Dear Doctors: I have read that older adults may not get enough vitamin D in winter. My father-in-law is 73 years old. He’s from Florida, but he’s spending a year with us here in Maine. He can’t be outdoors all the time like he is at home. How important is vitamin D? How do we know if he’s getting enough?

Dear Reader: Your question keys right into why vitamin D is often referred to as the “sunshine vitamin.” It’s a nutrient essential to human health and well-being and is produced by the body in response to exposure to sunlight. Vitamin D helps the body absorb calcium and maintain proper blood levels of both calcium and phosphorus. These functions are critical to the growth and maintenance of healthy teeth and bones. Newer research shows the vitamin has an anti-inflammatory effect, helps the body fight infection and can reduce cancer cell growth in some situations. Vitamin D receptors are found in tissues throughout the body and in several major organs. This suggests additional roles for the nutrient that have not yet been identified.

How much vitamin D someone needs depends on their age. Children, teens and adults up to the age of 70 are advised to get 600 international units, or IU, per day. Absorption becomes less efficient as we age, so older adults, like your father-in-law, are advised to get 800 IU per day. For infants up to 12 months old, the recommendation is 400 IU of vitamin D per day. While the nutrient is found in some foods, such as fatty fish, beef liver, egg yolks, cheeses and some mushrooms, most of us don’t eat enough of these to fulfill our daily requirement. To compensate, a range of commonly consumed prepared foods are fortified with the nutrient. These include dairy products, many breakfast cereals and some brands of prepared orange juice.

And then, of course, there’s sunshine. When the ultraviolet rays in sunlight hit the surface of the skin, they trigger a complex chemical reaction. With an assist from the kidneys, liver and other cellular structures, our bodies manufacture vitamin D. For those with light skin, 15 to 30 minutes of full sunlight on bare arms, legs or torso at least two or three times per week will do the trick. Melanin offers a protective effect, so people with darker skin need longer exposure. People with any kind of history or risk of skin cancer should rely on diet and supplements to get their daily allowance.

It’s important to note that several factors can interfere with natural vitamin D production. Clothing and sunscreen either partially or completely block UV light, which hinders or prevents vitamin D formation. In northern latitudes like your own, with shorter days, weaker sunlight and bad weather that keeps people indoors, getting enough vitamin D naturally can become a challenge. Your father-in-law can learn his vitamin D status with a simple blood test. If it’s low, his health care provider will offer guidance on supplementation. These can include fortified foods, vitamins or, in cases of serious deficit, vitamin D injections.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I had COVID-19, and now I’ve got long COVID. I have neuropathy and brain fog, and when I try to exercise, I get exhausted right away. My doctor said it just takes a while to get back on your feet, like after you’ve had the flu, but this feels different. What is the latest news on long COVID?

Dear Reader: As we enter the fourth year of living with COVID-19, it’s clear the initial illness isn’t the only health risk for those who contract the disease. They also face the chance of developing long COVID, the constellation of symptoms that can continue long after the initial illness has ended.

Symptoms include the physical and neurological effects that you have described. The condition can also adversely affect the heart, respiratory and circulatory systems, stamina, endurance, cognition and mental and emotional health. Long COVID does not appear to be linked to the severity of a case of COVID-19. And to make things even more complex, the symptoms can take a few weeks, or even months, to appear.

The newest data shows that 1 in 5 people who recovered from COVID-19 later develop long COVID. With more than 100 million cases of COVID-19 in the U.S. to date, that puts the number of long COVID patients well into the millions. Because the condition is so new, doctors are still struggling to understand what is happening to their patients. And, as with all new diseases, the learning curve is steep. This puts a burden on both patients seeking help and the medical workers trying to treat them. Here at UCLA, as is occurring in medical centers and hospitals throughout the U.S., long COVID treatment is rapidly emerging as a subspecialty of its own.

When it comes to physical activity, a new study backs up your experience -- that is, long COVID can severely reduce the ability to exercise, no matter the person’s prior level of fitness. It also offers clues into the mechanics of what is happening. Researchers at the University of California, San Francisco analyzed data from studies that compared the exercise performance of about 800 adults who have had COVID-19. Roughly half recovered completely, while the others developed long COVID. While running or cycling, the long COVID group had a diminished ability to use oxygen in the blood to fuel muscles. Additionally, their cardiac function, including heart rate, wasn’t able to meet the needs of the exercise. Why this happens is not yet clear, but each new bit of information suggests new paths forward.

At this time, the guidance for long COVID patients is to take a modulated return to exercise. This can mean adding as little as a few extra steps to a walk or run or a few extra strokes of a swim every few days or weeks. Research shows that pushing yourself can delay progress. Physical therapy is also emerging as an important tool for recovery. Long COVID continues to be the subject of many of the letters we receive. We’re keeping an eye on new developments and will continue to share them in future columns.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I am 75 years old, and two years ago, I was diagnosed with bullous pemphigoid. I understand this is an autoimmune problem. I also understand that this is more common in older people. I would like to know more about it. I have been prescribed minocycline, which has been helping.

Dear Reader: You are correct that bullous pemphigoid is an autoimmune disorder. That’s a condition in which certain cells in the immune system mistakenly identify the body’s own tissues as a threat. This causes them to go on the attack, which damages and destroys the tissues that they target. In the case of bullous pemphigoid, the immune system attacks the proteins that act as the glue between the two main layers of the skin. These are the epidermis, which is the outer layer of skin, and the dermis, the lower layer of skin that houses blood vessels, sweat and oil glands, nerves, hair follicles, connective tissue and other structures.

The initial symptom of bullous pemphigoid is often an itchy rash, which typically appears on the torso or the limbs. The inflammation that arises due to the immune cells’ rogue activity causes the skin to turn red. The areas of rash also become raised and swollen, as happens when someone has hives. In some cases, as bullous pemphigoid progresses, watery blisters begin to form on the surface of the skin. These can be numerous and can grow quite large. In some cases, they may contain blood. It’s possible for the condition to affect the mucous membranes, including those in the eyes, mouth, esophagus and genitals, but this is rare.

The causes of bullous pemphigoid, which typically develops at age 70 and older, are not fully understood. There is evidence that in some people, it may be triggered by medications. These include certain diuretics, antibiotics, blood pressure drugs and nonsteroidal anti-inflammatories. Again, the reasons for this response by the immune system are not yet known, but a genetic predisposition is suspected to play a role. The condition is somewhat rare, but cases have recently begun to increase. This may be due to the growing population of older adults who, with a wide range of health conditions, are exposed to numerous drugs. It may also be that growing awareness of the condition has led to an increase in diagnoses.

The goal of treatment is to limit the development of new lesions, promote skin healing, manage symptoms and prevent complications. Treatment plans are tailored to each patient’s specific case of bullous pemphigoid. They take into account the individual’s general health, as well as any preexisting conditions. Medications include systemic or topical steroids to manage inflammation and itching, antibiotics for infection and immunosuppressive drugs to calm the immune response.

Minocycline, which you are finding helpful, is an antibiotic used for bacterial infections. Episodes of bullous pemphigoid are often cyclic. They develop, peak, gradually recede and then return. The data shows that with treatment, the disease can go into remission. For about half of patients, this occurs after about two years. To remain in remission, continued treatment may be necessary.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)