Dear Doctor: My granddaughters are absolutely crazy about soccer. They’re constantly trying to outdo each other with their maneuvers, and now one is trying to learn to "head" the ball. But isn’t that dangerous, especially for girls?

Dear Reader: We’ve long been aware of the risk of head injuries posed by activities like football, rugby, ice hockey and boxing, where jolting physical contact is literally built into the sport. Now soccer, long thought of primarily as a running game, is also getting a second look. Although collisions with other players (and with the occasional goalpost) have been the most visible potential sources of head injury on the soccer field, new research confirms what many have suspected -- heading the ball takes a toll on the brain.

A study published last spring found that heading the ball -- in which a player deliberately moves into the path of a speeding soccer ball in order to control its trajectory with his or her head -- is responsible for a greater number of concussions than is contact with elbows, knees or feet. And according to a study published last summer, the adverse physical effects of that signature move are more pronounced in female soccer players than in their male counterparts.

The earlier study, published last April in the journal Frontiers in Neurology, gathered data from 308 active amateur soccer players, 78 percent of them male. Each player self-reported head impacts and any resulting symptoms during a series of two-week intervals. Players also underwent an in-person neuropsychological assessment during each two-week period. With a total of 741 complete data sets collected over the course of three years, researchers were able to correlate frequency of heading with reductions in attention span and reaction time. They found that working memory was also affected, but to a lesser degree.

The other study, published last July in the journal Radiology, found that heading the ball resulted in alterations to white matter in the brain. When this finding was broken down by the sex of the players, it was revealed that these alterations were more pronounced in the brains of the women in the study than in the brains of the men. White matter, which accounts for half of the human brain, is made up of millions of bundles of nerve fibers. Located beneath the gray matter cortex, they connect the different regions of the brain. The white color is due to myelin, a type of electrical insulation that coats the nerve fibers. Damage to white matter can interfere with the high-speed transmission of electrical impulses in the brain, which can result in impairment to sensory, motor and cognitive function. The researchers refer to this damage as "subconcussive head impacts," meaning that though these injuries don’t produce immediate or measurable symptoms, as concussion does, the effect can be cumulative and long-term.

Because the impact forces of heading a soccer ball come close to matching a helmet-to-helmet football tackle, U.S. Youth Soccer has banned the practice for players younger than 11 years old. You didn’t specify the ages of your granddaughters, but considering what’s potentially at stake, it’s probably a good idea to check in with their parents on the topic of heading.

(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: What is the link between smoking and bladder cancer? My urologist says smoking is one of the main causes, but until I got diagnosed with bladder cancer earlier this year, I had never heard about that connection.

Dear Reader: Your urologist is correct -- decades of studies have shown that smoking cigarettes is the greatest single risk factor for developing bladder cancer.

Bladder cancer is the fourth-most-common cancer in men, and it is less common in women. Of the 80,470 new cases of bladder cancer predicted for 2019, three-fourths will occur in men. The average age for diagnosis is 73, and the disease is less often seen in Latinos and African Americans.

Although earlier research suggested that smoking doubled or tripled an individual’s risk, the most recent data indicate that cigarette smokers face four times the risk of developing bladder cancer than do non-smokers. A study conducted in 2011 connected cigarette smoking to half of all cases of bladder cancer that arose in both women and men.

To understand this connection to cigarette smoking, we should first talk a bit about bladder function and anatomy. The bladder is a hollow and flexible organ that is part of the urinary tract, which is the body’s system for getting rid of waste and excess fluids. Each day the kidneys in adults filter up to 150 quarts of blood, which produces between 1 and 2 quarts of urine. In addition to water, salt and nitrogen products like urea, urine contains numerous additional waste products and impurities that the kidneys have stripped from the blood. The urine then moves from the kidneys to the bladder via a pair of tubes known as ureters. When the bladder begins to reach its capacity of 1.5 to 2 cups of urine, the brain receives signals that we recognize as the familiar urge to urinate. Urine exits the body via a duct known as the urethra.

Among the impurities filtered out of the blood by the kidneys are the thousands of byproducts, many of them toxic, contained in cigarette smoke. These include arylamines, which are known to be potent bladder carcinogens. And even as the number of smokers in the United States has dropped, cases of bladder cancer among smokers have increased. This has led researchers to draw a connection to changes in the array of chemicals used in the manufacturing of cigarettes.

When it comes to the chemical byproducts of smoking, they do their damage at least twice. The first time is in the respiratory system, particularly in the lungs. The second time is in the urinary tract, where they move from the kidneys to the bladder, where they are held in solution for hours at a time.

For people who have quit smoking, there’s both good and bad news. Smoking cessation will reduce your bladder cancer risk. But the data shows that former smokers still have double the incidence of bladder cancer of nonsmokers. Unfortunately, exposure to secondhand smoke has been linked to increased risk as well. We have had a number of questions about bladder cancer and will continue the topic, including symptoms, treatment and outlook, in an upcoming column.

(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: When my husband and I both got sick last month, he was diagnosed with a cold and I was diagnosed with the flu. What’s the difference?

Dear Reader: Both a cold and the flu are respiratory illnesses caused by a virus. The difference between the two is the type of virus that is involved.

The flu is caused by the influenza virus, of which there are four types -- A, B, C and D. The seasonal epidemics that make millions of people miserable each year are caused by human influenza A and B viruses. The type C influenza virus typically results in a shorter and milder illness and is not associated with epidemics. Influenza D viruses primarily affect cattle and are not known to infect or cause illness in people.

The influenza A virus is further broken down into subtypes based on the presence of two proteins on the surface of the virus. These are hemagglutinin (H), which has 18 known subtypes, and neuraminidase (N), which has 11 known subtypes. To make things even more challenging for epidemiologists, many different combinations of the H and N proteins are possible. That means influenza A viruses can be further divided into different strains, like H1N1 or H3N2. Although influenza B viruses are not broken down into subtypes, they are divided by lineage and strain.

Many viruses can cause the symptoms of the common cold, but the culprit in more than half of all cases is the human rhinovirus. As with the influenza virus, viruses that cause a cold can hang in the air for up to several hours in the mist of microscopic droplets that are released when an infected person coughs or sneezes. Breathe them in, and you can become infected. Similarly, when you touch something that has been contaminated by a virus, like a door handle, dishware or even a piece of fruit in the produce section, and then touch the soft tissues of your eyes, nose or mouth, the virus can enter your body.

Viruses are basically bare-bones genetic factories that exist to hijack cells and force them to quickly reproduce the virus in great numbers. As your immune system becomes aware of the presence of the intruders, it mounts the defense that we recognize as the symptoms of a cold or the flu. This commonly includes a sore throat, mucus production, cough, sneezing, fever, headache and body aches. Each of these arises from the immune system’s efforts to create an inhospitable environment for the new virus copies that the body has been forced to produce, and to expel them from the respiratory system.

Because there is so much overlap in symptoms, it can be hard to tell the difference between a cold and the flu. A cold typically inches in, symptom by symptom. But the flu hits hard and fast, with headache, body aches, fever and exhaustion, each of them often severe. The flu is also more likely to lead to complications that require prescription medications, or when serious enough, can lead to hospitalization. The only way to definitively diagnose a case of the flu is through a laboratory 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.)