminished. On the opposite end of the scale, some people gain weight as they consume more food in the desire to try and taste it. Food choices are often made based on texture and taste – the spicy heat  hot peppers offer (which is burning detected by mechanoreceptors rather than olfactory receptors) suddenly become far more desirable as they at least offer a sensation.

After hazards and food there are also issues with managing odours. Without a sense of smell, people are unable to tell whether they and their home or even their children and animals smell bad. If you’re a parent with a sense of smell, you will be all too familiar with the quick pass we do with our infants to see whether a diaper change is needed. Imagine not having that ability? Though, I admit there would be some benefits during the actual diaper changing process!

Finally, people with olfactory impairments suffer from social isolation. Someone who has body odour will be ostracised…a mother at a playgroup who seems to ignore her obviously soiled child will be judged…a roommate that can’t smell the stench coming from his bedroom could face eviction.

We have a tendency to trivialize our ability to smell – perhaps because we feel ourselves so inferior already next to our best friend the canine, therefore it seems no great loss. Yet, for those who have challenges with their ability to smell, it is no trivial matter. It has serious daily impacts on their quality of life. Perhaps if agnostics had some visual cues that showed their impairment, such as a cane or hearing aid or guide dog, we might be prompted to be tolerant and understanding.

With this in mind, I take great pleasure in the stinky cheeses in my fridge, the smell of my husband after a run and all the other important information about my environment I take in through my nose.

Sources:

[1]     Shepherd GM (2004). The human sense of smell: are we better than we think? PLoS Biol 2(5): e146. doi: 10.1371/journal.pbio.0020146.

[2]     Laska M, Seibt A, Weber A (2000). “Microsmatic” primates revisted: Olfactory sensitivity in the squirrel monkey. Chem Senses 25: 47-53.

[3]      Deibler KD, Delwiche J (Eds.) (2004). Handbook of Flavor Characterization: Sensory Analysis, Chemistry, and Physiology. Marcel Dekker, Inc.: Monticello, New York. ISBN: 0-8247-4703-8 (P.187).

[4]      Wedekind C, Seebeck T, Bettens F, Paepke AJ (1995). MHC-dependent mate preferences in humans. Proc R Soc Lond B 260 (1359): 245-249. doi: 10.1098/rspb.1995.0087

[5]      Milinski M, Croy I, Hummel T, Boehm T (2013). Major histocompatibility complex peptide ligands as olfactory cues in human body odour assessment. Proc  R Soc B: Biolog Sci, 280 (1755): doi: 10.1098/rspb.2012.2889

[6]      Science Blog (2003). Heavy drinking can impair one’s sense of smell. http://www3.scienceblog.com/community/older/2003/C/2003260.html

[7]      Roberts SC, Gosling LM, Carter V, Petrie M (2008). MHC-correlated odour preferences in humans and the use of oral contraceptives. Proc R Soc B: Biol Sci, 275 (1652): 2715-2722.

[8]      Kwak J, et al. (2013). Volatile biomarkers from human melanoma cells. Journal of Chromatography B, 931: 90-96.

[9]      Pickel D, et al. (2004). Evidence for canine olfactory detection of melanoma. Applied Animal Behaviour Science, 89 (1-2): 107-116. doi: http://dx.doi.org/10.1016/j.applanim.2004.04.008

[10]    Olsson MJ, et al. (2014). The scent of disease: human body odor contains an early chemosensory cue of sickness. Psychological Science, 25 (3): 817-823. doi: 10.1177/0956797613515681

[11]    Keller, A and D. Malaspina (2013). Hidden consequences of olfactory dysfunction: a patient report series. BMC Ear, Nose and Throat Disorders, 13:8 doi: 10.1186/1472-6815-13-8

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