Dear Doctor: I was a runner in college, but I stopped running after marriage and kids and work. I’m 62 now, and everyone says that if I resume running I’ll get hurt because I’m too old. What do you think?

Dear Reader: We believe it’s never too late to begin exercising and, with certain important caveats, this includes running.

Studies show that running offers a range of benefits to both physical and mental health. These include improvements to cardiovascular function, strength, endurance and bone health, as well as mood and cognition. An analysis of running studies conducted by British researchers published last year found that running as little as once a week was associated with increased longevity. A similar study published in 2017 found that runners reduced their risk of premature death by up to 40%. Several studies have associated running with an increase in metabolic markers associated with bone formation.

This potentially good news matters only if your approach to running matches your age and your fitness level. That includes taking into account muscle strength, balance, range of motion, heart and lung function, and any chronic health conditions you may have. Even if you are in great shape, your running life will be different today from how it was in your 20s. Based on your age, you’ll need to adjust your frequency, speed, intensity and mileage.

Since osteoporosis is a risk for post-menopausal women, and running is a high-impact activity, we believe it’s important get a bone density test. It’s a quick and painless scan that assesses bone health and can reveal any potential problems. It can also be a predictor of future injuries.

Also, please check with your health care provider before you get back to running. They can be a great resource to keep you strong and healthy, and to help track your progress. They can also make sure you’re being safe while getting back into running.

Do not try to do too much too soon. You’ve heard this before, but it really is important to ease in to your new activity. The most common running injuries, such as shin splints, stress fractures and Achilles tendon pain, arise from overuse. It’s tempting to set goals in terms of miles, but we recommend you think in terms of minutes. Begin by interspersing two or three minutes of gentle running (people used to call it jogging) with five or six minutes of running. Take a moment between cycles to stretch your quads and hamstrings, check your posture and control your breath. Give yourself a few months to gradually build up to a regular running schedule. Make sure to include strength and resistance training in your weekly exercise rotation, which will help prevent overuse injuries. Finally, finishing with gentle leg, hip and torso stretches will help your body recover and get you ready for your next run.

(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: I work in an open-plan office, and people aren’t good about staying home when they’re sick. Would wearing a surgical mask help keep me from catching someone’s cold or flu?

Dear Reader: It’s the height of our annual cold and flu season, so we understand your concern. Although the viruses responsible for respiratory illnesses are present year-round, they cause the largest numbers of illnesses in the winter months, when cold weather and short days keep people indoors. We spend a lot of time in close quarters, and in spaces that are often poorly ventilated. All of this makes it easier to transmit -- and to become infected by -- the viruses that cause influenza and the common cold.

Before discussing the pros and cons of a surgical mask, it’s useful to understand how viruses spread. A virus is a parasite that’s so tiny, it can only be viewed through an electron microscope. The influenza virus, for example, is 1,000 times smaller than a single grain of salt. It enters the body via the mucous membranes in the mouth, nose and eyes. Once inside a host, the virus injects its genetic material into a susceptible cell, hijacks that cell’s machinery and forces it to churn out millions of copies of the virus. The immune system promptly attacks the invaders, which causes the array of symptoms that make the flu such a misery -- fever, aches and pains, headache, exhaustion, coughing, sneezing and copious mucous production.

With every cough and sneeze, a sick person sends out a virus-packed aerosol mist that can travel 6 or 8 feet, bits of which can linger in the air for several hours. When a healthy person inhales the expelled droplets or picks them up from a contaminated surface and then touches their mouth, nose or eyes, they can become infected with the virus.

Unfortunately, when it comes to using a mask to protect against viruses, the evidence is mixed. Some studies, which focused on health care workers in hospital situations, found that masks can be effective at preventing infection when worn properly and used consistently. But the weave is too loose to filter all viral materials, and masks don’t always stay snug. Those same face masks may actually be more effective when worn by someone who is sick, since they block the spray of infectious matter from a cough or a sneeze.

If it makes you feel better, go ahead and wear a mask. But be sure to also follow additional precautions. Wash your hands frequently with soap and warm water, or use a hand sanitizer. Avoid touching your face since, as we mentioned, the mucous membranes of the nose, mouth and eyes are entry points for the virus. If you’re wearing a mask and then rub your eyes, you’ve defeated the purpose.

If you haven’t already done so, be sure to get a flu shot. Flu season typically peaks in January and February and lasts until the spring, so there’s still time for the vaccine to be helpful. And if you do get sick, please seek medical care.

(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: Can you please explain the keto diet? A bunch of my friends are on it, but it seems like they’re all doing it a different way.

Dear Reader: Your friends are among the millions of Americans who have jumped onto the keto bandwagon, making it the most popular diet in the United States right now.

The basic idea is simple: You cut carbohydrates to about 10% of your total food intake, typically fewer than 50 grams per day, and sometimes as low as 20 grams per day. This changes the way your body obtains energy. Instead of burning glucose, also known as blood sugar, your body is forced into Plan B -- burning stored fat. This is a metabolic state called ketosis. The name comes from compounds produced by the liver, known as ketone bodies, which the body burns for energy when glucose, its favorite energy source, is not available.

Any eating plan that causes this shift from burning glucose to burning stored fat is a ketogenic diet. You can check whether you are in ketosis with special test strips, available at your local drugstore, which detect the presence of ketones in your urine.

The high-fat, low-protein, low-carbohydrate formula may be the most widely accepted version of a keto diet right now, but it’s not the only one of its kind. Depending on your age, some of you may remember the Stillman diet from the 1960s, which eliminated carbs completely and focused instead on animal proteins such as beef, chicken, fish, eggs and cottage cheese. Part of the secret to the rapid weight loss on that diet? Ketosis. Ditto for the Atkins diet, which uses ketosis in certain phases of its eating plan, as do the paleo and Zone diets, both of which restrict carbs.

Today’s keto diet, in which up to 90% of calories come from fat, dates back to the 1920s. Sometimes referred to as “classic keto,” it was originally developed to help manage epilepsy, but soon fell out of favor with the advent of effective anti-seizure drugs. Other versions of the keto diet vary the percentages of fat, protein and carbohydrates, the trio of macronutrients our bodies require.

In addition to rapid weight loss, people in ketosis report decreased appetite. This makes sticking to such a restrictive way of eating a bit easier. The diet is also associated with improved insulin metabolism. However, while many people show improvements in blood lipids levels, the diet can raise levels of LDL cholesterol -- the so-called “bad” cholesterol -- in some people. Other challenges include headache, fatigue and irritability, particularly at the start of the diet. Many keto adherents also report dealing with chronic constipation, as well as food boredom.

A ketogenic diet can be a good way to jump-start weight loss. That means a focus on high-quality whole foods, including olive oil, nuts, avocados and fatty fish. But since this diet eliminates entire food groups, it makes getting needed nutrients a challenge. We think it’s wise to check with a registered dietician, or with your family doctor, for advice and guidance.

(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.)