Dear Doctor: I've always had headaches, from the time I was a teenager. But now I get them more often -- about one headache per week. Is this a warning sign of something?

Dear Reader: Headache is among the most common -- and ancient -- of physical complaints. You would be hard-pressed to find someone who hasn't suffered from a headache at some point in time.

Mentions of headache in written texts date back to 1200 B.C., and treatments over the centuries have included the use of leeches, garlic, oil in the ear, tying a dead mole to one's head (no, we're not making this up), magnets, electricity, psychotherapy and that universal refuge, the soothing bath.

Most headaches are periodic bouts of pain centered in the head and/or neck. It's not the brain tissue itself that's sending pain signals -- it's the surrounding tissues, blood vessels and nerves that serve the area. Many headaches respond to medication and lifestyle changes. Some, however, like migraines, may persist despite treatment. Some headaches can be warning signs of larger problems such as blood clot, stroke or tumor. Thankfully, those are rare.

Although there are many different types of headache, most fall into four major categories:

-- Tension headaches are the most common, marked by dull pressure, like a band tightening around your forehead.

-- Sinus headaches, which cause a deep, throbbing pain, occur when the cavities around the cheeks, nose and forehead become inflamed.

-- Cluster headaches, marked by severe pain in and around one eye, get their name because they often occur in clusters, but they are less common.

-- Migraines, which can last for hours or even days, cause debilitating pain, often on one side of the head. They can be accompanied by flashes of light, photosensitivity, a blind spot in the field of vision, tingling sensations on the skin and nausea.

Headaches can also be the result of factors such as eye strain, allergy, hunger or fasting, fever, fatigue, dental problems, lack of fluids and hangover, to name just a few. The truth is, the subject of headache is vast and extremely complex. And despite many strides forward (remember that dead-mole-on-the-head cure?), science still has more questions than answers.

What signals that a headache may be something to worry about?

-- A major change to the pattern of your headaches, such as going from a few per month to one per day.

-- The sudden onset of persistent headaches, particularly after age 50.

-- A headache that is the most severe you have ever had.

-- The onset of a headache after a traumatic event to the head.

-- Any headache that alters cognitive function, is accompanied by fever and a stiff neck, or includes symptoms like slurred speech, blurred vision, weakness, numbness or seizures.

These can be signs of underlying conditions such as head injury, infection, tumor, high blood pressure, fluid buildup in the skull, decrease in blood flow or bleeding in the brain.

This all sounds alarming, so let us assure you again that these conditions are rare. However, if any of the above happen to you, please be safe and seek immediate care.

(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 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.)