Dear Doctor: I'm reading a lot about a fasting-mimicking diet, which is supposed to help you live longer. What is it? I've been planning to lose some weight and wonder whether this diet might be worth trying.

Dear Reader: The approach you're referring to falls into an ever-expanding category of dieting known as intermittent fasting. It has its roots in research that has found a correlation between periodic fasting and increased longevity, as well as positive health outcomes. We live in a culture obsessed with the notion that we're just one ingredient or rule or combination of foods away from a perfect diet, one that will keep us (no pressure) lean and fit and healthy and happy, so it's not too surprising that the fasting-mimicking diet would have its moment.

Let's start with the background. A number of studies have found that intermittent fasting can bestow a range of metabolic benefits. These include a reduction in body fat and overall body weight, lower blood pressure, improved blood lipid levels and improved regulation of blood sugar. Several methodologies fall under the intermittent fasting umbrella, including 24-hour fasts, alternate-day fasting, restricted calorie diets and time-restricted diets, which allow eating only during certain hours of the day. The downside of this approach to eating is that it is quite restrictive and can be difficult to adhere to. In addition, there is concern that intermittent fasting, which entails a certain amount of deprivation, can lead to bingeing and other extreme eating behavior.

When it comes to the fasting-mimicking diet, the food restrictions kick in for just five days out of the month. That is, the diet mimics a fast without requiring daily deprivation. During those five days, calories are limited to about 800 per day. The nutrient profile of the diet is typically high in fat, low in protein, with carbohydrates falling somewhere in the middle. Some versions of this approach, which are more extreme, allocate nutrients in accordance with the high-fat ketogenic diet. In this approach, 80 percent of daily calories come from fat, with the rest divided equally between carbohydrates and protein.

A study published in the journal Science Translational Medicine last year followed 71 individuals who either followed a fasting-mimicking diet for five days each month for three months, or else switched to the fasting-mimicking diet after first following their normal diet for three months. Among both groups, researchers saw a drop in both body weight and body fat, as well as beneficial effects to blood pressure, fasting blood glucose and markers of inflammation. Interestingly, these benefits were more pronounced among individuals at greatest risk for disease than in those who were not at risk. And while the authors of this study concluded that the five-day fasting-mimicking diet is both safe and feasible for healthy adults, they also point out that larger studies are needed to see whether these results can be replicated.

As for whether this particular approach is one you should try, we believe that's a discussion you should have with either a dietician or your family physician. While the benefits of this approach to diet are appealing, it requires both discipline and consistency for optimal results.

(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: Being physically fit in middle age can decrease the risk of dementia, according to my sister-in-law. She walks four miles a day and runs on the weekends, so I imagine she's covered. But I work full-time and am a single mom to three busy kids, so I'm short on free time. Just how much exercise is enough?

Dear Reader: Your sister-in-law appears to be referring to a study conducted in Sweden, which has yielded some rather dramatic findings. According to the researchers, middle-aged women who had a very high degree of physical fitness were almost 90 percent less likely to develop dementia when they reached old age compared to women with medium or poor physical fitness. When women from the "very fit" group did develop dementia, it was at about age 90 rather than 79, a full 11 years later than the average age at which cognitive decline appeared in members of the less-fit groups.

The study, published online in the medical journal Neurology in March, began in 1968. That's when researchers tested the level of physical fitness of a group of 191 women between the ages of 38 and 60. The women's peak cardiovascular capacity was gauged via a test on a stationary bicycle, which they were asked to ride in several stages until they reached a point of exhaustion. They were then sorted into sub-groups based on their levels of cardiovascular fitness. Over the course of the next 44 years, the women who remained in the study underwent periodic cognitive screening.

During that time, 44 study participants went on to develop dementia. Of those, just 5 percent came from the group deemed "very fit." In the group of women who tested as moderately fit, 25 percent developed dementia. Among the women with the lowest degree of physical fitness, almost one-third of them went on to develop dementia.

When it comes to translating those results into daily life, it's important to note that the most pronounced reduction of dementia risk was observed among the women with the very highest levels of cardiovascular fitness. The women who were unable to complete the bicycle test without taking a break went on to have the highest rates of dementia. And lest the findings seem like a fluke, the authors refer to a number of other studies, both in the United States and in Europe, that associate increasing levels of physical fitness with lowered risk of cognitive decline.

As mothers ourselves, we understand the challenge of finding time for exercise. But a mounting body of research suggests that every minute you put into physical fitness now will pay off for years to come. For general health, current guidelines suggest at least 30 minutes of physical activity per day. When it comes to improved fitness and cardiovascular health, which was linked to lowered dementia rates in the study, the goal becomes up to 60 or 90 minutes per day. This can be broken up into 10- or 15-minute segments. It sounds daunting, but we know from personal experience that, with planning and determination, it's possible and worth the effort.

(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: A colleague just returned from a long hospital stay, where he battled a C. diff infection. He had asked for a fecal transplant but was refused. Will these ever become the standard of care? Are there uses for these procedures other than to fight C. diff infections?

Dear Reader: For those of you who may not be familiar with the topic, a fecal transplant is the introduction of stool from a healthy donor into the gastrointestinal tract of a recipient. You may also hear this referred to as a fecal microbiota transplant, or FMT. The goal is to repopulate the colon of the recipient with friendly bacteria that have been killed or depleted, often as a result of antibiotics.

The procedure has been most widely used in treating Clostridium difficile, or C. diff, a stubborn bacterium that can overpopulate the colon when one's own intestinal flora are wiped out. Symptoms range from ongoing diarrhea to colitis, an inflammation of the colon that can be life-threatening. The stool for a fecal transplant comes from donors who have been screened for a variety of criteria, including parasites and a wide range of pathogens that include E. coli, norovirus, HIV and hepatitis. The stool is then processed into a liquid and introduced into the recipient via an enema, endoscope, colonoscopy or nasogastric tube. As research into the procedure continues, freeze-dried fecal microbiota in capsule form are being developed.

While fecal transplants have entered the public conversation in just the last few years, they date back at least to the 4th century. Writings indicate they were used in China to treat diarrhea and food poisoning. A surge of interest in the practice among European scientists in the 18th century, who were intrigued by its use in other cultures, set the foundation for the modern-day study of microbiota. Following the rise of antibiotics in the 1940s, which cured infectious diseases but decimated the patient's own intestinal flora, scientists continued to experiment with therapeutic uses of FMT.

Today, FMT is used on a case-by-case basis. As your colleague has discovered, not every medical center offers the procedure, and not every patient qualifies for the treatment. According to the most recent FDA enforcement policy, FMT should be used only in C. diff. cases that are not responding to conventional drug therapy. However, considering the pace of research into the gut microbiome, and the enormous interest in the therapeutic potential of FMT, it's likely these guidelines will be continually revisited and amended.

Statistics about the success rate of FMT in treating C. diff infections are hard to pin down. Initial recovery rates are well above 90 percent. But treatment protocols have not yet been standardized. Depending on how donor stool has been processed and administered, relapse rates as high as 50 percent have been observed. Still, the procedure holds great promise. In addition to C. diff infections, it has been successful in treating Crohn's disease and ulcerative colitis. Ongoing research into the treatment of a range of metabolic and autoimmune diseases has yielded surprising results.

We expect that we'll all be hearing and reading about significant advances in the field in the years to come.

(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.)