Dear Doctor: I’m 19 years old, and I switched to a vegan diet six months ago. I’m careful about getting enough vitamin B12. However, some of my girlfriends say it’s not that important and your body makes all the B12 you need. Is that true?

Dear Reader: This is a serious issue, and the answer is that nothing your friends are telling you about vitamin B12 is correct. In fact, their advice puts you and any vegans who heed it in real danger. First, vitamin B12 is vital to good health. (More about that in a moment.) And no, your body does not manufacture it. That makes it imperative for everyone following a plant-based diet to make getting adequate B12 a priority.

For those not familiar with the specifics, a vegan diet excludes all animal-based food. That means no meat, fish, seafood, dairy products and eggs. The restrictions also cover honey, which comes from bees, and gelatin, a protein obtained from the bones and connective tissue of animals, often cows or pigs. Because B12 is naturally available only in the major animal products that vegans don’t eat, getting enough becomes a daily goal.

So what is B12 exactly? It’s the most complex of the 12 B vitamins, and it is categorized as an essential vitamin. That means the body requires it, but doesn’t make it. Instead, B12 is produced by certain bacteria that flourish in the gut microbiomes of animals. The B12 they make gets distributed throughout the animal’s body, where it binds to proteins. When you eat an egg, drink some milk or have a burger, the process of digestion releases the B12 from the animal protein and makes it available for your body to use. The vitamin is essential to a wide range of functions involved with the formation of normal red blood cells, the proper functioning of the nervous system and the synthesis of DNA.

A B12 deficiency can result in chronic weakness; mental issues such as poor thinking, confusion or depression; heart palpitations; vision loss; exhaustion; pain and tingling in the extremities; loss of balance; muscle weakness; or difficulty walking. And it’s not just vegans who are at risk. As people age, their digestive systems become less efficient at extracting the nutrient. Those with digestive issues such as Crohn’s disease or celiac disease, immune system disorders such as lupus or Graves’ disease, and those who have had gastrointestinal surgery are also at increased risk of a B12 deficiency.

So how much B12 do we need? For the average teenager and adult, the magic number is 2.4 micrograms a day. Women who are breastfeeding need 2.8 micrograms per day. The vitamin is available to vegans in vitamin supplements; fortified food products such as cereals, some plant milks and brewer’s yeast; and by prescription. The vitamin is not stored in the body, so it’s best to get it daily.

If you’re not sure about your B12 status, your health care provider can check your levels with a simple blood test.

(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 got a flu shot this fall, but I still got sick. Why did this happen?

Dear Reader: Considering how this year’s flu season is shaping up, we’re not surprised by your question. According to the Centers for Disease Control and Prevention, which tracks weekly flu statistics, the current season started early and has been unusually severe. Flu has been reported in all 50 states.

The typical flu season runs from mid-fall to early spring, usually October through April. At this point, 46 states have reported widespread flu activity. And an unusual strain of the influenza virus is hitting harder than expected, particularly among children.

Influenza types A and B, two of the four known strains of influenza virus, account for the majority of illnesses. Type C is a milder form of the virus that is not associated with epidemics; type D appears in pigs and cattle, but not in humans. Historically, influenza type A accounts for three-fourths of all confirmed cases of the flu, with type B only showing up late in the season. However, this year, type B is making a stronger showing than type A, particularly in the southeastern U.S. This is important because of how each year’s flu vaccine is developed.

Designated global partners, including the CDC, work with the World Health Organization to collect and analyze thousands of influenza virus samples. The goal is to spot trends, and, because influenza is a master at mutating, identify new strains. Based on this data, scientists develop a vaccine that targets the top three or four influenza viruses expected in the coming flu season. Sometimes the vaccine does not cover the flu strain that has made you sick. And when widespread anomalies occur, such as with this season’s robust and early appearance of influenza type B, the vaccine isn’t as widely effective as desired.

As for why you became ill despite getting a flu shot, there are several possibilities. It takes up to two weeks to build up immunity after a vaccine. If you’re exposed to the flu during that interim, it’s possible to get sick. The flu vaccine doesn’t protect against all respiratory illnesses, so if you had a cold, often caused by the rhinovirus, the vaccine won’t offer protection. And as we discussed earlier, you may have been infected with an influenza strain not covered by this year’s vaccine. Even when this does happen, it is still advised that you get the vaccine every year. If you are vaccinated but do get the flu, studies show that the illness is more likely to be both milder and shorter. That’s important because the main difference between a cold and the flu is that flu symptoms are more severe, last longer and can cause grave and even life-threatening complications.

For readers who haven’t been vaccinated yet, it’s not too late. With three or four months in an already challenging flu season, it would be wise to get your flu shot ASAP.

(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: Every winter I get these weird bumps on my skin, kind of like goosebumps, except that they won’t go away. What are they? Is there anything that can help?

Dear Reader: It sounds as though you’re describing a common and harmless skin condition known as keratosis pilaris. Symptoms typically include patches of small, hard bumps that are about the size of a grain of sand. They may match your skin tone, or can appear pink, reddish, white or brown. In some cases, the tops of the bumps are covered in dry skin scales.

The condition occurs when oil pores in the skin, which also contain tiny hairs, become clogged with dead skin cells. (The word “keratosis” refers to keratin, the main building block of the epidermis, or outer layer of the skin. The word “pilaris” pertains to hair.) The affected area becomes rough and in some cases may itch. However, keratosis pilaris doesn’t cause pain.

This is a condition that may be seasonal, as you have experienced, or can last for months or even years. It usually appears on the skin of the upper arm, upper thigh or buttocks, but can also develop on other areas of the body. It sometimes occurs on the face, most often on cheeks, where it resembles acne. It’s more common in young people, and often goes away as they get older. The cause of keratosis pilaris isn’t yet known, but since it’s more common in individuals with certain skin conditions, such as eczema, genes may play a role. It’s distinctive enough that it can usually be diagnosed with a visual and physical exam.

Keratosis pilaris is neither harmful nor dangerous, and it isn’t infectious. It can safely be left alone. But if you’re bothered by the appearance, or if you’re experiencing itchiness, you have several treatment options. Since damp skin absorbs moisturizer more easily than dry skin, and since the products form a barrier that protects the skin, make it a practice to apply moisturizer after bathing or showering. Creams that contain chemical exfoliators -- such as urea, salicylic and glycolic acid, and alpha hydroxy acids -- can help to break down excess keratin. Over-the-counter lotions such as Eucerin, AmLactin and CeraVe, which are gentle and fragrance-free, can be helpful in diminishing the size of the bumps and boosting moisture.

When showering and bathing, keep water on the cooler side, as hot water can contribute to dry skin and irritation. Using a loofah, washcloth or exfoliating mitt can also help minimize bumps, but don’t try to scrub them away. This can easily lead to irritated or even inflamed skin and make symptoms worse. Instead, treat your skin gently.

When toweling off after a shower or bath, dab your skin dry rather than rubbing. If you’re not getting results with this regimen of self-care, your health care provider can recommend prescription creams with vitamin A or Retin-A, which may help improve your skin’s appearance.

(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.)