Please Pass the Salt

Do you feel like your grandfather adds more salt to his soup each time you see him? Turns out this is actually one of the physiological effects of aging. In fact, Stevens et al. [1] reported that the elder required twice as much salt as the young before detecting it in their tomato soup.

What’s going on

Odourants are volatile chemicals (i.e. easily evaporated) which are carried in the air and inhaled into the Regio olfactoria (olfactory epthilelium). The Regio Olfactoria is located at the top of the two nasal cavities in the nose, in between the eyes [2]. The olfactory region in each nasal cavity is ~2.5 cm2 and contains ~50 million primary sensor receptor cells. However, there are only a few connections between the Regio Olfactory and the area of the brain where speech is generated [2]. Thus humans have difficulty in describing odours with words [2]. In contrast, there are many more connections between the Regio Olfactory and the area in the brain that produces emotions. Consequently, odours have a very strong link to our emotions and can evoke strong reactions (e.g. to perfumes, odour therapies, etc.) [2]

In the olfactory region, cilia project through the olfactory epithelium into a layer of mucous. This mucous layer is ~60 µm thick and contains a lipid-rich secretion that covers the surface of the receptor cells [3]. Odourants that are soluble in the mucous interact with the olfactory receptors. The receptors then transmit an electrical impulse to the brain to be interpreted as odour [2].

Figure 1. Schematic diagram illustrating the olfactory region in humans [4]

The olfactory epithelium also contains a supplementary sensory system, the Trigeminal Nerve receptors. The Trigeminal nerve is the largest cranial nerve and provides the nerve ends that are responsible for tactile, pressure, pain, and temperature sensations in the mouth, eyes, and nasal cavity [3]. Chemical trigeminal stimulants can trigger hot, cold, tingling or irritating sensations. For example low or high concentrations of leavo-methol in the nasal cavity produce cold or hot sensations, respectively [3]. Furthermore, the trigeminal nerve receptors serve as a back up system to the Olfactory Receptors [2]. For example, they can recognize ammonia gas (i.e. dangerous substance) even if the olfactory receptors are blocked [2].

As with most senses, our sense of smell (i.e. olfactory function) deteriorates with age. It is normal for the olfactory receptors undergo apoptosis (programmed cell death)—even in healthy individuals. While these cells get replaced, this process degenerates as we age [5]. Consequently, the surface area of olfactory epithelium and the number of olfactory receptors are reduced [6].

Although it is unknown how olfactory function deterioration directly causes changes to our taste buds, deterioration with age is for certain and does in fact impact our sense of taste. Interesting enough, through a survey of test subjects of varying age, it was found that the intensity of taste  indeed decreases with age, although the discrimination between different tastes does not [7]. However, when all the subjects were placed in taste tests with noise clips on (i.e. to impair the sense of smell), the gap between the taste intensity perceived by the young and the elder groups disappeared [7]. Clearly, smell is quite important to the intensity of taste we perceive.

Impact of olfactory deterioration

Gilbert and Wysocki [8] conducted a worldwide survey of 1.42 million people to determine the odour-sensing abilities of humans. The survey asked participants if they could detect, describe, and correctly identify the odour samples provided, including: i) androstenone (urinooous, sweaty, musky odour), ii) amyl acetate (banana or pear odour), iii) Galaxolide(R) (synthetic musk odour), iv) eugenol (clover oil odour), v) mercaptans, vi) synthetic rose fragrance [8]. They reported that elderly individuals show higher detection thresholds for odours compared to young adults and odours are rated as less intense [8]. Further analysis of this study by Boyce and Shone [6] found that elderly people with poor olfaction were more willing to sample foods with unpleasant odours (e.g. fishy or cheesy odours) compared to people with good olfaction. This means that people with decreasing olfactory ability are also less finicky [6].

More importantly, those on strict diets that limit their daily salt intake should take care when following recipes that suggest ‘adding salt to taste’. Who’s doing the cooking/tasting will indeed have a strong influence on how much salt is used.

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References

[1 ] J.C. Stevens et al., “On the Discrimination of missing Ingredients: Ageing and Salt Flavour,” Appetite, vol. 16, pp. 129-140, 1991.

[2] A. Dobmann. (Sept. 2001). Adhesives and Odours [Online]. Available: http://www.eastman.com/Markets/Tackifier_Center/Documents/AdhesivesandOdorLecture.pdf

[3] J.C. Leffingwell. (Apr. 1999). Olfaction [Online]. Available: http://www.leffingwell.com/olfaction.htm

[4] Merck Sharp & Dohme Corp. (2011). Smell and Taste Disorders [Online]. Available: http://www.merckmanuals.com/home/print/sec19/ch218/ch218j.html

[5]Boyce, J. (2006). Effects of ageing on smell and taste Postgraduate Medical Journal, 82 (966), 239-241 DOI: 10.1136/pgmj.2005.039453

[6] Wysocki CJ, & Pelchat ML (1993). The effects of aging on the human sense of smell and its relationship to food choice. Critical reviews in food science and nutrition, 33 (1), 63-82 PMID: 8424856

[7] Mojet, J. (2003). Taste Perception with Age: Generic or Specific Losses in Supra-threshold Intensities of Five Taste Qualities? Chemical Senses, 28 (5), 397-413 DOI: 10.1093/chemse/28.5.397[8] A.N. Gilbert and C.J. Wysocki, “The smell survey,” National Geographic, vol. 172, pp. 514, 1987.

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