Dear Doctor: What's the best type of exercise? I've been reading that high-intensity interval training can slow down the aging process. Is this really true? I'm not too keen on this approach, but I do walk at least a mile every day.

Dear Reader: First, congratulations on exercising regularly. Last year, 81 million Americans didn't take part in any type of regular physical activity at all. That means that 27.5 percent of the population was sedentary. That's terrible news because being active is an easy, affordable and effective way to maintain good health.

Our bodies change as we age. Cardiovascular capacity decreases, muscle mass is lost and body fat increases. Decades of studies show that regular exercise can prevent and even reverse these trends. Staying active boosts the immune system, improves muscle tone and balance, enhances cognitive function, improves the quality of sleep and has a positive effect on mood.

Now, a new study published in the journal Cell Metabolism adds a bit of a twist. While any amount of exercise beats sitting on the couch, a specific kind, known as high-intensity interval training, or HIIT, appears to change the body at a cellular level. Specifically, this type of exercise affects the mitochondria, which are structures within cells that make energy available to the body.

As we get older, the ability of mitochondria to generate energy decreases. HIIT reverses this trend by making mitochondria more efficient. Even more interesting, the older individuals in the study who took part in HIIT had a measurably larger increase in mitochondrial function than did the younger ones. The younger group had a 49 percent increase in mitochondrial capacity. The older group of participants showed a whopping 69 percent increase.

So what is high-intensity interval training? It's a brief workout performed several times per week. HIIT alternates bursts of intense exercise, during which muscles use more oxygen than the body has available, known as anaerobic exercise, with periods of slower activity in which the oxygen debt is repaid. Often it can be done at home with little or no specialized equipment. We recommend doing some reading and, of course, checking in with your primary care physician before you begin.

In basic terms, HIIT goes roughly like this:

-- Start with a three-minute warm-up, like jumping jacks or jogging in place. Don't rev up your heart rate just yet.

-- For the next 30 to 40 seconds, perform a high-intensity move such as scissor kicks, squat thrusts, lifting weights or running at a fast pace.

-- Slow down with 20 seconds of low- to medium-intensity activity.

-- Rest for 30 seconds, walking and breathing deeply, to recover.

-- Start the cycle again. Continue for 10 to 20 minutes.

Does HIIT slow the aging process? That depends on what you mean by aging. Yes, this study shows that in terms of mitochondrial efficiency, the clock can be turned back. But whether this translates into longevity remains to be seen. Still, in terms of potential health benefits, finding a way to make peace with your aversion to high-intensity interval training could be a worthwhile goal.

(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: Can you explain probiotics? Who should take them, and are they harmful in any way? They seem helpful to me in calming my stomach.

Dear Reader: For many generations, society feared bacteria, seeing only the devastation that bacterial disease created. Today, with the advent of antibiotics, we have been able to cure infections that once led to illness and death. However, our primary tool -- drugs known as antibiotics -- also indiscriminately kill bacteria that don't cause disease. Only recently have we come to realize how important these non-dangerous microorganisms are to our health. With an estimated 40 trillion bacteria in and on the human body, outnumbering the 30 trillion to 37 trillion cells that make up the body, it's clear that a symbiotic relationship with these organisms is important to our health.

To answer your question about probiotics, first let us look at gastrointestinal bacteria. Many probiotic preparations promise to help replenish beneficial bacteria in the colon, with the theory being that they increase the health of the intestinal lining, suppress inflammation caused by damaging bacteria, and reduce inflammation caused by an overactive immune system. Most probiotics contain species of the familiar bacterial strains known as Lactobacillus and Bifidobacterium, but some preparations also contain beneficial strains of Streptococcus and Clostridium, or even yeast (Saccharomyces boulardii).

In a 2008 study of 224 children with a diarrheal illness related to rotavirus, participants took either a probiotic preparation (containing various strains of Lactobacillis, Bifidobacterium and Streptococcus) or a placebo. On the second and third days of the illness, the group that took the probiotic had a decrease in stool frequency, improved stool consistency and less need for either oral or intravenous rehydration. A 2002 analysis of nine studies that used Lactobacillus for childhood diarrheal illness found an average reduction of illness of 0.7 days in children who took the probiotic.

A 2010 review of 63 studies assessed the impact of different probiotics on acute diarrheal illness; 56 of these studies looked only at children and infants. On average, probiotics decreased the duration of diarrhea by 24 hours and decreased stool frequency by the second day. Although the authors could definitely say there was benefit in children, they could not conclude if these benefits occurred in adults.

As for more chronic illnesses, a 2009 analysis of 16 studies of people with irritable bowel syndrome found that two studies using Bifidobacterium infantis showed a decrease in abdominal pain, bloating and bowel movements. The other studies were considered methodologically flawed, so the authors couldn't assess the probiotics' benefits.

Probiotics have had mixed results in Crohn's disease, but have shown benefit among people with mild to moderate ulcerative colitis, again through the probiotic preparation.

Similarly, some studies have found benefit from probiotics given with antibiotics, which kill the "good" bacteria as well as the "bad" and therefore increase the risk of Clostridium difficile infection. Not all studies show such benefit, but I would consider probiotics if you've developed diarrhea in the past when taking antibiotics.

One potential issue with probiotics is that, unlike medications, they're not regulated, so there is no true way to know exactly what you're getting. Some patients do develop bloating with probiotics, but for the most part the preparations are well-tolerated. If they're helping you, it seems prudent to continue using your current brand.

(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: How do I get the most out of my relationship with my primary care physician? The appointments are so short these days, and I want to be smart about the way I use the time.

Dear Reader: That's a great question. As fellow patients who see our own physicians, we share your concerns. And as primary care physicians, we are keenly aware of the need to structure an office visit to make every minute count.

Here, drawn from what we do in our own practices, as well as thoughts from fellow physicians, are some ideas to help maximize the time you have with your physician.

-- Consider timing: Be strategic when making your appointment. Monday and Friday tend to be busiest. The first appointments of the morning are least likely to have a wait time, and your physician won't be feeling as rushed. And please, arrive early!

-- Plan ahead: Write down and prioritize your health concerns. Be prepared to describe them succinctly. Symptoms can be physical, mental and emotional. Include details like when the symptom began, how long it lasts, anything that makes it better or worse, and what you are worried about.

-- Look at the big picture: Bring a complete list of the medications you are taking, including supplements. Be sure to include specific dosages. If it's easier, bring the bottles themselves. If you have recently stopped taking a medication, be sure to include it in the list as well. If you have undergone testing with other providers, tell your primary care physician what prompted the tests and provide a copy of the results.

-- Be a partner: Let your physician know your specific goals for the visit. This allows him or her to manage time wisely and efficiently. Take notes. A lot of information is imparted during a medical appointment, and it's easy to miss important details. Some patients bring a spouse, relative or friend to be another set of eyes and ears.

-- Be assertive: If your physician says something you don't understand, ask him or her to repeat it. If you feel you are not being understood or heard, say so. When you require more time than a visit allows, ask whether a nurse or physician's assistant in the office is available to further answer your questions.

-- Stay focused: Stick to the topic that brought you to the office. Spending the time delving thoroughly into your main health concerns will have the highest yield.

-- Stay connected: Ask your physician for the best way to reach him or her in the next few days, when new questions are likely to arise. Here at UCLA, we have a patient portal that allows our patients to reach us directly via email. Ask your physician whether that's an option. If not, learn your physician's preferred approach, which includes how to reach him or her in an emergency.

-- Keep the conversation going: If you feel your questions or concerns haven't been met, don't be afraid to schedule a follow-up visit.

(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.)