Nicotine on the Brain

Conventional wisdom holds that tobacco smoking ipso facto leads to a reduced sense of smell. Conventional wisdom is also wrong: the scientific evidence is mixed. Not to mention that some of the revered French perfumers of the past have smoked like chimneys.

A new study by researchers at the University of Dresden Medical School addresses the topic from a different angle. They used MRI scans to measure the size of the olfactory bulbs in smokers and non-smokers. It’s an interesting approach because we know that reducing neuronal input to the bulbs—by blocking a nostril or damaging the sensory epithelium in the nose—results in bulb shrinkage. Conversely, restoration of smell function makes them bigger.

In addition to bulb size estimates based on the brain scans, the researchers measured smell function in two ways: detection threshold for phenylethanol (rose alcohol), and odor identification ability with a 16-item test.

The results? There was no difference between smokers and non-smokers in odor detection or identification. Score another finding of no discernible effect of smoking on the sense of smell.

However, the scans showed that smokers had significantly smaller olfactory bulbs. “Aha!” say the anti-tobacco fascisti. “That proves smoking is harmful to the nose.”

But does it? The explanation for olfactory bulb shrinkage is that it results from peripheral damage to the sensory tissue in the nose. But if the noses of smokers in this study were damaged enough to cause their bulbs to shrink, how come they didn’t show reduced ability on the smell tests?

The authors of the new paper offer an intriguing alternative hypothesis. Their findings “could be a direct effect of nicotine on the neurogenesis/synaptogenesis of the olfactory bulb.” In other words, nicotine might have a damaging effects on these nerve cells quite independent of smoking.

That suggests a new study: compare sense of smell and olfactory bulb size in users and non-users of chewing tobacco. If chewers smell as well but have smaller bulbs, it would mean that nicotine, not smoking, is the chief factor.

Place your bets.

The study discussed here is “Olfactory bulb volume in smokers,” by Valentin A. Schriever, Nicole Reither, Johannes Gerber, Emilia Iannilli, Thomas Hummel, published in Experimental Brain Research 225:153-157, 2013.