Dear Doctor: I retired from a demanding desk job last year and, at age 65, have had to face the facts -- I have become sedentary and need to lose at least 20 pounds. My wife is helping me out with a more healthful diet. But my son says that in addition to the swimming and cycling program I'm on, I need to start lifting weights. Why aren't the pool and the bike enough?

Dear Reader: We'd like to congratulate you on your retirement and on your commitment to improving your health. It's all too easy to let the demands of work take precedence over self-care, and getting fit again can be a challenge. You're fortunate to have a supportive wife -- and a savvy son. Recent research shows that he's on the right track with his advice about adding weight training to your exercise regimen.

When it comes to body weight, it's not just what the scale says that matters. The percentage of lean muscle mass to body fat is an equally important metric. But as we age, the metabolic changes that take place mean we also lose muscle mass. So even if someone weighs the same at age 65 as they did at age 35, the composition of their body is not the same. Muscle mass has decreased and body fat has increased, a change that leads to a net drop in metabolic rate.

The challenge for older adults who want to lose weight is that the body burns both fat and muscle when in a calorie-deficit situation. For every pound you lose, up to one-third of it may be muscle. And since muscle is metabolically more active than fat, each pound of weight loss actually makes it a tiny bit harder to lose the next pound. The challenge becomes how to lose fat while preserving muscle.

According to the results of a study published last year, one possible answer is to pair a weight loss program with weight training. Researchers from Wake Forest University in North Carolina studied 249 adults, all in their 60s, all of them either overweight or obese. The participants were divided into three groups that focused on different weight-loss regimens -- diet alone, diet and walking, and diet and weight training.

After 18 months, the group that walked and the group that lifted weights lost 16 and 17 pounds, respectively. The diet-alone group lost 10 pounds. But when it came to loss of muscle mass, the walking group had the highest numbers. The diet-and-walk group lost 4 pounds of muscle mass. The diet-only and the weight training group each lost about 2 pounds of muscle mass.

Preserving muscle mass, which helps with balance and agility, becomes particularly important as we age. And weight lifting has other benefits as well: In addition to the metabolic boost, studies have shown that it can be as effective as yoga in conferring mental health benefits.

We think it's always a good idea to check in with your primary care physician before starting a new exercise program. And if you do take up weight lifting, please consider working with a trainer to be sure your workout is both effective and safe.

(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 had several concussions when I was younger due to various sports activities, not to mention one singularly bad bike ride. Now I read that a single concussion can raise the risk of Parkinson's disease. Just how serious is this increase in risk, and should I be worried?

Dear Reader: Your concern is understandable. The association between severe and moderate traumatic brain injury and Parkinson's has been recognized for some time.

Severe traumatic brain injury is an injury that leads to a loss of consciousness or coma that lasts for more than 24 hours and is evident on a brain imaging test. Moderate traumatic brain injury leads to a loss of consciousness for one to 24 hours and is evident via imaging. What you're describing sounds more like mild traumatic brain injury, in which the loss of consciousness lasts from seconds to minutes and brain imaging studies don't reveal brain injury. The majority of people describe this as a concussion. The question posed by the study you reference is whether mild traumatic brain injury can lead to Parkinson's disease.

The study looked at data from the Veterans Health Administration database. Researchers first gathered data on 162,935 veterans with a history of TBI and 162,935 veterans with no such history; they then classified the injury as mild, moderate or severe. The average age of both groups was about 48, and the veterans were followed for 4.64 years on average.

Overall, veterans with traumatic brain injury had a 71 percent relative increase in the risk of Parkinson's disease compared to those without TBI. The veterans with moderate or severe TBI had an 83 percent greater risk of Parkinson's, while those with mild TBI had a 56 percent relative increase in risk. When the authors looked further at those with mild TBI who had no loss of consciousness, they still found a 33 percent risk of Parkinson's. However, this last data point wasn't considered significant due to the low number of people diagnosed with Parkinson's in this group. Of note is that people with TBI had greater rates of psychiatric disorders.

Now let's look at why traumatic brain injury would lead to Parkinson's disease -- and let's start with Lewy bodies. These abnormal accumulations of protein in the brain have been known to contribute to Parkinson's, and a component of these proteins, called alpha-synuclein, is seen in the cerebrospinal fluid of those who have had severe traumatic brain injury. In addition, autopsy studies have found an association between early-life traumatic brain injury and Lewy bodies in the brain.

This is an important study for veterans. Of the 20 million veterans alive today, an estimated 40 percent have some history of TBI and 82 percent of those are considered mild TBI.

That brings us back to you and the concussions you had in sports and the biking accident. Although the study showed a 56 percent relative increased risk of Parkinson's with mild TBI, let us think of this another way. After the age of 60, Parkinson's affects 1 in 100 people, so if we extrapolate the data for those with mild TBI, the rate of Parkinson's would go up only to 1.56 in 100 people.

Still, it's good to understand the association between TBI and Parkinson's because it may lead to earlier recognition and treatment of the disease. But no one should panic just yet over the risk of Parkinson's caused by prior traumatic brain injuries.

(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: Like practically everyone else in the United States, we threw out our romaine lettuce because of the E. coli outbreak. What is E. coli and how do we avoid it?

Dear Reader: E. coli is a type of bacteria that lives in the intestines of humans and some animals. It exits the body in feces and can survive outside of the intestines for several hours. Most strains of E. coli (Escherichia coli, for our fellow science nerds) are harmless. In fact, they're a natural part of our gut microbiome. The "good" E. coli perform helpful functions, like the synthesis of certain vitamins and keeping certain pathogens from colonizing the colon.

However, a few strains of E. coli cause disease, including gastroenteritis, which is what we're seeing with the present outbreak. And the potential dangers of the "bad" versions of E. coli don't end there. Pathogenic strains can cause illnesses outside of the gastrointestinal tract, including respiratory illness, pneumonia, urinary tract infections and neonatal meningitis.

When it comes to the current outbreak, laboratory tests show that the strain involved, identified as E. coli O157:H7, produces a particularly nasty toxin; in fact, the hospitalization rate has been higher than in previous outbreaks. As you noted in your question, this outbreak centers around romaine lettuce, which has been contaminated with fecal material containing the pathogenic E. coli.

Infection with E. coli begins anywhere from two to five days after exposure. The first symptoms are abdominal pain, cramping and tenderness, which within 24 hours are followed by diarrhea. As the infection progresses, the diarrhea becomes increasingly watery and -- this can be the scary part -- visibly bloody. That's because the toxin in this particular strain damages the lining of the small intestine. Nausea and headache may accompany the diarrhea, and some people may experience chills and fever. The disease typically runs its course in a week or so. However, in some vulnerable populations, such as young children, older adults and those with weakened immune systems, something called hemolytic uremic syndrome, which is a type of kidney failure, can occur.

The federal Centers for Disease Control and Prevention has warned consumers against romaine lettuce grown in the Yuma, Arizona, region. Unless you know specifically where the romaine lettuce in your fridge originated, it is safest to throw it away. This includes whole heads, hearts, pre-packaged lettuce mixes and premade salads. These same precautions apply to romaine served in restaurant.

In general, the best precaution against infection with E. coli is good hand hygiene. Always wash hands after using the bathroom, changing a diaper or helping someone else use the toilet. Wash hands before handling and preparing food, and after contact with animals.

Always wash fruit and vegetables thoroughly under running water; cook meats to their recommended internal temperatures; and be careful about cross-contamination in the kitchen. Anything that has come into contact with raw meat -- whether it's your hands, countertops, knives, cutting boards or utensils -- is a potential source of disease-causing E. coli.

(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.)