Dear Doctor: How much sunshine do I need to get my daily dose of vitamin D? And does wearing sunscreen interfere with my body's production of vitamin D?

Dear Reader: These are good questions. The major source of our body's vitamin D comes from our own production in the skin. This requires the ultraviolet rays from sunlight to form vitamin D3; both the liver and the kidneys are needed to then create the active form of vitamin D.

Those with inadequate sun exposure -- including disabled people, infants, the elderly, dark-skinned people, and those who live at northern latitudes during the winter -- are at risk of low vitamin D3 production, which can lead to a loss of bone density and an increased risk of fractures. Disabled people and infants are often less likely to go outdoors, and people over the age of 70 don't produce vitamin D3 from their skin as effectively. As for people with darker skin, they have more melanin, so less UV light gets absorbed to create vitamin D3. They need more sun exposure to produce vitamin D3 than those with lighter skin.

So to begin to answer your question, we have to look at all the variables that affect vitamin D3 production: your skin pigmentation, your general age, your latitude, the time of day when you go out in the sun and the season of the year.

A study done in Valencia, Spain, measured the amount of sunlight necessary to produce a sufficient amount of vitamin D in those with lighter skin. (Valencia is about the same latitude as Kansas City, Missouri.) The researchers took into account the amount of clothing and the season of the year. In spring and summer, 25 percent of the body (the hands, face, neck and arms) is exposed to the sun, and in these seasons, about 8 to 10 minutes of sun exposure at noon produces the recommended amount of vitamin D. In the winter, only 10 percent of the body is exposed, and nearly 2 hours of sun exposure at noon is needed to produce a sufficient amount of vitamin D.

Another study compared the geographic extremes of Miami and Boston. Researchers studied people who tanned well, but who still burned when exposed to sun. In the summer in Miami -- with 25 percent of the body exposed to the sun -- a person would need only 3 minutes of sun exposure to make a sufficient amount of vitamin D. That same person -- when placed in Boston in the winter -- would need 23 minutes at noon to produce enough vitamin D.

Then again, Boston in the winter is really cold, so you would probably have only 5 percent or less of your body exposed to the sun. Thus, 23 minutes in the sun in Boston would need to be stretched to more than 2 hours in order to ensure sufficient sun exposure. In addition, if you have darker skin pigment, the time needed to produce sufficient vitamin D would be even longer.

As for sunscreen, it can decrease the formation of vitamin D3 by the skin, but again there are many variables, such as how much of your body has sunscreen, how thick the layer of sunscreen and the level of SPF. There is a balance -- a Goldilocks zone -- between sufficient sun exposure to make vitamin D3 and the risk of getting skin cancer.

Lastly, because our fat cells can store vitamin D for months, you don't need to worry if there are days when you don't get enough sun exposure. You'll still be safe from the detrimental bone effects of low vitamin D if you get enough sun other days. So take some time to be outside.

(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.)

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

Dear Doctor: Our elderly father is now seeing several specialists, all of whom have prescribed medications. My brothers and I are worried about how many pills Dad has been told to take, and the fact that these doctors don't seem to be consulting with one another. How do we make sure the drugs are safe and appropriate?

Dear Reader: You've hit on a problem that is both serious and widespread, particularly among the elderly. It's called polypharmacy, and it describes the simultaneous use of multiple drugs to treat one or more ailments or conditions. The risks of polypharmacy range from drug interactions that may make a person feel unwell, to those that are dangerous and may cause physical harm or even death.

It's estimated that more than 40 percent of men and close to 60 percent of women over the age of 65 now take five or more medications per week. This includes prescription medications from physicians; over-the-counter medications such as aspirin or antacids; vitamins, homeopathic or herbal remedies; and nutritional supplements. Any of these, in the wrong combination, can cause problems.

When it comes to medications, these are often prescribed by several different specialists who may not be familiar with a patient's medical history. When taken in combination, some of these drugs may cause problems such as dizziness, bleeding, confusion, low blood pressure or respiratory distress. In some cases, adverse drug interactions can send patients to the hospital.

