Dear Doctor: I'm having trouble keeping my New Year's resolution to get more exercise, and one of my co-workers swears that an activity tracker or an exercise app will help me stay focused. What are they, exactly, and why would they work?

Dear Reader: First, let us reassure you that you're not alone. By deciding to exercise more, you've committed to creating a new habit, and that's always a challenge. There's the honeymoon phase, when you're buoyed by elation and optimism, and when getting out every day for a walk or a run or a trip to the gym is a snap.

However, this is often followed by a period of struggle as reality sets in. No matter the scope of the goal, you've committed to a process that moves gradually, in increments, and requires consistent willpower. Suddenly, finding an excuse to skip that yoga class is all too easy.

But take heart. For those who don't give up, who power through the disruption and uncertainty that a new activity can bring, there comes a magic moment. Suddenly, the new behavior has become second nature. It's a habit, a part of your life.

For those who decide to make exercise a new habit, an activity tracker or an exercise app can make that difficult middle phase bearable. They help you set a goal, track your progress and keep a diary of your effort. A recent study found that 75 percent of the participants who used an exercise app said they were more active. They also reported a drop in their BMI, or body mass index.

Depending on the product, you can track your heart rate, calories burned, distance you have run and the number of steps taken in a day. Some apps and devices help you with food choices and let you log your food intake. Dive deep into the technology, and you'll find devices that can offer feedback on the duration and quality of your sleep.

As to specifics, a fitness app is simply a program that you download onto your smartphone. You can use it to log information about your workouts and, depending on the app, receive data that tells you how well you're doing in achieving your goals. Some popular apps, such as MyFitnessPal, are free.

Fitness apps are highly personal. They help you stay aware of what you're eating, how much you're moving and whether you're improving. Your long-term goal is to exercise more. What an app can do is provide enough detail to set daily goals. It can make incremental progress interesting and even fun, which is both the heart and challenge of creating a new habit.

An activity tracker, like the popular Fitbit, is basically an exercise app that you wear. It uses sensors to detect your movement and, with an algorithm, translates the raw data into statistics about your progress. You then download the data onto your computer to use as you wish.

Whether you choose an app for your smartphone or the wearable tech of a fitness tracker, what you're getting is a helpful blend of coach, trainer and cheerleader.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I've been taking thyroid medication for several years, and my doctor says my blood tests are where they should be, but my face feels like sandpaper, my nails are brittle, and I'm losing so much hair that I can see my scalp. Could there be some underlying problem my physician is missing?

Dear Reader: Hair loss can be a distressing symptom, made all the more so when its cause is a mystery.

Your thyroid hormone levels are an obvious place to start, because both low and high thyroid levels can lead to hair and nail changes. Symptoms of high thyroid levels include hair loss, skin that is unusually smooth and warm, and nails that soften and loosen from the nail bed. The remaining hair becomes thinner, softer and does not hold a wave. Symptoms of low thyroid levels also include hair loss, including in the armpits and genital area, but the hair in this scenario is dull, coarse and fragile. As for the nails, they tend to be delicate, thin and have multiple grooves. That said, if your physician has done a complete panel of thyroid tests and the results have been normal, then most likely the function of your thyroid gland is not the cause of the brittle nails nor the hair loss.

That doesn't mean the thyroid isn't a factor. Autoimmune thyroid disease can lead to hair loss, both patchy and more diffuse, as well as inflammatory conditions of the skin. Such disease isn't always reflected in thyroid hormone levels. Checking anti-thyroid antibodies in the blood can identify autoimmune thyroid disease, and point you and your doctor in a clearer direction.

Hair loss also can be caused by androgenic alopecia, linked to an excess of androgens, a type of male hormone. These hormones are present in both men and women, but they're higher than normal in some women, such as those with congenital adrenal hyperplasia or polycystic ovarian disease, which is relatively common. Simply checking levels of testosterone and dehydroepiandrosterone (DHEA) can either rule out androgenic alopecia or suggest that it be explored further.

Another potential cause is medication. Some medications can lead to hair loss, so if your symptoms seem coincidental to starting a new medication, there might be an association.

Biotin deficiency, which is rare, can also cause hair loss and inflammation of the facial skin. But if you have a normal diet and eat eggs, you have a low likelihood of this condition. Nonetheless, it's something to rule out.

Iron deficiency also can lead to both brittle nails and hair loss. This doesn't explain the skin manifestations that you have, but if you are looking at other possibilities, checking the iron level of the blood should be part of the workup.

Any major illness can lead to hair loss and nail changes, and psychological stress can lead to hair loss. So, if there have been major stressors in your life, either physical or psychological, consider that a potential culprit.

In summary, if your thyroid levels are normal, it would be wise to check your thyroid antibodies, androgens and iron levels -- and your level of stress.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Our family went out to dinner the other night, and when I urged our kids to order a salad instead of french fries because of the dietary fiber in salads, they challenged me to explain why. I was embarrassed that all I could come up with about the benefits of fiber was "because it's good for you." Can you help?

Dear Reader: Don't feel bad! With your brief answer, you got right to the heart of the matter -- dietary fiber is integral to a well-balanced diet and to a healthy life. It lowers your risk of heart disease, diabetes and certain cancers, prevents constipation, stabilizes blood sugar and can help you maintain a healthy weight.

But kids are often natural skeptics when it comes to any kind of rules, so we're happy to help you out with the science of why dietary fiber matters.

Let's start with what we're referring to when we talk about fiber. Dietary fiber is the part of the fruit, vegetable or grain that your body cannot digest and absorb. It falls into two basic categories. Fiber that can dissolve in water is known as soluble fiber. Fiber that cannot dissolve in water, and which passes through the body pretty much intact, is known as insoluble fiber.

Each type of fiber offers a specific health benefit. Soluble fiber dissolves in water to form a kind of gel matrix, which puts the brakes on the entire digestive process. The presence of soluble fiber slows the absorption of sugars, which stabilizes blood glucose levels. It binds cholesterol so that instead of being absorbed by the body, cholesterol is excreted. And because the stomach takes longer to empty when soluble fiber is present, you wind up feeling fuller for longer.

Insoluble fiber, meanwhile, has an equally important job. Not only do the texture and "chew" that insoluble fiber adds to food make a meal more interesting, it travels relatively unchanged through the digestive tract and gives much-needed bulk to stool. This results in easier and more regular elimination. Insoluble fiber also helps to control the pH of the colon, and helps to prevent microbes from producing cancerous substances.

How much fiber do you need? Current recommendations put the number at 25 grams per day for women under 50, and 38 grams per day for men under 50. For adults over 50, the number is 21 grams per day for women, and 30 grams per day for men.

Although all kinds of fiber supplements are available, they lack vitamins, minerals and micronutrients, so it's better to eat whole foods. Most fruits, vegetables, beans, legumes and grains contain both soluble and insoluble fiber, but in differing amounts. For the best results, eat from a wide range of foods.

If fiber has been missing from your diet, it's wise to add it slowly. Make the change to a high-fiber diet too quickly and you run the risk of unpleasant side effects, such as gas. And be sure to drink plenty of liquid to keep the fiber -- and your digestive tract -- moving smoothly.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)