Why the smell of food makes you hungrier

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Fasting for 24 hours improves the sense of smell

What matters:There was a correlation between the odour threshold and body weight.— photo: special arrangement
What matters:There was a correlation between the odour threshold and body weight.— photo: special arrangement

Traditional families regularly practice days of fasting, during which they would skip meals — be it on Maha Sivaratri, days of the Ramzan month, or some even on a New Moon Day.

Some of these people who do such fasting claim that they end up eating more than the usual meal when they break the fast — and that the very smell of food makes them hungrier and to eat more than the normal portion. Is this true or just a feeling of an emptied stomach?

Branch of science

Well the branch of science called “hedonics” (note its connection with the phrase ‘hedonistic’), which is a study of pleasant and unpleasant sensations and states of the mind, seems to have an answer.

A recent paper in the professional journal titled Appetite (how appropriately named!) reports that fasting for 24 hours improves nasal chemosensory performance (meaning improves the sense of smell) and food palatability (meaning makes you eat more than usual and enjoy it as well). In other words, changes in smell function can modify feeding behaviour.

This group of scientists from Canada inducted a group of nine men and six women as volunteers and tested their threshold of smell by asking them to smell a set of “sniffing sticks” on two separate occasions — first when they had a proper meal and so were “fed”, and later when they were asked to go without any food for 24 hours, or “fasted”.

The odour threshold and discriminatory power were higher in the “fasted” group than in the “fed” group. So too was the palatability of the food.

Body weight

There was also a correlation between the odour threshold and the body weight of the volunteers, but this was more so in women than in men. The researchers concluded that fasting for 24 hours improves smell function and increases palatability.

It is still not clear why this is related to the body weight and the sex of the individuals. More research is needed to confirm this correlation, but we cannot yet dismiss their work as “fishy”!

Now comes a recent paper in the February 9 issue of the rather more prosaically named journal Nature Neuroscience , from a group of European scientists led by Dr Giovanni Marsicano of Bordeaux, France.

They wanted to unravel the mechanisms behind why hunger arouses sensory perception and increased food uptake.

To this end, they worked with mice because one can do direct in vivo testing on the neuronal circuits connected with sensory perceptions in the regions of the brain of live mice.

One can also work with such mice which are genetically devoid of genes which are connected with the odour perception circuit in the brain (genetically manipulated mice in which these genes are “knocked out”, so as to make them lose odour perception).

Earlier work by the Marsicano group showed that the molecule in the neurons in the mouse brain which triggers odour perception is called CB1 receptor.

Marijuana effect

This receptor, incidentally, responds to the smell of marijuana (also called hemp, hashish, or bhang, ganja, bangi, kanjavu, bangiaku in various Indian languages), and is technically called cannabinoid receptor 1 or CB1R. Though called cannabinoid receptor, CB1R responds to a variety of smells.

The group now started working with two sets of mice, one termed the control group and the other, the experimental group, members of which were given a bit of marijuana (actually the active molecule from this is called tetrahydrocannabiniol or THC) to activate their CB1R.

Now they analysed the smelling and eating patterns of the two groups for comparison. First they were both made to sniff almond oil.

The THC administered mice were seen to sniff longer and deeper, and also to eat more feed than the control group of mice. Thus, the connection between activation of the odour perception circuit in the brain and greater intake of food was seen to be true.

No difference

Next, they recruited knock-out mice in which CB1R was absent and hence no smell perception. Administering them with THC made no difference.

Thus, the role of odour perception to feeding became clarified.

Then, they made a group fast for 24 hours with no feed at all and these behaved the same way as the ones treated with THC in the first set of experiments. Those ‘fasted’ mice smelt the food longer and ate more than the control group mice.

Thus what was seen in humans by the Canadians is true with mice as well, plus we now implicate the role of the odour sensor neurons and the CB1 receptor in the process.

So, now we know why the some food, when hot and steamy tastes better than when it is cold.

May be weight watchers should watch the temperature of their daily diet as well (and may be eat cold food?), and maybe those with anorexia should check their odour perception first, and repair that first rather than go on long binges of fasting.




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