Often overlooked in favor of cooler, sexier senses such as sight or touch, smell and taste are more complicated than many people might think and have a surprisingly sweeping impact on behavior, perception and overall health.
For instance, the sense of smell might provide clues to some of the mysteries of Alzheimer's and Parkinson's, while genetic differences in taste could hold the key to predicting what we eat, how well metabolism works, and even whether or not we're overweight, according to new research.
At the same time, experts say taste and smell do work together, in ways you might not realize, to produce some of the basic sensations of everyday life.
"The sensation of flavor is actually a combination of taste and smell," said Tom Finger, a professor at the University of Colorado-Denver Medical School and chairman of the 2008 International Symposium on Olfaction and Taste, held last month in San Francisco. "If you hold your nose and start chewing a jelly bean taste is limited, but open your nose midway through chewing and then you suddenly recognize apple or watermelon."
That's because as you chew, you're forcing air through your nasal passages, carrying the smell of the food along with it. Without that interplay of taste and smell, you wouldn't be able to grasp complex flavors, Finger said. Instead you'd be limited to the basic taste sensations picked up chemically by the tongue: salty, sour, sweet, bitter and umami — a savory sensation frequently associated with the additive MSG.
Because of this connection, losing your sense of smell can end up being devastating. Food no longer tastes as good, and these eaters miss many scent-related emotional connections as well. For instance, studies have shown that people, particularly women, can identify the specific smell of their romantic partners, Finger said. And, because scents are often more novel than, for instance, shapes or other things you might see, scent often gets intertwined with our memories of places and events.
"It's the novel things we recall," said Richard Doty, professor and director of the Smell and Taste Center at the University of Pennsylvania School of Medicine. "So an odor similar to that of your grandmother's pantry might be more quickly associated with your memories of that place than a similar sight, which might be more generalized."
Smell and memory fade together
Doty also thinks that smell and memory may be connected in another, more disturbing way. Loss of smell is one of the initial symptoms in degenerative neurological diseases such as Parkinson's and Alzheimer's. In fact, he said studies have shown a big connection between lowered sense of smell and the likelihood that a person will develop such diseases later.
"They've done studies measuring the sense of smell in people who have no signs or symptoms of Parkinson's, then taken the top 10 percent and the bottom 10 percent and followed them for years," he said. "All the people who were ultimately diagnosed with Parkinson's later in life came from the group who had smell problems when they were younger."
Why would these seemingly disparate things be connected? Doty said there's a possibility that a significant number of Alzheimer's and Parkinson's cases are caused by environmental factors. In that case, the nose could serve as an entry point for whatever prions, viruses or toxins are getting into the brain and damaging it.
This theory hasn't been proven yet, but there's lots of circumstantial evidence tying the nose and olfactory system to both diseases. Doty outlined some of that evidence earlier this year in a Jan. 29 article in the journal Annals of Neurobiology. He said if the theory does prove to be true, it could lead to better ways to prevent the diseases, including drugs or filters that block intruder access to the brain through the nose.
Taste of genetics
Meanwhile, other studies presented at the San Francisco symposium showed some big connections between what people are genetically programmed to taste and how they eat — connections that could possibly even explain some of the genetic components of obesity.
One example is GLP-1, a hormone long known to be produced by the stomach and to control the production of insulin in the pancreas. In the July issue of the Journal of Neurochemstry, Steven Munger, assistant professor of anatomy and neurobiology at the University of Maryland School of Medicine, presented evidence that GLP-1 is produced by some cells in the tongue as well. Munger's team was able to prove that GLP-1 helps the tongue communicate with the brain, particularly when it comes to sweet tastes. When they bred mice that lacked a GLP-1 receptor, the mice lost much of their ability to taste sweet foods.
Munger said there's a big opportunity for GLP-1 to play a role in the choices people make about food, particularly if, as is very possible, some people have a better ability to produce or detect GLP-1 than others. More importantly, he said, the research shows some definite connections between taste and metabolism.
"GLP-1 isn't the only hormone acting on the taste system. Leptin from fat is there as well, and GLP-1 and leptin seem to act in opposition to each other. One is an up regulator and one is a down regulator, and the result is some fine control on metabolism that's related to taste," he said.
Another compound that seems to play an even larger role in what you choose to eat is PROP, a chemical marker for the ability to taste certain bitter flavors. About 70 percent of people can taste PROP, said Beverly Tepper, professor of food science at Rutgers University, but bitterness isn't the only flavor it seems to effect.
"People who are tasters find other bitter compounds to be more intensely bitter than people who are non-tasters. They find sweet things to be more sweet, hot things to be more hot. And they also perceive the fattiness of certain fats as being more intense," she said. "Non-tasters taste all these things, but with less intensity."
She said non-tasters were more likely to seek out foods that have intensified versions of the qualities they didn't perceive very well: More heat, more sweet, more fat. "Non-tasters, we have evidence that they consume more added fats, salad dressings, spreads, and margarines. We've even seen in some studies that non-tasters also consume more calories," she said.
In the 2008 "Annual Review of Nutrition," a volume due out in August, Tepper outlined the implications PROP sensitivity can have on individual nutrition. She said the next step for her research is to study whether and how knowledge of a person's PROP-tasting status could be used to help them create a more personalized diet plan.
"I'd like to figure out what kind of advice we can give to people who want to lose weight and how that would differ between tasters and nons," she said. "For instance, we're often asked to reduce the amount of fat we consume, but that could be more difficult for nons. So maybe we turn up the volume on the foods we're offering them to make up for the lack of sensory stimulation that they'd be getting from a low-fat diet, maybe adding non-caloric flavor enhancers, like mustards and vinegars."