Dear Doctor: Could one's sense of smell really be used to predict Alzheimer's? I've never had a very good sense of smell, so does that mean I'm doomed?

Dear Reader: It's true that recent research has suggested a link between an impaired sense of smell and an increased likelihood of developing dementia. But before we get into the details, it's important to note the researchers agree that this dysfunction is not a risk factor for, and not a cause of, dementia. Instead, the results of the study suggest that an impaired sense of smell may be an early warning sign that someone is in the process of developing the condition.

In the study, which was published last year in the Journal of the American Geriatrics Society, about 3,000 participants between the ages of 57 and 85 had their sense of smell evaluated. They were asked to sniff a series of "odor pens," which look somewhat like a felt-tip marker. They were then asked to choose the scent they detected from four possible options. The five scents they were asked to identify were orange, rose, peppermint, fish and leather. A solid majority -- 78 percent -- correctly identified four out of the five scents. (Of the five scents, leather proved to be the most challenging.) Three scents were correctly identified by 13.8 percent of participants, 4.9 percent got two scents right, and 2.2 percent correctly named one scent. One percent of study participants were unable to identify any of the five scents.

Five years later, researchers conducted home visits with either the study participants themselves, or a proxy, such as a friend or relative, if the individual was too ill or had passed away. They discovered that those individuals who could not correctly identify at least four of the five scents were twice as likely to have developed dementia. There was also a correlation between the fewer scents identified and an increase in the likelihood of a dementia diagnosis.

What's going on? The researchers suspect that our sense of smell is so closely connected to brain function that when it declines, it can be an indicator of more serious and complex problems. Our sense of smell takes place via the olfactory nerve, which leads from the mechanisms in the nose that identify odors and connects directly to the brain. In addition, the scent sensors in the nose are constantly being replaced, with olfactory stem cells producing new cells when the old ones die off. This has led scientists to wonder whether the decrease in the ability to smell indicates that the brain's ability to rebuild certain vital components may be in decline and could be related to changes connected to dementia.

As for your own poor sense of smell, that appears to be a condition you've lived with for some time. As we mentioned earlier, flunking the smell test doesn't amount to either a risk factor or a cause of dementia. Instead, it means that in your case, the smell test would not be an appropriate diagnostic predictor.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I smoked for more than 20 years. And although I stopped about 20 years ago -- I'm now in my early 60s -- I worry about what changes it might have wrought in my cells. Should I get a CT scan just to be sure?

Dear Reader: Your concern is understandable. Tobacco smoke is a potent carcinogen, and lung cancer is the leading cause of cancer death among both men and women in the United States. On the plus side, detection of budding lung cancers saves lives. Among people diagnosed with Stage 1 lung cancer, the survival rate is 92 percent at five years; the survival rate for those with Stage 4 lung cancer is 6 to 8 percent.

As for whether CT scan screening specifically can reduce deaths from lung cancer, let's look at the numbers. A 2011 study analyzed data of 53,454 men and women who had a 30 pack-year or greater smoking history; a 30 pack-year is the equivalent of smoking one pack of cigarettes daily for 30 years. The participants were either current smokers or had stopped within the last 15 years. The study compared the results of having either yearly CT scans or yearly X-rays for three years, and the participants were followed on average for just over six years.

Screening X-rays had no real benefit in reducing the lung cancer death rate, but CT screening did -- a 20 percent reduction, in fact. That's because CT scans were much more likely to find cancers. Previous trials of CT screening had not shown the same survival benefits because those trials were much smaller.

CT screening for lung cancer is not without risk. For starters, CT scans detect many nodules that are not cancer. This can lead to additional scans to evaluate the nodule or to a biopsy of the lesion, potentially requiring major surgery. In the 2011 study, 96 percent of nodules 4 millimeters or greater -- considered a positive finding in this trial -- were not lung cancer. Nonetheless, invasive procedures were needed to address 11 percent of those nodules, creating the associated risks of lung collapse, bleeding and death.

Also, the chest radiation incurred through such scans could potentially increase the risk of radiation-induced cancers. (This concern is somewhat tempered by the use of low-dose CT scans for screening.) Finally, some slow-growing lung cancers found by CT screenings may never have led to a problem.

That said, CT screening for lung cancer does improve overall mortality rates as well as mortality rates from lung cancer. But strict guidelines -- based on the 2011 study -- address who can be screened as part of their insurance coverage. The U.S. Preventive Services Task Force, the American Association for Thoracic Surgery and the American College of Chest Physicians recommend CT screening for those adults ages 55 to 79 who have a 30-pack year history of smoking and who are either current smokers or smokers who have stopped in the last 15 years. Medicare and other insurers follow these guidelines as well.

That's not to say you aren't at risk for lung cancer, but the benefits of lung cancer screening may not be greater than the risks.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Apparently, wounds that occur at night don't heal as well as those that occur in the daytime. Why would that be -- and what can people do if they're hurt at night?

Dear Reader: An intriguing study that looked at wounds and the rates at which they healed reinforces the idea that the circadian clock, the inner timekeeper in virtually every living thing, is even more complex than was realized. Considering the circadian clock in humans is already known to manage sleep-wake cycles, hormone production, brain wave activity, body temperature and mood, to name just a few of its functions, that's pretty impressive.

The circadian cycle is a period of approximately 24 hours. During that time, and keyed to the daily shift from light to dark and back again, the circadian clock influences rhythmic changes in both physiology and behavior. Disruption to these cycles, whether short- or long-term, is known to adversely affect health and well-being. Research has shown that the cycle of sleep and wakefulness in mammals is keyed to visible light. A specific region of the brain receives and interprets the visual input from the eyes and sends out the appropriate cues of whether to sleep or wake up.

But recent studies have surprised scientists by suggesting that certain parts of the body, including the liver and lungs, have circadian rhythms that are independent of patterns of dark and light, and of the brain center that interprets those patterns. All of which (finally!) leads us to your question. According to research published in the journal Science Translational Medicine, burns that were sustained during the day took about 17 days to heal. The same types of injuries that were sustained at night required an additional 11 days -- or 28 days total -- to heal.

The reason for this turns out to be the behavior of fibroblasts, which are among the first cells to rush to the rescue at the site of an injury. These take various forms, depending on their location within the body. But what fibroblasts have in common is the ability to generate a matrix of specialized proteins that help a wound to contract and heal. How quickly they get to work depends on whether it's day or night.

When researchers looked at cells grown in a petri dish, at wounds in mice and at data from the International Burn Injury Database, which includes the time that a patient was injured, they discovered that daytime wounds healed a startling 60 percent faster than the same types of wounds when sustained at night. Because fibroblasts obey their own circadian clocks, which are independent of a person's own master clock, it turns out that they work harder and more efficiently during the daytime hours.

At this time there's no known way to override the fibroblasts' circadian clock. When you become injured during the nighttime hours, the healing process will be slower. However, researchers are already looking into how these insights might be used to improve medical care -- surgical outcomes, for example -- and to explore the impact on other forms of treatment.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)