Effect of Anosmia on Hunger
In the literature with which I am familiar anosmia is frequently associated with a loss of anosmia. If food has no more pleasurable flavor, why eat. But I have corresponded with many anosmic persons who say that after becoming anosmic they ate more than they did earlier, apparently in an attempt to get the pleasure that anosmia robbed them of. Another way to think of this is that the pleasure of eating is reduced but not totally eliminated by anosmia, so they overeat in an attempt to get more pleasure. I count myself as one of those overeaters. Also, when anosmic I increased my use of hot peppers to get more sensation from eating. One possible explanation for overeating in some anosmic persons is "afferent inhibition of hunger."
The regulation of hunger is quite complex. When the brains detects that levels of nutrients in the blood decline or there is a decline in leptin, which is secreted by fat cells, the brain turns hunger on. But what turns hunger off? The brain could just wait until levels of leptin or blood nutrients returned to their normal levels, but if we kept on eating until that happened we would be eating too much. The brain needs to be able to turn off hunger after we have eaten just enough, which is generally well before the food is digested and excess nutrients converted to fat. There are a number of afferent inhibition systems that monitor eating and turn off hunger after enough has been eaten but before it is digested. “Afferent” means from the outside to the brain, so afferent pathways are sensory pathways. Signals from the muscles involved in chewing and swallowing and from stretch and food detectors in the stomach tell the brain how much has been eaten, and, when enough has been eaten, the brain turns off hunger, temporarily. If normal levels of leptin and nutrients in the blood are not restored after a while, then hunger is turned back on. Researchers cut an esophageal fistula (hole) in the throats of each of several dogs, so that food that they ate would not reach their stomachs. Such dogs did stop eating after eating about as much as they normally would - the afferent inhibition from chewing and swallowing was effective in turning off hunger - but, since the nutrients were never absorbed into the blood, the hunger returned after a half hour or so.
Humans sometimes use such afferent inhibition to try to keep hunger turned off, by chewing gum or by swallowing diet-aids that swell up in the stomach. Overeating in some persons with anosmia may be related to the fact that they do not get afferent inhibition from the smell of foods they eat. That is, since anosmic persons do not get the sensations of smell and taste when they eat, afferent inhibition normally produced by such sensations does not occur, so there hunger lasts longs, resulting in overeating.
Not everybody starts overeating when they lose their sense of smell. Some people have the opposite problem -- they no longer enjoy eating and their caloric intake is greatly reduced. This suggests that the hunger control system varies considerably among individuals, as previously demonstrated by psychologist Stanley Schachter. Schachter showed that obese individuals' hunger seems to be controlled primarily by "external" factors, the smell and taste of the available food, while normal-weight individuals' hunger is controlled primarily by "internal" <physiological> factors. Denied of tasty foods, but provided with a bland diet that was nutritionally complete, normal-weight individuals' caloric intake was unchanged, but that of obese individuals dropped drastically.
Karl L. Wuensch, 16-October-2022