Dear Doctor: I'm going sailing with friends soon. Since I tend to get motion sickness in cars and airplanes, I'm worried that means I'll get seasick as well. Are there any natural remedies?

Dear Reader: If it helps at all, you're far from alone. A large portion of the population suffers some degree of motion sickness during travel. The most common effects -- dizziness, nausea, even vomiting -- are sure to ruin your day. For some people, additional symptoms like cold sweats, a throbbing headache or a bout of anxiety only deepen the misery.

The culprit in motion sickness is believed to be (scientists still aren't sure) the disconnect between what your eyes see and what the sensitive mechanism in your inner ear, which controls balance, feels.

When you walk or run or twist or bend down, those structures in your inner ear, known as the vestibular system, are in sync with what your eyes are telling you. You are moving, but the ground you stand on is not.

On a boat, however, your eyes and your inner ear are sending seriously mixed messages to your brain. Your eyes know you are sitting still, but according to your vestibular system, you're in motion. For reasons that are still unclear, it is this sensory disconnect that lights up pathways in the brain, causing the symptoms of motion sickness. And all you want is relief.

Some sufferers rely on antihistamines such as Dramamine, an over-the-counter medication, which can control nausea and vomiting. Prescription medications, including the Transderm Scop patch and promethazine, can also help with the symptoms of seasickness. But these medications can have side effects such as dry mouth or drowsiness.

Since you want to go the natural route, let's take a look at your options.

According to the Centers for Disease Control and Prevention, simple behaviors can give you a leg up. Stay hydrated, skip the beer and cocktails, don't drink any caffeine and limit eating to small meals. If possible, gaze out at a fixed point on the horizon. This lets your brain confirm the input that it's getting from your eyes and your inner ear -- yep, you're definitely in motion. (For people who tend to get carsick, riding in the front seat can help in the same way.)

As for natural remedies, a lot of travelers swear by ginger, which has been shown to ease nausea associated with motion sickness. You can find ginger in powdered form in many natural food stores and pharmacies. Dramamine also offers what it says is a clinically tested full dose of powdered ginger, packaged in a single capsule. Candied ginger and ginger tea are also options.

Although there is conflicting evidence over its effectiveness, acupressure has its proponents. In this method, constant pressure is applied to the insides of the wrists via special elastic wristbands.

When you do find an effective natural cure, there's a bonus. You're now prepared for the latest frontier in motion sickness -- virtual reality!

(Eve Glazier, M.D., MBA, is an internist and assistant professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and primary care physician at UCLA Health.)

(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.)

Dear Doctor: Should I take large doses of vitamin D to prevent osteoporosis?

Dear Reader: This is an excellent question. Vitamin D appears to be a wonder vitamin in regard to health. For whatever ails you, it seems, vitamin D is the cure. But that's not necessarily true.

Vitamin D is so crucial for healthy bones that deficiencies in children lead to the malformed bones seen in rickets. Because vitamin D helps the body absorb calcium, it's vital for a host of other biological functions as well. In fact, vitamin D acts more like a hormone in that way.

The recommended daily allowance of vitamin D is 600 to 800 units. It can be found in foods fortified with vitamin D -- and the numbers of those products seem to be increasing -- or from fatty fish or eggs. But the majority of the vitamin D that our bodies need begins with UVB light; our bodies use this light to produce the precursors of vitamin D.

Adults who are out in the sun regularly and have normal vitamin D levels probably don't need a vitamin D supplement. A meta-analysis published in the renowned medical journal The Lancet reviewed 24 studies that followed patients who took vitamin D. The research found little change in bone density with supplementation of vitamin D. Some studies found that vitamin D reduced the risk of bone fracture -- but only when it was combined with calcium.

Calcium on its own has been shown repeatedly to decrease the risk of fractures. Vitamin D on its own has not been shown to decrease fracture risk.

Some practitioners recommend mega-doses of vitamin D to their patients, causing very high blood levels of it. But high levels of vitamin D in the bloodstream might actually increase the rate of bone breakdown.

The highest daily intake considered safe by the Institute for Medicine is 4,000 units. If you are taking vitamin D, have your blood levels checked. For the prevention of osteoporosis, the Institute of Medicine recommends a vitamin D blood level greater than 20 ng/mL (nanograms per milliliter). The majority of people can achieve this level without supplementation, but if you do supplement, 600 units is more than sufficient.

As for all the other purported health benefits of vitamin D, such as its ability to decrease the risk of cancer and heart attacks, there may be benefit to vitamin D intake, but we're not sure yet. We need good long-term studies that compare people who take vitamin D daily against those who don't.

Until then, if you don't have osteoporosis and you do have sufficient vitamin D levels in your blood, there appears to be little additional benefit to taking a vitamin D supplement.

(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)

(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.)

Dear Doctor: I love to go for long walks and lately have noticed that I'm developing pain in my hip joints. I think that's just part of aging, but my daughter is urging me to try a glucosamine supplement. Can this really be helpful?

Dear Reader: Glucosamine is a compound that is naturally produced within your body. It plays a key role in building and maintaining healthy cartilage, the connective tissue between your joints. Tough and flexible, cartilage provides a smooth, lubricated surface over which bones can easily move and glide without friction or pain.

But cartilage has a limited ability to repair itself. When it is damaged, or when it wears away over time as we age, the resulting joint pain and stiffness can turn even the simplest tasks into an ordeal. Loss of cartilage, which leads to degeneration of the underlying bone, is what makes osteoarthritis so painful.

The use of glucosamine, a supplement sold in capsule and tablet form, became popular in the 1990s after several best-selling books touted it as a hedge against osteoarthritis. Several studies had suggested that glucosamine has anti-inflammatory properties and might even aid in the regeneration of cartilage. Sales of the dietary supplement spiked.

However, subsequent research has offered conflicting medical data about the benefits of glucosamine. In the largest study thus far, which looked at more than 1,500 adults with osteoarthritis of the knee, glucosamine proved to be no more effective than a placebo. Only the participants who were given acetaminophen for pain reported ongoing relief.

In several other studies, meanwhile, participants with osteoarthritis of the knee and hip who took glucosamine reported reduced pain and swelling, as well as increased range of motion. Little wonder that the glucosamine debate is lively and occasionally contentious.

If you do decide to give glucosamine a try, the National Institutes of Health report that glucosamine appears to be safe and well tolerated when taken in suggested doses over a two-year period of time. Although infrequent, reported side effects are headache, indigestion, heartburn and constipation. Since glucosamine is made from shellfish, including shrimp and crab, individuals with shellfish allergies or who are sensitive to iodine are warned to avoid it.

It's also an excellent idea to also follow recommendations put forth by the American College of Rheumatology. Their panel of experts -- primary care physicians, rheumatologists, geriatricians, surgeons, physical therapists and other health care professionals -- suggests the following:

-- Exercise: Thanks to increased muscle strength and toning, individuals with arthritis who exercise regularly have more energy, report less pain, sleep better and are better able to carry out daily tasks.

-- Lose weight: Every extra pound that you carry puts additional stress on your knees, hips and spine.

-- Medication: Check with your primary care physician whether oral or topical pain relievers might be helpful. Acetaminophen is a common first treatment for osteoarthritis. Nonsteroidal anti-inflammatory drugs, also known as NSAIDs, can also help with pain and swelling.

As with any treatment plan, make your primary care physician your partner. Whether it's giving your daughter's idea about glucosamine a try, or following through with the American College of Rheumatology's recommendations, your doctor is there to help, encourage and guide you.

(Eve Glazier, M.D., MBA, is an internist and assistant professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and primary care physician at UCLA Health.)

(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.)