Dear Doctor: Why is it that vitamin D is so important? I know you can get it from food and supplements, but I'd like to get it the old-fashioned way -- from the sun. Is that too dangerous?

Dear Reader: It can't be overstated: Vitamin D is critical to good health. It's needed for calcium absorption from the intestinal tract, and for the regulation of blood phosphorus levels. Both of these are essential to developing and maintaining strong and healthy bones. Even when you get enough calcium and phosphorus in your diet, you can't absorb it unless you're also getting enough vitamin D.

Without adequate vitamin D, bones become thin and brittle and can easily bend or break. Rickets was once a widespread disorder in which children's bones were so thin and spongy from a lack of the vitamin that their skeletons failed to develop properly. In some cases, their legs became visibly bowed. Since the addition of D to milk, yogurt, breakfast cereals and orange juice, rickets has largely been wiped out.

Vitamin D also plays a vital role in brain development, muscle function, maintaining a healthy respiratory and immune system, and in optimal cardiac function. Recent research has also uncovered a potential link between a deficiency of vitamin D and depression. In fact, we now know that vitamin D receptors are present in virtually every tissue, so we fully expect research to continue to discover even more ways in which the vitamin is essential.

As you point out in your question, our bodies manufacture vitamin D in response to sunlight. That's why it's also known as the "sunshine vitamin." Specifically -- and this is a bit of a deep dive, but we think it's fascinating -- the process of vitamin D synthesis begins when ultraviolet rays strike the skin. This triggers a chemical process known as hydroxylation, which involves the liver, kidneys and certain cellular structures, and ultimately creates the chemical compound that we refer to as vitamin D. This process of synthesis also takes place with the vitamin D that we get from food and supplements.

The challenge is that getting enough D from sun exposure can take vigilance. Variables include air pollution, cloud cover, latitude, time of year, time of day, clothing, the use of sunscreen, and an individual's skin type, age and lifestyle. Each of these can make it more difficult to get enough D.

To trigger vitamin D synthesis in the body, the general recommendation is to spend between 10 to 30 minutes with arms, legs and/or torso exposed to sunlight during peak hours -- without sunscreen -- two to three times per week. Melanin protects the skin from sun damage, so darker skin requires longer exposure. For those who prefer not to deal with the potential risks of sun exposure, good dietary sources of D include the fortified foods we just mentioned, as well as fatty fish such as salmon, tuna and mackerel, and fish liver oils. The vitamin is also present in beef liver and egg yolks, but in small amounts. And if you decide to take supplements, please be sure to follow the dosage guidelines. For all its many benefits, it's possible to get too much vitamin D.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I've been taking a drug for blood pressure issues for a couple of years, but now I hear it's being recalled. What's the risk? Should I stop taking it?

Dear Reader: You're referring to valsartan, an oral medication that belongs to a class of drugs known as angiotensin receptor blockers, often referred to simply as ARBs. The drug, which works by blocking a certain chemical that causes blood vessels to constrict, is prescribed for individuals with high blood pressure. It is also used to treat congestive heart failure and may be prescribed following a heart attack. Last summer, the U.S. Food and Drug Administration announced a voluntary recall of several blood pressure medications that contain valsartan, due to the presence of a particular chemical which has been identified as a probable human carcinogen. After trace amounts of another type of potential carcinogen were detected, the FDA not only widened the recall to include more blood pressure medications, but it also issued a warning to the manufacturer, Zhejiang Huahai Pharmaceuticals in China.

Since the start of the initial recall in July 2018, investigators uncovered additional concerns, which have resulted in an ever-widening FDA recall. At this time, the agency has increased the recall list to include dozens of drugs used to treat hypertension, all due to the presence of trace impurities that are associated with cancer risk. In addition to many types of valsartan, the recall now includes losartan potassium tablets USP, as well as irbesartan tablets, which are also used to treat hypertension.

The FDA has set up several web pages to help consumers identify whether or not their particular medications are affected.

-- The recalled valsartan drugs are numerous. The list is 11 pages long and the print is pretty small, so get ready to use the zoom function on your device. You can find this list at fda.gov/downloads/Drugs/DrugSafety/UCM615703.pdf.

-- The FDA has also created a website that lists valsartan drugs that are OK and have not been recalled. You can find that one at fda.gov/downloads/Drugs/DrugSafety/UCM615704.pdf.

-- To check whether your type of potassium tablets is on the recall list, visit fda.gov/Safety/Recalls/ucm629261.htm.

-- Information about the recall of irbesartan tablets is available at fda.gov/Safety/Recalls/ucm624593.htm.

The affected drugs are being recalled due to the presence of trace amounts of either N-nitrosodimethylamine (NDMA) or N-nitrosodiethylamine (NDEA), chemical compounds that have been identified as probable carcinogens. The latter chemical has been the subject of research that also associates it with liver and blood cell damage. According to the FDA, the chance of developing cancer as a result of taking these drugs is very small.

If your particular medication is on the recall list, do not stop taking it. The medical risk of suddenly going unmedicated is far higher than the cancer risk posed by the drug. Instead, contact your physician immediately. Let him or her know your prescription is on the FDA recall list and make arrangements to find an alternative as quickly as possible.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My wife read that a man had his legs amputated after he got an infection from being licked by his dog, and now she's freaking out. She's afraid to even touch our dog anymore, lest it somehow get saliva on her. Please talk some sense into her.

Dear Reader: We can understand your wife's initial sense of alarm regarding this story and suspect her reaction has been shared by quite a few others. The idea that just a lick from a beloved family pet can have such dire consequences is indeed disturbing. But before we get to the specifics, we're glad to offer your wife reassurance that what happened in this case is extremely rare.

According to news reports, a 48-year-old man from Wisconsin was rushed to the hospital after symptoms similar to the flu -- fever, vomiting, and aches and pains -- quickly escalated. His fever spiked high enough that he became delirious, and bruises suddenly began to appear on his limbs, face and torso. The doctors in Wisconsin fought the infection with antibiotics for a week but were unable to stop its progress. In order to save the man's life, parts of his legs and hands had to be amputated. The man was diagnosed with a rare blood infection caused by the bacterium Capnocytophaga canimorsus, which is found in the saliva of many healthy dogs. Prior to becoming ill, the man had contact with several dogs, including the family pet.

The vast majority of the time, Capnocytophaga isn't harmful to humans. In rare instances, however, an infection with the bacterium can cause the immune system to go into overdrive. This happens most often in individuals whose immune systems have been weakened due to certain medical conditions like HIV or lupus, chemotherapy drugs or having had a splenectomy.

Transmission of Capnocytophaga canimorsus most commonly occurs via a dog bite, and less often through close contact in which saliva may be transferred. Although cats are also carriers of the bacterium, infection is most often linked to dogs.

Most people who become ill due to Capnocytophaga will begin to exhibit symptoms between three and five days after contact. In some cases, it can be as fast as one day, and can take up to two weeks. Symptoms of illness can include:

-- The formation of small blisters around the bite wound, sometimes beginning within an hour of the bite

-- Pain, swelling, redness or pus at the site of the wound

-- Elevated temperature, which may spike to a high fever

-- Stomach pain, often accompanied by diarrhea

-- Nausea and vomiting

-- Headache, sometimes accompanied by confusion

-- Aches and pains in the muscles and joints

If you have any of these symptoms following a dog bite, seek immediate medical help. In fact, any time that you get bitten by an animal, you should first thoroughly wash the area with soap and water, then immediately see a doctor. Not only is there the risk of infection, but animal bites can put you at risk for rabies as well.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)