Dear Doctor: One of the yoga teachers at our gym always goes on and on about the importance of the vagus nerve whenever we do deep-breathing exercises to relax. What is she talking about, and why is it important?

Dear Reader: Your yoga instructor is referring to one of the attributes of the ever-fascinating vagus nerve, which is the most complex of the 12 cranial nerves. Like each of the cranial nerves, the vagus nerve arises directly from the brain rather than the spinal cord. It’s the longest of these nerves, and it travels a meandering path through the body, beginning in the brain, through the face, neck and upper body, and to the abdomen, where it ends in the colon. In fact, its name derives from the Latin word for "wandering."

Along the way, the vagus nerve divides, branches out and exchanges fibers with numerous other nerves, thus playing a role in an impressive array of our sensory and motor functions. It’s one of the crucial links between the brain and the neck, lungs, heart and abdomen, and it helps to regulate mood, heart rate, digestion, anti-inflammatory and immune response. The gut and brain communicate via the vagus nerve, as do many other parts of the body.

There’s so much to tell about this remarkable structure that we could easily fill several columns and not scratch the surface. So let’s get right to your question about the vagus nerve and deep breathing.

One of the functions of the vagus nerve is to communicate with the diaphragm. That means that when you’re taking the slow, deep and controlled breaths associated with yoga, which engage the muscles of the abdomen and the diaphragm, you’re stimulating the vagus nerve. And since vagal stimulation has been found to help lift mood, lessen anxiety and even lower blood pressure, it follows that you can trigger this relax-and-restore response by doing some deep breathing exercises.

What’s great about diaphragmatic deep breathing is that it’s portable. You can do it anywhere, at any time, without any special equipment. And the bonus is that you can practice it in public without anyone knowing you’re doing it. If you have a home-based blood pressure device, you can do your own experiment about the benefits. Start by taking a baseline blood pressure reading without any special preparation. Follow this with a few minutes of diaphragmatic breathing. For anyone who has never tried it, it’s slow, deep breaths that fill the lungs from bottom to top. Be sure to relax and soften the muscles of the belly, which will pop out a bit as you breathe, to get the full beneficial effect.

Holding the breath for five to 10 seconds also stimulates the vagus nerve. So does adding a bit of resistance to the exhale, which can be achieved by blowing out through pursed lips. Not only do these breathing techniques have the potential to impart a sense of calm and relaxation, they quiet the fight-or-flight response in our bodies. And as you’ll see if you try the blood pressure monitor experiment, they will often yield a lower blood pressure reading.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Are those wireless earbuds everyone seems to be wearing nowadays safe? I heard that some international researchers are petitioning the World Health Organization about them because the signals that they emit may be hurting our brains. Should we be concerned?

Dear Reader: When it comes to wireless devices, which have been swiftly and near-universally adopted over the last two decades, advances have far outpaced our ability to monitor or even understand the potential health effects and consequences.

Wi-Fi, which first became commercially available in 1997, is one of the fastest-growing technologies in history. Cellphones quickly moved from a pricey niche product to the ubiquitous pocket computers of today, their telephone functions now barely an afterthought. (Fun fact -- the first truly wireless phone call was made in 1973 on a "mobile" phone that weighed 2.5 pounds.) And now, with the advent of wireless earbuds, phone manufacturers are bringing wireless tech into even closer and more sustained contact with the body.

At issue are EMFs, or electric and magnetic fields. These fall into two categories -- ionizing and non-ionizing. Ionizing EMFs are high-level radiation with the potential to cause damage to cells and DNA. Non-ionizing EMFs, which are low-level radiation, are the ones generated by wireless devices. Non-ionizing EMFs haven’t been proven to be harmful to humans. But they haven’t been shown to be harmless, either, and so the debate rages on.

As you point out in your letter, some scientists and researchers are raising an alarm. In 2015, a group of 250 doctors and scientists who specialize in EMF research signed a petition directed at the WHO and the United Nations. In it, they state their belief that the non-ionizing EMFs released by wireless devices pose a range of health hazards. These include cancer, memory problems and reproductive and genetic disorders. The U.S. Food and Drug Administration, meanwhile, does not consider the low-level EMFs released by cellphones and wireless devices to be a health hazard.

The debate about non-ionizing EMFs doesn’t hinge on the power of the energy fields generated by our wireless devices, which everyone agrees are on the low end of the non-ionizing EMF spectrum. Instead, it focuses on the effects of sustained exposure from the growing number of wireless devices we’re letting into our lives. Now, with wireless earbuds being placed into the ear canal, the concern is that we have moved from being bathed in non-ionizing EMFs from a distance to directing them into our bodies and close to our brains.

As we mentioned earlier, the results of ongoing research into the health effects of low-level EMFs remain mixed at this time. Scientists point out that Bluetooth devices, which include wireless earbuds, give off less than 10% of the radiation of cellphones. But if you’re at all worried, skip the wireless earbuds and stick with the old-school wired ones. Yes, it’s true that a number of cellphone manufacturers have ditched the headphone jack in their devices in recent years, but you can get an adapter that lets you used wired earbuds or headphones.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Could energy drinks really lead to stimulant use? How can that be? No one’s ever linked coffee or cola consumption to an increased risk of drug abuse. This sounds like some kind of urban legend.

Dear Reader: Considering the centuries-long history of coffee drinking, and the estimated 2.5 billion (!) cups of the stuff consumed each day worldwide, your skepticism is understandable. However, a 2009 study by researchers at the University of Maryland indeed suggested a link between the sustained use of energy drinks by young adults and the chance of future abuse of certain types of drugs.

The study began with 1,060 first-year college students with an average age of 18. Over the course of the following three years, the students completed three annual interviews regarding their use of energy drinks, any other sources of caffeine, and alcohol and tobacco use. The students also reported the use of prescription drugs, and any illegal drugs. In a series of post-college interviews, which took place when the participants were between the ages of 21 and 25, the researchers assessed their drug use.

Over the course of the first three years of the study, about half of the students either diminished or stopped using energy drinks. The other half continued to consume them in the same large quantities. When they followed up several years later, researchers found that students who continued robust energy drink use went on to have a markedly higher risk of using cocaine or illicit stimulants after the age of 25 than did the group of occasional or nonusers. The high-use group was also found to have an increased risk of alcohol abuse.

Whether the energy drinks themselves set the stage for the future drug abuse, or whether the individuals who were naturally susceptible to drug or alcohol abuse were also drawn to the effects of the energy drinks, is not known. However, the researchers concluded that the implied connection between sustained energy drink usage among young adults and the potential for future illicit stimulant use deserves further study. With an estimated one-third of teens between the ages of 12 and 17 now regularly downing several energy drinks per day, we heartily agree.

Addiction aside, ongoing studies continue to identify a range of health risks associated with energy drinks. These include increased blood pressure, heart arrhythmias, sleep disturbance, damage to the heart and blood vessels, kidney damage and weight gain. This last may seem counterintuitive for a product engineered to rev you up, but many varieties of energy drinks contain excessive amounts of sugar, up to 2 or more tablespoons per 8-ounce can. That means just one energy drink will put you over the recommended allotment of sugar for the day.

In addition to caffeine and sugar, energy drinks contain a range of herbs, enzymes, vitamins and other additives. And because the products are not regulated, you can’t always know exactly what you’re ingesting. Bottom line -- research repeatedly shows that energy drinks can have serious health effects, particularly in children, teenagers and young adults.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)