Dear Doctor: I'm hearing that vitamin D can reduce the risk of colds and flu -- and that more foods should be fortified with it. What's the evidence for that?

Dear Reader: As a doctor, I'll confess I'm skeptical about such a broad claim. As I wrote in a prior column, vitamin D is not so much a vitamin as a hormone within the body. It's produced by the body and, further, many foods are rich in the substance. If your vitamin D levels are normal, ingesting additional vitamin D wouldn't seem to help prevent the common cold.

That's not to say such thinking is new. The famed English general practitioner Robert Edgar Hope-Simpson, noting that the influenza virus occurred predominately in the winter, surmised that lack of exposure to the sun may make people more susceptible to influenza. He also noted that in tropical climates, influenza occurs more often during the rainy season when there is a decrease in exposure to the sun. To produce vitamin D, the human body needs sun exposure, so the purported connection between vitamin D and colds seems to have started with that correlation.

Further, a deficiency of vitamin D can lead to decreased white blood cell function, impeding the body's ability to fight off infection. Additional vitamin D stimulates the production of antimicrobial peptides.

As for scientific research on the topic, a 2012 study of 30 patients with chronic lung disease and very low levels of vitamin D (less than 10 ng/ml) showed a 43 percent reduction in exacerbations of the disease when they were given 100,000 units of vitamin D monthly (average 3,200 units per day) for one year. Commonly, these exacerbations are related to infection. A 2015 study showed similar findings of a 43 percent reduction in moderate to severe exacerbations among lung disease patients with vitamin D levels below 20 ng/ml.

However, vitamin D supplementation did not change the upper respiratory infection rate. Also, in both studies, vitamin D supplementation had no effect on people with normal vitamin D levels, i.e., greater than 20 ng/ml.

In another 2012 study, patients with normal vitamin D levels were randomized to receive 100,000 units of vitamin D or a placebo monthly for 18 months. The authors concluded that there was no difference in the rates, or the severity, of upper respiratory infections between those who took vitamin D and those who took a placebo.

The latest study, to which I believe you are referring, was a combined analysis of 25 studies from around the world, including Afghanistan and Mongolia. This analysis linked vitamin D supplementation to a 12 percent reduction in the number of people who developed at least one respiratory infection.

However, in further analyzing the data, the authors found that most of the benefit for vitamin D supplementation was seen in those with levels less than 10 ng/dl, which is very low. The authors did not find any statistical benefit in vitamin D supplementation among people with levels greater than 10 ng/ml. The authors also found that daily or weekly intake of vitamin D had a greater benefit than larger doses given monthly.

In summary, vitamin D does seem to have benefits in decreasing colds and flu in those who have exceedingly low levels of vitamin D -- numbers not seen in the majority of the population. Further, patients with chronic illnesses, elderly people and residents with long, cloudy winters may find that vitamin D supplementation can decrease the frequency of upper respiratory infections.

(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. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I'm trying to lower my blood glucose levels, but I have a sweet tooth. What raises blood glucose more -- the sugar from fruit, or foods with refined sugar? Is a healthy diet the only way to keep blood glucose steady?

Dear Reader: Managing your levels of blood glucose, which is the measurement of how much of a certain sugar is dissolved in the blood, is important to good health. Glucose, which comes from the foods we eat, is a major source of energy to cells throughout the body. However, blood glucose levels that remain consistently high can lead to a variety of health problems, including prediabetes, a condition that frequently leads to Type 2 diabetes.

Blood glucose starts its journey in the form of carbohydrates, which are the main nutrients in foods like bread, pasta, rice, fruits, vegetables, legumes, grains and some dairy products. When we eat these foods, the digestive process frees the sugars within the food and makes them easily available to the body.

Glucose, the smallest sugar molecule, moves from your small intestine into your blood. There, it is distributed throughout the body to provide energy to the cells. To get from the blood into the cells, glucose needs the help of insulin, a hormone produced by the pancreas, a long, flattened gland that sits behind the stomach.

As your question notes, not all foods release glucose in the same amounts or at the same rate. Foods like sugary breakfast cereals, pastries and candy are basically glucose bombs. But naturally sweet foods like apples, strawberries or yams, which contain carbohydrates but are also high in fiber, release glucose more slowly.

However, if you turn that apple into juice, you've just accelerated the rate at which your blood glucose will rise. But if the candy bar you're eating contains a handful of nuts, the fiber they contain will slow the rate of glucose absorption.

It's a little confusing, we know. Fortunately, a handy tool known as the "glycemic index" has taken much of the guesswork out of maintaining healthy blood glucose levels. The glycemic index, or GI, ranks each food relative to how it will affect your blood glucose.

Foods high on the GI release their sugars more rapidly, which can cause blood sugar to spike. Foods lower on the scale release their energy more gradually, and blood glucose levels remain steady.

Interestingly, research has shown that exercise affects blood glucose levels. When you increase your amount of activity, your body works harder and your muscles' demand for glucose increases. The insulin your body produces becomes more effective as well. The key here, as with diet, is moderation.

A brisk walk, a dance class, an aerobic bout of house cleaning or yard work -- any of these can add balance to the blood glucose equation. The good news is that even a little bit of regular exercise, say, 30 minutes three times a week, will also help your heart, lungs and even your mood.

(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. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: My daughter's school recently sent home a flier about potential exposure to hand-foot-and-mouth disease. How worried should I be?

Dear Reader: Hand-foot-and-mouth disease (HFMD) is very common. It occurs most often in infants and children younger than 7 years of age and is caused by one of 16 types of enterovirus. Outbreaks are more likely in the late summer and early fall, when children are starting a new year of school or preschool.

Because the viruses that lead to HFMD are largely intestinal, they're usually passed via stool. Small amounts of the virus then make it onto the hands of the child or someone changing the child's diaper. The virus passes from one individual to another when it ends up on food, the fingers or the pacifier of another child, ending up in the mouth. The fecal-oral transmission is the most common way the virus infects others, but it can also be passed through oral secretions, through coughing, and through the fluid from the blisters seen in hand-foot-and-mouth disease.

Regardless, the virus makes its way down to the lower intestine, spreading to the lymph nodes and from there to the rest of the body. The typical time that it takes for the enterovirus to be ingested and for the first symptoms to appear is three to five days.

Symptoms of the disease begin with mouth or throat pain or the refusal to eat. The most striking symptom of hand-foot-and-mouth disease is the blisterlike rash that occurs both within the mouth and on the hands and feet; such blisters can also appear on the legs, arms and buttocks. The lesions are normally not painful and resolve in three or four days. Some species of enterovirus also cause fever, nausea and vomiting. Rarely, enteroviruses can lead to dehydration, viral meningitis or heart inflammation.

I can understand the worry that your school -- and you -- have regarding this infection. As I noted, the incubation period is typically three to five days, but one study of an outbreak at a day care showed that children were infectious up to seven days. And, not to make you overly worried, but some enteroviruses can be passed in the stool up to 10 weeks after infection.

I would ask school officials at which date the infected child began having symptoms of HFMD. If it was more than five days ago, I would be less concerned about your child now developing the disease.

However, because the virus can be shed long after disease, I would stress the importance of hand-washing. If your daughter does get hand-foot-and-mouth disease, the symptoms likely will not last long, and any discomfort can be treated with acetaminophen or ibuprofen. Watch for warning signs of potential complications, such as listlessness, severe headache or neck stiffness. Consult your pediatrician if your child has these symptoms.

Also, if your daughter does become infected, make sure to practice good hygiene at home, so the virus does not pass to you or other family members.

(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. Owing to the volume of mail, personal replies cannot be provided.)