The Nose Knows
By Kate Jackson
Vol. 6 No. 1 p. 34

The answer to the obesity epidemic in the United States, suggests neurobiologist and psychiatrist Alan Hirsch, MD, may be right under our noses. While billions of research dollars are earmarked for studies that attempt to pinpoint the causes of weight gain and devise strategies with which to combat it, Hirsch believes that we have much to learn—and many pounds to lose—by following our noses.

When it comes to exploring the reasons individuals overeat and investigating the methods by which they can lose weight, researchers at the Smell and Taste Treatment and Research Foundation, Ltd., in Chicago believe that the nose is overlooked or underappreciated. Giving the nose its due, Hirsch, who heads the foundation, has been exploring the use of aromas and flavor intensifiers as tools that can help individuals lose weight without dieting. Instead of restricting certain foods or imposing limitations on the quantity of foods, his approach focuses on enhancing the smell and taste of food, which, his studies indicate, alters perceptions of satiety.

The foundation’s work is based upon the fundamental knowledge that the regulation of weight is influenced by hunger and satiety and the perception of hunger and satiety is modulated by emotional, physiological, and environmental stimuli. The sense of smell is both a physical and environmental stimuli, and associated with the limbic system, it is inextricably linked to emotion. Support for the notion that aroma influences feelings of fullness is found, says Hirsch, in the anatomic link between the ventromedial nucleus of the hypothalamus, which controls satiety, and the olfactory bulb. Also present as a neurotransmitter in the olfactory bulb, he observes, is cholecystokinin, a factor in gastric satiety. Additional credence to the belief in an olfactory-satiety feedback mechanism, he suggests, is lent by the fact that weight gain often occurs in individuals with severely faulty olfaction.

More than anything else, it was his awareness of the link between weight gain and a diminished sense of smell that led Hirsch on the path toward investigating the nose’s role in weight regulation. His clinical work as a neurologist and psychiatrist involved treating individuals whose sense of smell was impaired by illness, head trauma, or the use of medications. He noticed that many of these patients gained weight when their sense of taste was disrupted. In the preface to his book, Scentsational Weight Loss, he comments, “It’s as if they overate in an attempt to regain some of the pleasure from eating that had been lost along with the sense of smell. Since 90% of taste is really smell, if we can't smell food, we don’t taste it. So, these people were consuming more food but most certainly enjoying it less. It’s difficult, if not impossible, to enjoy food that has no taste.”

Hirsch continues in his book, “Assuming that the mechanism is working well, the brain can correlate the amount of food that you have taken into your mouth with the amount of odor that has reached the nose. In other words, the satiety center uses the odor molecules to operate its signals of ‘You’ve eaten enough, you are satisfied now.’ The message might actually be, ‘I’ve smelled it, therefore I’ve eaten it.’”

Hirsch’s initial studies focused on the role of olfaction in weight loss. He recruited 3,193 volunteers to test the premise that inhaling certain odors would facilitate weight loss in overweight individuals. Volunteers were required to be between the ages of 18 and 64, at least 10 pounds overweight, and could not be asthmatic, breast-feeding, pregnant, or planning to become pregnant during the six-month study. Participants were instructed to eat normally and exercise according to their habit. They were provided with inhalers containing aromatic blends and told to inhale in each nostril three times whenever they became hungry, which ranged from three to 48 times per day. Each month, the subjects were given new inhalers with different flavors, including peppermint, green apple, and banana.

Hirsch indicates, “Those subjects whose test scores showed that they had normal olfactory abilities and who used their inhalers frequently, ate two to four meals a day, felt bad about overeating, but did not feel bad about themselves lost nearly five pounds or 2% of body weight per month.” The study, published in the Journal of Neurological and Orthopaedic Medicine and Surgery in 1995, underscored that the frequency of inhaler use correlated directly to the amount of weight lost. Individuals with strong olfactory ability—those who were able to recognize the odorant in the inhalers—tended to lose more weight, with an average weight loss of 4.7 pounds per month. The aromatic blends are generally considered to be safe for most individuals. Hirsch, however, advises those with asthma to avoid using inhalant devices for weight loss and cautions that the leading side effect of inhalant use is excessive weight loss, so care should be taken and weight loss should be monitored continuously.

A serendipitous finding of this study, notes Hirsch in Scentsational Weight Loss, is that individuals can short-circuit the way in which an appealing food aroma arouses a desire or hunger for the food. Through the use of inhalants and flavor enhancers, he believes, individuals can be deconditioned to develop an appetite for a food based upon an enticing smell. In his studies, he’s seen that by inhaling the scent of a food but not actually eating that food, patterns of associations and hunger triggers were broken.

