Dear Doctors: My heart rate was 99 when I went for my physical recently, and my doctor said that’s too high. I explained it’s usually about 80, but medical appointments make me nervous. Still, it got me to thinking: What’s a normal resting heart rate? How do I get it to be lower?

Dear Reader: Heart rate refers to how many times your heart beats each minute. Add in the word “resting,” and you’re talking about a heart rate measured when someone is calm and relaxed. Along with blood pressure, an individual’s resting heart rate is often considered to be a window on their general health. And, just so you know, it’s not that unusual for someone’s heart rate, along with their blood pressure, to become elevated in the sometimes-stressful setting of a medical office.

According to the American Heart Association, a resting heart rate that ranges from 60 to 100 beats per minute is normal for an adult. Factors such as age, gender, height, weight, physical fitness, lifestyle and general health each play a role in a person’s resting heart rate. Research has shown that being a cigarette smoker often elevates heart rate. So can stress, depression or being sick with a cold or the flu.

Elite athletes and people who are physically fit tend to have lower resting heart rates, sometimes measuring in the 40s or 50s. Medications can have an effect on heart rate, as well. For example, stimulants such as Ritalin and certain antidepressants have been shown to raise resting heart rate. Other drugs such as calcium channel blockers or beta blockers can slow the heart rate down. Some studies have associated a rise in resting heart rate with an increased risk of complications such as atrial fibrillation or coronary artery disease.

Although resting heart rates vary widely from person to person, for each individual they tend to remain fairly stable over time. If you should ever notice a sudden and lasting change, check in with your doctor. Becoming familiar with your heart rate, and how it changes when you’re relaxed and when you’re active, can help you stay aware of your own general health. Start by measuring your heart rate before you get out of bed in the morning. Just locate the pulse point on your wrist, count the number of beats for 30 seconds and multiply by two. Repeat the process throughout the day, during various activities and while at rest, and you’ll develop a sense of how your heart behaves.

One of the best ways to a lower resting heart rate is through physical activity. A 2018 analysis of studies on the topic found that exercises that build endurance, as well as yoga, had a beneficial effect on resting heart rate on both women and men. Deep breathing and meditation are also helpful. Substances like caffeine, nicotine and sugar all increase heart rate, so be aware of what you ingest. And, as with all types of change, be realistic. Start slow and gradually build toward a goal so you’re in control and stay safe.

(Send your questions to askthedoctors@mednet.ucla.edu. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I’ve heard that a lot of people don’t get enough vitamin D in their diet. What about getting it from the sun? Is that too risky? Also, what happens if you don’t get enough?

Dear Reader: Vitamin D is a fat-soluble nutrient that our bodies require in order to absorb calcium from the intestines. That calcium is then used to harden the tissues of the skeleton, a process known as mineralization. Vitamin D is also crucial to bone growth, plays a role in managing inflammation, helps prevent involuntary muscle spasms and aids in the regulation of blood phosphorus levels.

Not getting enough vitamin D makes it difficult to maintain adequate levels of calcium and phosphorus, which can cause bones to gradually become thin and brittle. Together with calcium, vitamin D also helps protect older adults from osteoporosis. In children, a lack of the vitamin can lead to rickets, a disease that interferes with mineralization. Rickets is a sometimes painful disease, marked by poor skeletal formation and soft, weak bones that can become malformed.

When our skin is exposed to sunlight, our bodies become natural vitamin D factories. The energy in ultraviolet rays triggers a complex chemical reaction that, along with help from the liver, kidneys and certain cellular structures, results in the formation of vitamin D. The nutrient is also naturally available in fatty fish, including tuna, mackerel, salmon and sardines, and in smaller amounts in certain foods such as beef liver, cheeses, egg yolks and some mushrooms. However, we don’t eat these foods in large enough quantities to satisfy our body’s ongoing need for vitamin D. To help make up for a potential deficit, a range of prepared foods are fortified with the nutrient. These include orange juice, breakfast cereals and dairy products, as well as some soy, rice and noodle products.

The amount of vitamin D that you need to maintain optimal bone health depends on your age. Infants up to 1 year old should get 400 international units (IU) per day. Children, teens and all adults up to the age of 70 are advised to get 600 IU. After age 70, when our bodies become less efficient, the recommendation increases to 800 IU per day.

It’s rare, but possible, to get too much vitamin D. The addition of the nutrient to prepared foods is carefully regulated, and our bodies naturally limit the amount they manufacture from sun exposure, so you’re not going to run into problems there. If you’re going to rely on vitamins or supplements for your vitamin D intake, be sure to check with your health care provider.

Getting enough vitamin D from the sun depends on location, weather and skin type. Also, sunscreen limits how much a person can produce. For those with light skin, 15 to 30 minutes of full sunlight on bare arms, legs or torso a total of two to three times per week is adequate. Due to the protective effects of melanin, darker skin requires longer exposure. People with any kind of skin cancer history or risk should rely on diet and supplements to get their daily allowance.

(Send your questions to askthedoctors@mednet.ucla.edu. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: Can you explain herd immunity? We have it with diseases like smallpox and polio, but when it comes to the coronavirus, everyone keeps saying it’s a bad idea. Why wouldn’t it work for COVID-19?

Dear Reader: It’s true that we’re hearing a lot about herd immunity, which is also sometimes known as community immunity, from a variety of sources these days. There’s quite a bit of conjecture, and even misinformation, in some of these discussions, so we’re glad you’ve broached the topic.

Herd immunity refers to the point at which a large enough portion of a population becomes immune to an infectious disease that the illness can no longer be easily spread. This offers protection to the population at large, including to the minority who are not themselves yet immune to the infectious agent.

And you’re correct that we have herd immunity to thank for the eradication of smallpox and the extremely low incidence of polio throughout the world. The reason for this is the success of widespread vaccination efforts, which helped to make the vast majority of people immune to what had previously been serious health threats. In fact, thanks to worldwide smallpox vaccination programs, that disease was declared eradicated in 1980.

Researchers estimate that at least 60% to 70% of the population will have to become immune to the novel coronavirus in order to achieve herd immunity. Unfortunately, we don’t yet have a vaccine to achieve that. That means the only path to herd immunity at this time is for a significant portion of the populace to become infected. Based on the current U.S. population, we’re talking about almost 200 million adults and children becoming infected with the novel coronavirus in order to achieve herd immunity.

As we write this, the total number of novel coronavirus infections in the U.S. has just passed 6.5 million, far short of the 200 million needed for herd immunity. The disease has already caused more than 200,000 deaths since the start of the year, and it has left many survivors with serious and lingering health conditions. So you can see that, while achieving herd immunity through natural infection theoretically is possible, it would come at an unbearable price.

It’s true that the health risks of COVID-19 may recede as we continue to learn about the disease and, hopefully, develop new and more effective treatments. That would make becoming infected with the virus less dangerous. However, another important question remains unanswered. That is, how long do people who have recovered from COVID-19 remain immune? Unfortunately, we just don’t know yet.

All of which brings us back to the discussion we’ve been having for most of this past year. Until there’s a safe and effective vaccine, we need to do our best to slow transmission of the virus. Wear a cloth face covering in public, avoid large gatherings, maintain physical distancing, wash your hands or use a hand sanitizer, avoid touching your face, and disinfect high-touch surfaces. These are challenging times, so please, remember to be kind to one another.

(Send your questions to askthedoctors@mednet.ucla.edu. Owing to the volume of mail, personal replies cannot be provided.)