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.)

Dear Doctor: I'm 55 years old and am going through menopause. My grandmother had osteoporosis and so does my mother, so I'm worried. My dad read that probiotics can help, which sounds a little nutty. What on earth do a bunch of bacteria have to do with your bones?

Dear Reader: Although our skeletons have taken on their final size and shape once we reach adulthood, bones are living tissue continuously undergoing change. This occurs in a process known as remodeling, which is cellular activity in which old bone is removed and new bone grows in its place. Remodeling continues throughout our lifetimes and, (fun fact) in the process, most of the adult skeleton is replaced by new growth every decade. However, as we age, the removal of bone happens more quickly than replacement. Other factors -- like hormonal changes that come with menopause, being sedentary, inadequate diet, certain medical conditions and smoking -- are also associated with a decrease in bone mass and strength.

This net loss of skeletal mass, known as osteoporosis, results in such structural abnormalities as increased porosity and thinning of the bones, which make them weaker and more fragile. People with osteoporosis are at increased risk of stooped posture, loss of overall height, fractured or collapsed vertebrae, and fractures or breaks due to even a minor fall.

Although there is no cure at this time, medications, hormone therapy and some lifestyle changes have been shown to slow the rate of bone loss and to lower the risk of fracture. But now -- and maybe this is what your dad is referring to -- a new study puts forth the possibility that certain probiotic supplements may help balance the bone loss/bone growth equation in people with osteoporosis. According to the results of the study, which were published in June in the Journal of Internal Medicine, the bacterium Lactobacillus reuteri reduced bone loss in older women diagnosed with low bone mineral density.

Researchers measured the bone density of 90 women ages 75 to 80, all of them in good health but with low bone mineral density. The women were then randomly assigned to two groups. One group took a twice-daily supplement of freeze-dried Lactobacillus reuteri, which is a strain of lactic acid bacteria. The other group was given a placebo. None of the women knew which group they had been assigned to. After a year, the bone density in the shins of all of the study participants was measured again. It turned out that the women taking the probiotic supplement experienced half as much bone loss during that year as did the women in the placebo group. And while that's a rather remarkable outcome, the authors of the study have a few words of caution. First, since this is the first human study of its kind, the data set is quite small. And second, the specific reasons behind the outcome of the study are not yet understood.

Still, the results are promising enough to ensure new and expanded studies. They add to both the interest in and the promise of the gut microbiome as a potential therapeutic target.

(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.)