Sniffing out the truth

There was some discussion here last week about the integration of multiple senses -- in that case, hearing and sight -- and now there's news of a study published in the journal Chemical Sense that reports that a heightened sense of smell affects how much we eat.  This is important because, well, we'll let them explain. 

The relationship between hunger state and olfactory sensitivity is important for both olfactory and appetite research. This connection has a more urgent need due to the obesity epidemic and the demonstration that obese adults have reduced olfactory sensitivity.

Of course, this is a science story so it's real headline is the 'urgent need' for more research.  That pressing priority notwithstanding, previous studies have found that people have a heightened sense of smell either before or after they have eaten -- according to the authors of the current study, this discrepancy may be because of methodological differences in the studies.  Previous studies have also found that people with a high body mass index (BMI) have lower olfactory acuity than those with lower BMI -- unless it's the other way around.

So the authors of the present study wanted to determine which was right (Yin or Yang), and further, whether people have a heightened sense of smell when they are hungry or full, and whether it differs by BMI.   Or mood.  Or alertness. Or just a way to get another grant.  Maybe the answer is 'maybe'?  But fortunately for these authors, whatever they find, the answer will agree with some previous study and so can be said to be furthering the field!   

This study looked at a huge total of 64 people, and assessed their sense of smell before and after lunch, which was chosen as described below (the study was in the UK). 

Participants were asked to choose between 2 sandwich options, either chicken and bacon (470 Kcal) or cheese and celery (480 Kcal) (Marks and Spencer). They were also given a packet of Hula Hoops cheese and onion crisps (129 Kcal) and a bowl of chocolate chip cookies (126 Kcal) (Sainsburys). In order to ensure the lunches were acceptable, a pilot study was completed where 6 participants (3 females and 3 males) were presented with a selection of sandwiches with different fillings and a variety of sweet and savory snacks. The savory and sweet snacks with the highest pleasantness ratings were selected for the study. For the sandwiches, in order to cater for vegetarian and meat options, the 2 sandwiches with the highest pleasantness ratings that were also most similar on this dimension were selected.

They did personality measures, mood measures, hunger and alertness measures, the latter as described here:

Ratings of hunger were made using 100 mm unmarked line scales end-anchored “not at all” and “extremely,” with the adjective “hungry” centered above the line. In addition to this adjective, other mood adjectives were also used (alert and drowsy), mainly to divert attention away from the real purpose of the study but also to provide data on how temporary suspension of lunch might affect behavior.

And, they measured olfactory threshold and discrimination (from the Sniffin Sticks battery), before and after lunch.

And what did they find?  That people's smell acuity doesn't vary for non-food related odors, but that they can smell food odors somewhat better after lunch than before.  Especially if their BMI is high. 

At first glance, this seems counter intuitive because on the basis of evolutionary theory, we might well expect the ability to detect foods that are edible and ripe to be more advantageous in a hungry compared with satiated state. Though it could be theorized that better olfactory acuity following a meal might in fact aid in the regulation of food intake, that is, as it is then easier to detect and reject foods that are no longer required. 

 
One response to this can't be printed in a PG-13 post.  Olfactory sensation is based on combinatorial detection among hundreds of highly variable olfactory receptor genes so, like the immune system  we don't have to be pre-programmed for specific odors or recognize specific pathogens.  Whatever we're able to detect does seem to be in major chemical families that certainly relate to food.....and predators, and so on.  But humans have largely lost specific pheromone sense, again pointing to the idea that we learn smells, and are born with the ability to detect an open-ended set of odors.

There may of course be modifiers of acuity, and this study seems to claim that there are, and that they are environmentally contextual.   Of course, those who for whatever reason get into McFoods are likely to come to like their aroma, and perhaps be more sensitive to them.  Afterwards, all they need is a good belch and a nap.  Skinnier people need to keep thinking "Food!"  Do we need Darwin and lethal fitness differences to account for that?  To imagine what, in Alley Oop's day, smelled like Big McMax, but only to those who had just tagged along at the hunt and were only thrown a bone or two to gnaw?

The bow to Just-So stories -- er, evolutionary explanations -- aside, here's what the lead author had to say to the BBC about his results:

[Dr Stafford's] team found that people who are overweight - those with a higher body mass index or BMI - have a far heightened sense of smell for food compared to slim people, particularly after they have eaten a full meal.

Dr Stafford believes this keener sense of smell might compel the individual to carry on eating, even when they are full.

He said: "It could be speculated that for those with a propensity to gain weight, their higher sense of smell for food related odours might actually play a more active role in food intake.

Is this a disguised 'thrifty genotype' argument, that those with the better sense of smell ate more, stored more, and starved less?   It doesn't seem to be.

But clearly more research is needed.  But in fact, why?  If slim people stop eating when their sense of smell is more acute than when they are hungry -- or rather, when they stop eating their sense of smell is slightly heightened -- how does it follow that the fact that obese people might have a more heightened sense of smell after eating make them eat more?   And why would we think that this correlation (keeping in mind that correlation is not causation) has anything to do with why people eat, or why they eat too much?

Why can't the press smell useless science by now? We report these items as a service to our readers.