Dear Doctor: French fries are absolutely one of my favorite foods, but because of their calorie content, I try to limit them to twice a week. Now I just read that eating them even that rarely is linked to a higher risk of death. Just how problematic are a couple of servings of fries each week?

Dear Reader: In my younger days, I also loved french fries. It didn't take me long to figure out that potatoes fried in oil were not the healthiest of foods, so I gave them up. I didn't rely on any studies to make this decision, but the research to which you're referring raises some interesting questions about the potential risks of potatoes, especially fried potatoes.

The study, published recently in the American Journal of Clinical Nutrition, assessed 4,400 people who were at high risk of osteoarthritis of the knees. All participants filled out a food questionnaire; two of the questions were about consumption of fried potatoes and un-fried potatoes. Fried potatoes included french fries, hash browns and tater tots.

Participants were then divided into sub-groups, depending on how often they ate potatoes: less than once a month, two to three times a month, once a week, twice per week or, lastly, three or more times per week. Over the eight years of the study, 236 people died. People who ate fried potatoes twice a week had almost double the risk of death, compared to people who ate fried potatoes less than once a month. Those who ate fried potatoes three or more times per week had more than twice the risk of death, compared to the group who ate the fewest fried potatoes. The authors did not find a correlation between death risk and un-fried potatoes.

Note that there were significant limitations to this study. One is that it was relatively small. Second, the authors did not make mention of other foods that participants ate, such as processed meats, trans fats or sodas. So to take fried potatoes in isolation -- and then draw conclusions -- is difficult. Lastly, the groups who ate the fewest fried potatoes included a greater proportion of women, who have a lower death rate within any given time frame as compared to men.

That said, let's assume the data are reliable. If so, why are fries so risky? A 2016 study combined data from three large trials with a total of 199,181 people. In that analysis, three servings per week of french fries correlated with a 19 percent increased risk of diabetes. This increased rate of diabetes was also seen in two other studies. Further, a 13-year Swedish study found a 22 percent increase in the cardiovascular death rate among people who ate fries -- not other fried potatoes -- three days per week. Something to consider: At the time of these studies, fries may have contained greater amounts of trans fats, which have been associated with increased death rates.

The research correlating fries and death rate is not perfect, but there does appear to be a correlation. I would recommend decreasing your intake. Now I have to convince my kids to do the same.

(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: We live in Orlando, and last year a teenager here was infected with that brain-eating amoeba. Now we're reading that it's in the drinking water in Louisiana. How common is it? Should we be worried about an epidemic?

Dear Reader: There's nothing quite like the words "brain-eating amoeba" to get the news cycle humming. And the reassurance we're about to give you -- it is extremely, exceedingly rare -- is not necessarily the most attention-grabbing. But that's where we're going to begin.

Between 2007 and 2016, there have been just 40 cases of the so-called brain-eating amoeba reported in all of the United States, according to the Centers for Disease Control and Prevention. To put that into even clearer perspective, that's 40 cases in 10 years among 324 million people.

The microscopic creature that's been making headlines is called Naegleria fowleri. Trace amounts of it were discovered in two community water systems in southern Louisiana during routine testing last June. Customers of the affected water facilities were promptly alerted by the Louisiana Department of Health, and disinfection procedures were implemented.

So what is it?

Naegleria fowleri is a single-celled organism found throughout the world in warm freshwater, like lakes, streams, rivers, ponds and hot springs. It enters the body through the nose, typically when people accidentally inhale water while swimming or diving. The amoeba then travels through the nasal passages and enters the brain via the olfactory nerve. Once there, it begins to destroy brain tissues.

You can't become infected by drinking contaminated water, or by swimming in water that has been adequately chlorinated. There have been no known cases of transmission through water vapor, as during a shower or through a humidifier.

The majority of infections reported since 1962 have been in the southern U.S., with half of those in Florida and Texas. But infections have occurred as far north as Minnesota and as far west as Nevada and California. The specific disease the organism causes is called primary amebic meningoencephalitis, or PAM. It's diagnosed using specific laboratory tests that, because the disease is so rare, are not widely available.

Symptoms begin one to nine days after infection, and are divided into two stages. They're similar to those of bacterial meningitis, which adds a level of difficulty to an accurate diagnosis.

The first stage of the disease is marked by a severe headache in the front of the head, often with fever, nausea and vomiting. This is followed by stiffness in the neck, altered mental state, seizures and coma.

Although in most cases the disease is fatal, the Orlando teenager you're referring to survived. It is believed that a swift diagnosis -- the infection was identified within hours of the onset of symptoms -- was key to his beating the odds. So was aggressive treatment, which included an investigational drug, and cooling his body to well below normal body temperature, a process known as therapeutic hypothermia.

Unlike other survivors, who had permanent neurological damage, the Orlando teen recovered fully and is back in school. The hope is that early diagnosis and novel therapeutics may lead to higher survival rates in the future.

(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 read that you can use your own stem cells to rejuvenate worn-out knees. Does this really work?

Dear Reader: "Worn out" is a good way to term what happens to the knee joint with prolonged use. Let's look at how this happens, starting with cartilage.

The lower portion of the knee joint (at the tibia) contains shock absorbers -- called menisci -- made of cartilage. You have one on the inner portion and another on the outer portion of each knee. The upper portion of the knee joint (at the femur) is lined with cartilage as well. All of this cartilage helps protect the bones at the joint -- but it doesn't heal or regenerate well due to limited blood supply. When severe, worn cartilage leads to arthritis of the knee. In knee X-rays of people over the age of 60, 37 percent have shown evidence of arthritis of the knees.

The intriguing thing about stem cells is that they have the ability to become any type of cell that the body needs. The cells used for stem cell injections in the knees are called mesenchymal stem cells, and they can differentiate into bone, fat or cartilage cells. These stem cells can come from the fat cells of your body, from your bone marrow or from the inner lining of your knee joint; they're then replicated in the laboratory and injected into the knee joint.

Here's what the research shows so far ...

In a 2013 study, 32 patients with meniscal tears of the knee were injected with a combination of stem cells, platelet-rich plasma and hyaluronic acid. The study reported improved symptoms and even MRI evidence of meniscal cartilage regeneration.

In a 2014 study, 55 patients who had surgery for meniscal tears of the knees were separated into three groups, with two of the groups receiving stem cell injections. Researchers found that, after six weeks, pain had decreased substantially in the two groups that received stem cell injections and that the decrease was even greater at one and two years after the injection.

In a 2017 study in the British Journal of Sports Medicine, researchers analyzed six studies that used stem cells for osteoarthritis of the knees. In five of the studies, stem cells were given after surgery to the knee; in the other study, stem cells from a donor were administered without surgery. All the studies showed reduced pain and improved knee function. Further, in three of the four trials, MRIs corroborated the cartilage improvements. However, the authors noted, five of the six studies were of such poor methodology that an overall conclusion about the stem cells' effectiveness could not be made.

In all these studies, the most common side effect was knee swelling and stiffness, which improved over time.

There may be benefit to stem cell injections for cartilage loss of the knees, but more data are needed, especially in those who aren't having surgery of the knee. I'd also like to see more data on this type of therapy as a preventive measure for younger patients -- before their knees are worn out.

(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.)