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.)

Dear Doctor: I always thought fiber was fiber, but it seems that the FDA is about to crack down on food manufacturers who add weird ingredients to bulk up processed foods. Does it really matter whether the fiber we're eating is added or is a natural part of our food?

Dear Reader: For anyone not familiar with this topic, 26 ingredients that food manufacturers add to their products in order to bump up the dietary fiber numbers on food labels are currently under review by the Food and Drug Administration. These include fibers that have been extracted from plant-based sources, as well as synthetic fibers cooked up in a lab.

For example, inulin, frequently seen on processed food labels, is a naturally occurring carbohydrate that resists digestion. (It's the "resists digestion" that makes it, or any carbohydrate, qualify as dietary fiber.) Found in more than 36,000 plant species, inulin is most often sourced from chicory when it is used in food production. Polydextrose, on the other hand, another common entry on food labels, is a synthetic fiber. It may sound familiar if you've read the food labels on products ranging from breakfast cereals and baked goods to ice cream, salad dressing and even certain beverages.

All of which leads to the point behind the FDA's move to reconsider what, specifically, can count as a dietary fiber on food labels. The argument isn't whether or not these substances qualify as fiber. They are carbohydrates that can't be fully digested -- so in the broadest sense, they do. However, their presence in foods that are otherwise nutritional black holes can, in truth, allow manufacturers to use language like "high in fiber." And that, according to those behind the push to bump certain types of fiber from food labels, can mislead consumers as to the nutritional value of certain foods.

The FDA's goal, as stated in a rule published in 2016, is that "only certain naturally occurring dietary fibers such as those found in fruits, vegetables and whole grains, and added isolated or synthetic fibers that FDA has determined have a physiological effect that is beneficial to human health, could be declared on the label under 'Dietary Fiber.'"

And the truth is, whether or not those oddball added fibers meet the "beneficial to human health" criteria is not yet known.

Research has proven the beneficial effects of naturally occurring dietary fiber for decades now. Soluble fiber, which dissolves in water, forms a gel-like substance during digestion and can help slow the absorption of simple sugars. Insoluble fiber, which does not dissolve, adds bulk to the materials passing through the digestive system and helps with elimination. Diets high in fiber are associated with improved bowel health, lower blood cholesterol levels and regulation of blood sugar levels. In the bigger picture, studies associate high-fiber diets with lower rates of cardiovascular disease and certain cancers.

At this time, the FDA has hit the "pause" button on the food label decision when it comes to fiber. Meanwhile, if that sugary carton of yogurt is claiming to be high in fiber, we think a glance at the small print on the food label could be educational.

(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.)