Dear Doctor: My son, who plays several sports, recently broke his right arm. Now he's worried that the muscles will waste away while it heals. I read that exercising his left arm could help his broken arm. When I told him this, he scoffed. It does seem crazy, but is it true?

Dear Reader: Improbable as it sounds, a new study found that when a limb on one side of the body is immobilized, as in the case of your son's broken arm, it's possible to preserve muscle strength and size by exercising the corresponding limb on the other side of the body.

This is a phenomenon known as "cross education," which has been written about since at least 1894. Since then, a number of studies have backed up the existence of cross education. Whether through voluntary muscle contractions through exercise, imagined contractions via targeted visualization or contractions that were electrically stimulated, activating the muscles of the opposite limb preserved both strength and muscle mass in the immobilized body part.

In this newest study, researchers in Canada studied the forearms of 16 male and female college students for four weeks. All were right-handed, and none had recent experience with resistance training. Using CT scans, ultrasound and a weight machine, the researchers measured the dimensions and strength of the muscles in the participants' forearms. The students were then fitted with casts on their left forearms to immobilize their wrists, hands, thumbs and fingers up to the knuckles. Each was randomly assigned to either a training group or to a control group. The training group took exercise classes three times a week that focused on certain muscle groups in the wrist, hand and forearm of the free limb. The control group did not take part in those classes and was asked to refrain from any other exercise as well.

A month later, after the casts were removed, measurements showed that participants in the control group had lost about 20 percent of the strength in their left forearms and had lost 3 percent of their muscle mass. For the participants who had exercised those specific muscle groups in their right forearms, however, both strength and mass measurements in their immobilized forearms were virtually unchanged. Even more interesting was the fact that the muscle groups in the right hand that were deliberately not exercised had measurably atrophied in the left hand. Only the same muscles that were exercised in the free limb maintained their strength and mass in the limb that was in a cast.

How and why cross education takes place is not yet known. Although sensors placed inside the casts of the resistance training group showed that there was indeed some muscular contraction taking place while the students exercised their free hands, these were too weak to have a physical effect.

So what's going on? Most popular among the many theories that have been floated is the idea that something is happening in the neural circuits of our brains. In the meantime, let your son know that, thanks to cross education, he has a way to emerge from his cast stronger than he expected.

(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 know that plain old soap and water are best for washing your hands, but sometimes it's either do nothing, or use an alcohol-based hand sanitizer. So, I'm wondering, though they promise to kill "99.9 percent of germs," just how effective are hand sanitizers with respect to disease-causing viruses like the flu?

Dear Reader: A hand sanitizer is a liquid made with a specific chemical composition to decrease the presence of potentially infectious agents like bacteria, fungi and viruses. You've asked about alcohol-based hand sanitizers, and since it's widely agreed that formulations with an alcohol content of 60 percent or higher are the most effective, these are the ones we'll be referring to in our answer. When it comes to specifics regarding which infectious agents these hand sanitizers kill or abate, though, things can become a bit murky.

A study done in Boston in 2005, said to be among the first into the efficacy of alcohol-based hand sanitizers, found no difference in the spread of respiratory infections between the families who used hand sanitizers and those in the control group who didn't. Subsequent other studies over the years, as well as a repeat of the Boston study, found the same thing. As we've discussed here in previous columns, infections like the influenza virus are most often spread via the aerosolized droplets from an infected person's cough or sneeze. These hang in the air, where they can be breathed in and thus cause a new infection.

Although hand sanitizers can neutralize an impressive range of microbes, they don't work against everything. Even when you've used the right amount and the proper technique, germs like Cryptosporidium, norovirus and Clostridium difficile may persist. Studies have shown that hand washing with soap and water is most effective for these types of germs.

The federal Centers for Disease Control and Prevention agree with you that soap and water is the best hand-washing method. However, when you need a Plan B, do reach for the alcohol-based hand sanitizer. Those with an alcohol content lower than 60 percent, however, and those that are not alcohol-based don't work as well for all germs. They may only reduce and not eliminate the germs on your hands and may encourage the infectious agents to develop resistance to the sanitizer.

As with hand washing, proper technique matters when you're using a liquid hand sanitizer. Be sure to read the label of your particular product and use the full amount indicated. Dispense into one palm, then rub continuously to spread the liquid on all of the surfaces of your hands until the alcohol has evaporated, and your hands are dry.

When you do use a liquid hand sanitizer, it's important to start out with clean hands. When hands are dirty or greasy, the products are less effective. It may seem self-evident, but never ingest a hand sanitizer. And while they can be effective against a range of microbes, when it comes to cleaning off noxious chemicals, like a bug spray or weed killer, skip the hand sanitizer. Instead, go for a thorough wash and rinse (and repeat) with good old soap and warm water.

(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 switched jobs just over a year ago to an office big on birthday cakes, group lunches and sharing homemade treats. I've now managed to gain almost 20 pounds and have started feeling low. My mom says she heard on television that it's because of my gut bacteria. Is that true? Should I try probiotics?

Dear Reader: These days, with the rise in research into the billions of microorganisms that live in our intestines -- bacteria, fungi, viruses and others -- it can seem that all of the workings of the human body eventually lead to the gut. We already know that our gut bacteria play a significant role in countless bodily processes. The ones we understand the best at this time have to do with metabolism and immune function. But as results of new research emerge, it becomes increasingly clear that the gut-body-mind connection is both complex and far-reaching. A number of these studies, which have explored whether the gut microbiome can affect how people think and feel, have made persuasive cases that there may indeed be a connection.

With that in mind, the idea that the composition of the colonies of microbiome could have an effect on mood doesn't seem like such a stretch. And that turns out to be the takeaway from one of the newer studies, conducted by researchers from the Joslin Diabetes Center of the Harvard Medical School. In a paper published in June in the journal Molecular Psychiatry, they came to some intriguing conclusions about what happens to the gut microbiome when its host -- that's you and me -- has a sudden weight gain brought on by a high-fat diet.

The researchers began with mice that became obese after eating a high-fat diet. This increase in body weight and body fat triggered a spike in depression and anxiety, which was ascertained through a series of behavioral tests. The next step was to dose the mice with antibiotics in their drinking water, which altered their gut microbiomes. The result was a return to normal behavior patterns. When gut bacteria from the stressed-out mice were transferred to the bowels of a control group of mice, they too began to exhibit increased levels of depression and anxiety. The researchers concluded that eating a high-fat diet not only can result in obesity, but it also appears to cause changes to gut bacteria that influence brain chemistry and may lead to anxiety and depression.

While this experiment further opens a door into our understanding of the gut-mood connection, it's not meant as a prescriptive for humans. The mice improved not because they were fed probiotics, but because antibiotics wiped out a wide swath of their gut bacteria, including those associated with low mood and increased anxiety. When the researchers achieve their next stated goal, to identify the specific microbes involved, we'll be a step closer to understanding the specifics of the mood-microbiome connection.

In the meantime, this onset of low feelings is something you should take seriously. We also think losing the weight you gained is important, both for your physical and mental health. We hope you will consider consulting with your family doctor, who can assess the situation and advise you.

(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.)