Dear Doctors: I’ve heard that when you’re on a diet and you decrease caloric intake by a lot, your body will start to consume muscle for energy. It that really true? How can you lose weight and not wind up losing muscle?

Dear Reader: Discussions about losing weight typically focus on pounds, as in, “I want to lose 10 pounds.” The important follow-up question here is: pounds of what? Work up a sweat with a vigorous set of tennis, and the scale will show you’ve immediately dropped a few pounds -- of water weight. You’ll gain it right back with your next beverage. (A quart of water weighs 2 pounds, in case you were curious.)

Diets that involve a drastic calorie cut do lead to weight loss, but participants wind up burning not only fat, but also significant amounts of lean muscle. That’s a bad idea, because we rely on our muscles for both strength and endurance. Muscle tissue also plays an important role in resting metabolic rate.

The answer to the “pounds of what?” weight-loss question is, of course, fat. When we set a weight-loss target, the goal is to lose fat. Or, to view it in more useful terms, we want to achieve a more healthful ratio of lean tissue to fat. Achieving and maintaining a healthful weight has many benefits, including reducing the risk of diabetes, high blood pressure, coronary artery disease, depression and even certain cancers. And, while it’s tempting to try some of the more extreme low-carb and high-fat diets that are now popular, which promise swift and painless weight loss, we believe a moderate approach yields better and more sustainable results.

In order to maximize fat loss and minimize the loss of lean muscle mass, you need a three-pronged approach. This begins with cutting back calories enough that you’re operating at a slight deficit, but not to the point that your body thinks it’s starving. A recent study in the Netherlands followed volunteers on a five-week diet of 500 calories per day, and those on a 12-week plan of 1,250 calories per day. Both groups lost the same amount of weight -- about 19 pounds. However, the crash dieters lost almost three times as much muscle mass as those who followed the more moderate weight-loss plan.

To stay healthy as you lose weight, eat a wide range of fresh foods, with a focus on lean proteins, healthy fats, and fresh fruits and vegetables. Take care to avoid simple carbohydrates and processed foods. You want a diet that, once you’ve reached your goal weight, you can continue to sustain.

Finally, it’s important to incorporate two kinds of exercise into your daily life: cardio and resistance. Cardio helps to burn fat, and resistance maintains and builds the lean muscle mass that you want to conserve. Again, the aim here is a new set of habits that you’re willing and able to maintain even after you’ve reached your goal weight.

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

Dear Doctors: Our 16-year-old daughter wants to become a vegan. Her father and I think that may be a bit extreme, so we’ve compromised, and first she’s going to try being a vegetarian. What’s a good way for a growing teen to safely make the transition?

Dear Reader: We’re both parents ourselves, so we understand your concerns about meeting your daughter’s nutritional needs. A vegan diet, which cuts out all foods derived from living creatures -- including eggs, dairy products, gelatin and honey -- can send you on a steep learning curve. Even the more lenient parameters of a vegetarian diet take care and planning to be healthful and well-balanced.

As with any diet, the goal is to get enough calories, protein, vitamins and minerals from a wide array of fresh and healthful foods. The good news is that vegetarian and vegan diets are quite popular. That means the information and products your daughter needs to be a healthy vegetarian are widely available.

When following a vegetarian diet, your daughter will no longer eat red meat, poultry, fish or other seafood. Vegetarians may choose whether or not to eat eggs and dairy products. (Some, referred to as pescatarians, include fish in their diets.)

Although it can be tempting to dive into the deep end with a new lifestyle choice, we suggest a gradual transition. Instead of eliminating meat, start by adding an array of foods to your daughter’s existing diet. This includes the tofu, tempeh, seitan, beans, legumes, nuts, seeds and whole grains that will become staples of her new way of eating. Once she’s familiar and comfortable with these new foods, she can start eating them instead of the meat-based meal the rest of the family is having. A vegetarian we know made an easy transition by eliminating one category of meat at a time. She started with beef and, every few weeks, stopped eating another type of meat. Within a few months, she had achieved her goal of becoming a vegetarian.

When it comes to nutrition, you want to keep an eye on B12, a vitamin that’s essential to the proper function of the body's nerve and blood cells. B12 also plays a role in the synthesis of DNA, and it helps prevent megaloblastic anemia, which causes weakness and exhaustion. In addition to meat and fish, B12 is found in milk, cheese, eggs and some fortified cereals. Several studies have found that even vegetarians who consume eggs and milk can become deficient in B12, so a supplement may be a good idea.

People who no longer consume meat also have to take care to get adequate calcium, iron, zinc and protein. Protein needs can be satisfied by eating a variety of beans, legumes, nuts, seeds and soy products, as well as eggs and dairy products.

A few good vegetarian cookbooks will be essential tools for coming up with varied and healthful meals and snacks. We also think it would be wise for your daughter to meet with a registered dietitian to learn the nutritional guidelines of her new lifestyle.

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

Dear Doctors: Can you please explain about masks again? Why do we need them, what kind should we wear, and what is the proper way to wear them? It would really help me to explain to my friends and family so they can be safer.

Dear Reader: We continue to get letters from all parts of the country asking these same questions. We’ve written about masks before, but new information often takes repetition to sink in. It also takes time for new habits to take hold. We’re happy to do a recap for those of you who are still unsure about the why and how of using a mask during the COVID-19 pandemic.

Now that we have learned so much more about how this particular coronavirus spreads, wearing a mask turns out to be a crucial part of protecting our families and communities. The bottom line is that you wear a mask to keep your breath to yourself. Research shows that the main way this virus travels is via the droplets and aerosols we emit when we sneeze, cough, speak and sing, and even when we breathe. When you wear a mask, you create a barrier that catches some of these particles and limits the distance your breath can travel. Combine that with the 6-foot buffer zone we have come to know as social distancing, and the odds of transmission are significantly reduced.

Even if you’re not worried about the coronavirus on your own behalf, you can be sure that there are vulnerable people all around you. This one simple habit -- which, let’s face it, isn’t a real burden for the majority of us -- can literally save a life. In fact, the life you save may be your own. The latest research shows that a good mask, worn properly, can also help to protect the person who is wearing it.

A mask works by providing a network of fibers that both slow the force of an exhale and capture respiratory droplets. The closer the weave, the more effective the filtering effect of the mask. Tightly woven cotton masks with multiple layers are more efficient than looser fabrics, such as a scarf or a bandanna. The nonwoven materials of the N-95 masks are the most effective. Shape and fit also play an important role. A good mask has a snug but comfortable fit without any gaps around the perimeter. It fits over the nose and under the chin, and it leaves enough space around the nose and mouth for easier breathing. Masks with valves don’t slow or trap any aerosols and, therefore, do not protect the people around you.

When you breathe out through a mask, the fibers capture many of the droplets you generate. The rest travel a much shorter distance than they would in a maskless exhale. When someone wearing a mask exhales near you, the load of aerosols headed your way is reduced. If they do reach you, your own mask filters them, again reducing the number of particles that reach you. When everyone wears a mask, we’re working together to reduce one another's level of risk.

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