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.)

Dear Doctor: I'm 81 and usually go to bed by 10:30 p.m. Although I would like to sleep until 7 or 7:30 a.m., I wake up after 5 or 5 1/2 hours. I've tried 3 milligrams of melatonin, then 5 milligrams. What else can I do?

Dear Reader: Generally, sleep problems get worse as we get older, meaning we have more difficulty falling asleep or staying asleep and our total sleep time is lessened. The elderly in particular have a greatly reduced percentage of deep, or restorative, sleep -- that is, stages 3 and 4. This lack of high-quality sleep causes older people to feel more tired during the day and feel an increased need to take naps. Those naps can significantly affect the sleep-wake cycle at night.

If you're already forgoing daytime sleeping, you'll need to focus on other ways to improve your ability to fall asleep and stay asleep. Start by increasing your amount of exercise and by spending more time outside. Also, assess how much time you spend in the bed not sleeping. If you use the bed as an entertainment platform for watching television or some other form of media, removing the television from the bedroom would be helpful.

Pain and problems with urination or your bowels can also affect sleep, as can caffeine and alcohol. If you have the former, talk to a doctor. If you indulge in the latter, limit their intake.

In a 1999 study published in JAMA, researchers divided 78 people with insomnia into two groups. One group had eight weeks of behavioral therapy that included learning to stop daytime napping, getting rid of media prior to sleep and meditation techniques for relaxation. They compared this group with people who used drug therapy alone for eight weeks. The authors found that those who did the behavioral changes were able to fall asleep more quickly than those who used drug therapy, 55 percent compared to 46.5 percent. Moreover, the behavioral changes had sustained benefit even 12 and 24 months after the therapy, while medication had no sustained benefits.

The medication used in that study was temazepam (Restoril), but many other medications are also available to help people sleep. The problem is that many have side effects, especially for those over the age of 65. The most common side effects are seen with sedative hypnotics such as temazepam, zolpidem (Ambien) and eszopiclone (Lunesta), but also with benzodiazepines such as Xanax, Ativan, Valium and Klonopin. I would caution you not to start these medications.

As for melatonin, it appears to be safe, but doesn't seem to be working that well for you. Medications that work differently than the ones above might provide more relief with fewer side effects. One drug specifically for sleep is Ramelteon, and it binds to the melatonin receptors in the body. Another is the antidepressant Trazodone, which works by increasing levels of serotonin and can cause drowsiness.

But start by improving your sleep hygiene. Don't take naps during the day; decrease your intake of alcohol and caffeine; use the bed only for sleeping; get outside regularly; and exercise. If these methods don't work, a cognitive behavioral therapist who focuses on sleep might be able to help.

(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 reading that choosing foods that are low on the glycemic index is better for your health. What is the glycemic index and why should I pay attention to it?

Dear Reader: The glycemic index, also referred to as the GI, is a system that rates foods based on how quickly the glucose contained within them is absorbed into the bloodstream. Developed to help people living with diabetes to maintain steady blood glucose levels, the GI has become a useful tool for anyone who wants to avoid blood sugar spikes.

The food we eat is broken up into three main categories -- protein, fat and carbohydrates. Of the three, carbohydrates are our main source of energy. The body turns carbohydrates into glucose, a type of sugar, which powers bodily functions. It gives you energy for both mental (your brain is a heavy user of glucose) and physical activity.

However, not all carbohydrates behave the same way after they are ingested. Some, like sweets, baked goods and some fruits and cereals, will cause a sharp rise in blood glucose levels. More complex carbohydrates, like beans or legumes and most vegetables, are digested and absorbed slowly due to their fiber content. When you eat foods that are low on the glycemic index, it leads to a gradual and controlled rise in blood sugar. Foods high on the glycemic index cause blood sugar levels to rise and fall quickly.

The GI is based on a scale of 0 to 100, with pure glucose assigned a value of 100. The more rapidly that foods release their load of glucose into the bloodstream, the higher they rank on the scale. Candy and processed cereals have high glycemic index values. The lowest values are assigned to foods in which glucose is digested and absorbed slowly. For example, cauliflower, spinach, green beans and mushrooms all have a GI of 0.

Why does this all matter?

Chronically high blood sugar levels have been tied to a wide range of health problems, including the risk of developing diabetes, heart disease, nerve damage, cataracts and kidney disease. By learning the GI values of the foods that you eat, you can take control of your diet. You can add foods low on the GI scale that will steady your blood sugar, and eliminate -- or at least limit -- the foods that make your blood sugar jump.

It's important to note that a balanced and healthful diet will be made up of foods from all parts of the GI spectrum. There are several nutritious foods with a high GI value, such as sweet potatoes, which have a GI of 70. Meanwhile, foods with little nutritional value, such as milk chocolate, will rank lower on the GI scale, at about 40.

Foods high in carbohydrates are essential to a healthy diet, but quality matters. Stick with fresh vegetables, fresh fruit, whole grains, legumes and beans. Steer clear of processed foods, which are usually high in refined carbohydrates like white sugar or white flour. And before you make a radical change to the way you eat, be sure to check with your physician.

(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.)