While I was in Orlando, and particularly as I have just suffered from having a cold, I read a review article on Menthol and Related Cooling Compounds. I previously wrote about menthol as a cooling agent and when I did background reading for the earlier blog entry this seemed to be a key review but it wasn’t available to me at that time.
Menthol (C10H20O, MW: 156.27) is a cyclic terpene alcohol. There are four pairs of optical isomers (Wikipedia image). The isomer, (-)-menthol is the most widely occurring and has the classic sensory effects I talked about earlier. While the other 7 isomers apparently have a similar smell to (-)-menthol, they do not have the same sensory cooling action.
Menthol is obtained from members of the mint (Mentha) genus and it is the presence of menthol that gives them their characteristic smell and flavor. Mmm – has anyone else tried chocolate-mint tea? It’s delicious.
Cornmint oil is used as the primary source of (-)-menthol as it contains 70-80% (-)-menthol. Peppermint oil is another important source but it typically used directly as a flavor for toothpastes, chewing gums and candies. Peppermint oil was in the 1980s [I don’t know where to find recent numbers for this] the world’s third most popular flavoring beaten only by vanilla and citrus flavors.
The sensation of coolness is the major effect of menthol when it is applied to the skin or mucosal surface. This is a specific action of menthol on the sensory nerves as menthol alters the movement of calcium. Menthol also acts as an irritant and as a local anaesthetic. The anaesthetic response is probably also caused by menthol’s effect on calcium movement in the sensory nerves, but the irritant response is most likely a nonspecific action.
Menthol’s effect on calcium transport is probably responsible for the fact that menthol can be used to treat the spasming of the smooth muscle large bowel as observed with irritable bowel syndrome (IBS).
Menthol has a complex sensory effect in the mouth as it influences both taste and temperature receptors as well as smell. Prolonged exposure of the taste receptors to menthol anaesthetised them. While the nerve response after a dose of menthol lasted 2.5 seconds, the tongue receptors remained insensitive to menthol for up to 10 minutes. This was not observed for salt (NaCl) or other solutions.
Menthol is widely used in cold cures especially to reduce nasal congestion and coughing. However, there is little evidence that menthol actually has this effect. It seems that menthol, camphor or eucalyptus vapors have no effect on nasal resistance to airflow and menthol probably increases the production of nasal mucus – that is it makes your nose run more! However, menthol does increase the nasal sensation of airflow. My theory, I don’t know how you would test this, is that the strong smells of these oils means that you realise that you can breath as you can smell them.
Despite the fact that menthol has no effect, I’m going to keep using menthol lozenges and vapor until my cold is gone.
R.Eccles (1994) Menthol and Related Cooling Compounds J. Pharm. Pharmacol. 46:618-639