Having demonstrated that orthonasal olfactory stimuli produced weight loss, Hirsch designed research to determine whether or not retronasal olfaction would similarly influence weight loss. “After we finished our studies looking at the effects of odors on weight loss, we began to explore the concept that maybe there’s a better way for us to deliver it because inhaling throughout the day isn’t a very physiologic way of doing things.” Neither, he adds, was it convenient for people. They started to explore the concept of influencing retronasal smell. “The smell that comes from outside the nose and up to the top of the nose,” he explains, “we call smell. On the other hand, the smell that comes in from the mouth, goes up the back of the throat to the oropharynx to the top of the nose, we call that taste, and it’s really a form of olfaction.” It’s a trick of perception that’s much the same, he observes, as that which results if you close your eyes and push in on your eyeball. “You see light, yet there’s no light. You misperceive this pressure as light. It’s the same thing with smell and taste all the time. Smell in our mouth is perceived as taste, so we decided to use that route to try to facilitate weight loss.”

Subjects used two different powders each month—one salty powder to shake on salty foods, and one sweet powder to shake on sweet foods—for six months. The salty/sweet combinations included cheddar cheese/cocoa; onion/spearmint; horseradish/banana; taco/raspberry; ranch/strawberry; and parmesan cheese/malt. “The idea,” explains Hirsch, “is that the powders would intensify the flavors of the food, which would make people feel that they’ve eaten more, and they would then actually eat less.” Subjects’ weight and hedonics were measured monthly and compared with a control group of 100 individuals enrolled in a traditional diet program. The control group gained weight, while those who sprinkled tastants lost weight.

While the experiment clearly worked, the reasons for its success are fuzzy, and researchers point to a number of possible mechanisms of action. One of the theories to explain the weight loss, Hirsch says, is that the odors act on the retromedial nucleus of the hypothalamus—the satiety sensor—to make people feel full. And presumably, when they feel full, they would eat less. But the mechanism may be much more simple than that, he muses. “It may have worked in other ways. In the same way that one writes in a food diary and the act of writing reminds them that they really shouldn’t eat, maybe the act of shaking reminded them not to eat.” Or perhaps, he suggests, the physical act of shaking induces kinesthetic memory that reminds them that the desire to overeat is negative.

Another possibility—and one Hirsch seems more inclined to believe—is that the odors don’t influence the hypothalamus but act as a counterhedonic mechanism. “When you eat a piece of cheesecake, the first bite tastes really good, but by the last bite, it doesn’t taste as good. If you eat your fourth cheesecake, you can’t stand the taste anymore, and the reason is that because your hedonics for a food varies inversely to how much of it you’ve eaten.” Even if you love a particular food, he notes, you lose your taste for it if you eat it repeatedly. This phenomenon may come into play with flavor intensifiers, he explains. “By providing the same taste to all of the food through shaking the powders, subjects may have become hedonically negative toward all the food they ate. Maybe that’s why it works. I’m not sure.”

The whys are unclear, but the use of flavor intensifiers does help subjects lose weight. Hirsch points to the results of the first 108 people involved in the study. Participants used the different powders for six months, and the researchers noted an average weight loss of 34.7 pounds in that time period. Individuals in the control group, on the other hand, gained an average of 1.1 pounds. This kind of use of odors and tastes might have some greater utility in weight-loss efforts than most diets, he suggests, because it doesn’t focus on denial or require self-control, but rather on satiety. “Most diets are ineffective because they’re based on restriction, upon telling people not to eat something. In our research, we don’t restrict anything. We tell people to eat whatever they want, but to put these powders on whatever they eat.”

Dietitians interested in helping their clients lose weight, says Hirsch, should tell them to use their noses. “Tell your clients to sniff their food before they eat it, which lets the odor molecules stimulate the olfactory nerve. Advise overweight individuals to chew their food a lot, not only because it slows the eating process, but because it aerates more of the molecules so more molecules reach the top of the nose, which in turn creates the impression that you’ve eaten more than you actually have.” In addition, he advises, tell clients to heat their food whenever possible because heating also aerates more of the molecules. In addition, blowing bubbles or whipping food may also increase aeration and let you perceive that you’ve eaten more than you’ve really consumed.

Hirsch is broadening his studies to include a larger group of people and seeking several thousand volunteers. Participants must be between the ages of 18 and 64, at least 10 pounds overweight, like to eat between meals, and have a normal sense of taste. They may not be pregnant or breast-feeding, and may not have diabetes, uncontrolled hypertension, or Sjögren’s syndrome. Subjects may not exercise for more than 40 minutes per day, and they must like either ice cream or pickles. Potential volunteers may contact the Smell and Taste Treatment and Research Foundation, LTD, at 800-664-3880 or visit www.smellandtaste.org.

Hirsch’s hope is that this research will open greater inquiry into sensory phenomenon in the realm of food research. The ways in which taste and odors influence the desire to eat, he claims, has never really been explored. “We’ve done all sorts of things. We’ve operated on people’s stomachs, stapled them, banded them, and given them all sorts of medications that can cause cardiac side effects. But, no one until now has looked at the effects of chemosensation on diet and appetite.”

— Kate Jackson is a staff writer for Today’s Dietitian.

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