So how do you make sure this doesn't happen to your dad? If you and your brothers are helping him keep track of things, these are tasks for you. If your father is handling his own health care, these are steps he should take:

-- Start by getting organized. Make a complete list of all medications, including any over-the-counter medications as well as herbal and dietary supplements. Write down the dose of each drug and how often and at what time he takes it.

-- If your father has a primary care physician, ask her or him to check the list. It can be run through a drug interactions database to identify potential problems. If a primary care physician is not part of your father's health care, you can get the same help from his pharmacist. To keep things simple, it's a good idea to have just a single pharmacy dispense all of your father's medications.

-- If any problems come to light, contact each of the prescribing physicians and explain the issue. By coordinating with all of the doctors, you can arrive at a list of drugs that are both necessary and safe for your father to take.

-- Going forward, your father should bring this new list of medications with him to every medical appointment. He should share it with each doctor he sees so future drug interactions can be avoided.

-- When a new drug is prescribed, your father should ask what the drug is for, what is the proper dosage, how long it should be taken for, how he will know it is working and whether there are any side effects or risks associated with the drug.

-- Always add any new drug, along with its dosage, to the medications list.

-- Periodically check with the prescribing physicians regarding the drugs they have recommended. Perhaps a dosage has changed, a drug can be eliminated, or it should be changed.

(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: My son is obsessed with electronic gaming. He spends hours a day doing this. Is this unhealthy?

Dear Reader: I have a teenage son as well, so this is a question with which both you and I struggle. When I was younger, in the infancy of computer games, the games were of low resolution and the goal was simply to develop enough skill to master them. To me, this was a fun activity, often separating me from the mundane and stressful activities of everyday life.

Today's games are more complex, requiring a variety of skills in complex reasoning and hand-eye coordination. Proficient players record their techniques on YouTube, and the truly elite compete in auditoriums, watched online by hundreds of thousands of people. Is there a downside to this? Definitely -- if parents don't manage it properly.

The first potential problem with these games is the prolonged sitting required. This type of sedentary behavior can lead to obesity.

A 2004 study performed in Switzerland compared obesity rates and the hours that people spent either playing electronic games or watching television. Researchers found a significant correlation between playing electronic games and obesity, with a connection especially likely among people who spent more than two hours a day playing games. On the plus side, the obesity rates were not as high among game-players as among people who watched television, supposedly because of the passive nature of watching television and the greater amounts of unhealthy food consumed while watching it.

Other studies have not shown this obesity risk, but then they've only compared the risk of more than one hour of gaming per day versus less than one hour of gaming per day.

In a study by the American Academy of Pediatrics in 2009, children and teens who said they played more than 45 minutes of video games per day were more likely to report poorer health and increased psychological distress than those who played less than that amount. Those gamers also were found to have more behavior problems.

And consider this: In an Australian survey study of 3,000 adolescents ages 11 through 17, those who had the highest levels of psychological distress were those who spent the most time playing electronic games or roaming the internet. It may be that gaming itself is not the full problem. Rather, the gaming may be an outlet for children with emotional issues.

However, the biggest problem with gaming is that it can be addictive. The Diagnostic and Statistical Manual of Mental Disorders (DSM 5) identifies Internet Gaming Disorder as a condition that warrants further studies.

My feeling is that electronic games can have benefits for children and adolescents, but exposure to them must be limited.

I recommend that you watch for addictive behavior from your son in connection to these games and that you limit the amount of time he spends with them each day. The American Academy of Pediatrics recommends no more than two hours of screen time per day. If your son is playing these games and in addition watches television, he may have an increased risk of obesity, poorer health and behavior problems.

When it comes to activity, keep two other pieces of advice in mind. One, encourage your son to substitute more active video games over the more passive variety and, even better, encourage him to pursue sports and other forms of exercise.

(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.)

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