Dear Doctors: I’ve been an endurance athlete my whole adult life. After having COVID-19, I’ve gone from running marathons at an 8-minute mile pace to needing walking breaks whenever I run. Are there studies about long COVID and endurance? How do I find one I can join?

Dear Reader: You have introduced an important topic that will be of interest to many readers. The issue of long COVID -- and of COVID itself -- has largely vanished from public discussion.

However, in the nearly six years since the start of the COVID-19 pandemic, there have been more than 111 million verified cases of the disease in the United States, and well over 1 million deaths. Those who survive are at risk of developing long COVID, an ongoing collection of symptoms that persist for weeks, months and even years after the initial infection has passed.

Long COVID affects everyone differently. The symptoms, as well as their severity and duration, vary from person to person. Some people experience only one or two symptoms, while the condition affects others in multiple ways.

These symptoms include heart palpitations, changes to blood pressure, insomnia, headache, brain fog, an ongoing loss of the sense of smell and taste, depression, gastric disturbance, bowel problems, anxiety and exhaustion. Unfortunately, the decrease in physical stamina that you have experienced is common. It is referred to as “exercise intolerance,” and sometimes as “post-exertional malaise,” or PEM.

Studies into exercise intolerance among people who have recovered from COVID reveal it to be a complex and multifaceted condition. It involves changes to how the muscles maintain strength, how they use energy, how the cardiovascular system maintains aerobic capacity and responds to stress and how the autonomic nervous system regulates heart rate and blood pressure. Researchers have also observed changes to immune function that affect endurance.

Because long COVID varies so greatly from person to person, there is no single approach to managing exercise intolerance. For some people, pushing through the exhaustion only makes symptoms worse. For others, a gradual and measured approach of low-intensity and short-duration exercise yields gradual improvement.

The current guidance for people living with COVID-related exercise intolerance recommends working with, and being monitored by, your health care provider or an experienced physical therapist.

As for research into this aspect of long COVID, the good news is that it is widespread and ongoing. The best source for finding an appropriate study is clinicaltrials.gov, which is maintained by the National Institutes of Health, and the U.S. National Library of Medicine.

The homepage includes a form into which you would enter the search terms “long COVID” and “exercise intolerance.”

Another resource for anyone living with long COVID is the NIH RECOVER initiative, at recovercovid.org. It includes research, resources and news updates about the syndrome.

For information about ongoing long COVID studies, click the link at the top of the page on the right-hand side. This information includes all aspects of long COVID, including exercise intolerance, cognitive issues, cardiovascular and other physical issues and sleep disturbance.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

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Dear Doctors: I’ve noticed as I’m getting older -- I just turned 63 -- that I have more episodes of dry mouth. I’m told this isn’t unusual for older people. Why would that be? Chewing gum helps, but I don’t enjoy it. Are there things I should eat or avoid that can help this go away?

Dear Reader: Dry mouth is just as it sounds, a shortage of saliva that leaves the tongue, teeth and inner surfaces of the mouth without adequate hydration and lubrication. Also known as xerostomia, this occurs when the salivary glands don’t produce enough saliva.

For most people, dry mouth is something that happens occasionally, often when they are nervous, anxious or under stress. But for an estimated 10% of the population, the condition becomes chronic.

Symptoms include a perceptible drop in saliva production; saliva that is thick, foamy or stringy; a sticky sensation inside the mouth; a rough, dry tongue; bad breath; and dry or cracked lips. This decrease in saliva can cause the tongue, cheeks and inner surfaces of the mouth to stick to each other and to the teeth.

It may become difficult to chew and swallow dry or crumbly foods. Because a lack of saliva can change the optimal pH inside the mouth, the risk of tooth decay, gum disease and fungal infections increases.

It’s true that the condition can occur more often in older adults, with an estimated 30% of people over 65 affected by the condition. However, this is not due to age-related changes in the body.

Rather, dry mouth is most often a side effect of certain health conditions, medical treatments and medications. Diabetes, salivary stones, some autoimmune conditions and some neurodegenerative diseases can adversely affect the salivary glands. Tobacco use, alcohol consumption and even behaviors such as prolonged open-mouth breathing can affect moisture levels within the mouth.

Dry mouth is associated with a wide range of prescription and over-the-counter medications. These include certain blood pressure medications, antidepressants, bladder control medications, antihistamines, anti-inflammatories, diuretics, opioids, GLP-1 drugs, nausea medications, muscle relaxers and antipsychotics. The condition is also a common side effect of radiation, chemotherapy and immunotherapy.

There are several dietary steps that you can take to ease the symptoms. It is important to stay hydrated, but to avoid carbonated and sugary drinks, which can increase acidity in the mouth. Caffeine and alcohol can contribute to fluid loss, so limit or avoid them. Xylitol, an artificial sweetener, triggers saliva production and output.

Since you don’t enjoy chewing gum, you might consider using hard candies to help keep your mouth moist. Fresh fruits and vegetables with high water content, such as melons, cucumber and celery can also help.

If you take any medications, your pharmacist can help you pinpoint if dry mouth is a common side effect. And because dry mouth can be a symptom of an underlying medical condition, it would be wise to check with your doctor if your episodes become more frequent.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I just read about these electronic mouthguards that can show if someone has a concussion during contact sports games. Apparently, if you get hit really hard, they will flash and let the coaches know that you need to get out of the game. This seems like a great idea. What took so long?

Dear Reader: You are asking about the news that an international women’s rugby tournament will be introducing the use of an “instrumented” mouthguard that can calibrate the force with which a player is tackled. If that force is powerful enough to cause a head injury, it will trigger a flashing red light within the mouthguard. This, as you say, alerts the coaching staff that the player should exit the game for a medical evaluation.

While contact sports such as rugby and football can be exciting to watch, the repeated tackles and collisions during the game put players at significant risk of head, face and neck injuries. These include concussion, fractures, intracranial bleeds, cerebral contusion, dental and eye injuries, and cervical spine injuries.

Even soccer, in which players use their heads to direct the ball, is coming under new scrutiny. For decades, due in part to cultural attitudes and economic pressures, the issue of head and neck injuries in contact sports flew under the radar.

With the emergence of high-profile cases of chronic traumatic encephalopathy, or CTE, the issue of head injuries in contact sports became a public health concern. CTE is a progressive disease in which repeated blows to the head damage the brain and cause cognitive decline.

Tragically, CTE can be diagnosed only after death, via an autopsy. The awareness of CTE, along with pressure from players, families and the medical community, has led to renewed efforts to protect athletes from playing while injured. That is where the flashing mouthguard enters the picture.

Like a traditional mouthguard, this new so-called “smart” version is made from food-grade thermoplastic that is resilient yet flexible. What makes it different are the tiny motion sensors embedded in the plastic. These sensors use accelerometers to detect the force with which a player is hit head-on and gyroscopes to measure angular and rotational forces.

Because our upper teeth are in a fixed position, they absorb the same impact as the rest of the head. That means the instruments in the mouthguard can provide an accurate picture of what has happened to a player’s head during contact.

The upper limits of an acceptable collision are calibrated using medical studies and biometric and biomechanical data. When one of the smart mouthguards registers contact that exceeds the preestablished limit, it triggers a flashing red light. Due to the difference in the physical size and bone and muscle structures of women and men, the danger zones of the mouthguards are calibrated accordingly.

With this kind of real-time data, players can be pulled from a game even when symptoms are not immediately present. Cumulative hits, another danger of contact sports, can also be tracked. Flashing mouthguards don’t eliminate the risks of contact sports, but they do represent a significant step forward in player safety.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)