Dear Doctor: I know that all sorts of studies show that fiber is good for you, but has anyone figured out the reason why?

Dear Reader: You're right -- the upside of fiber has been with us for quite a while now. Not only has the list of proven health benefits continued to grow, but also the language itself has changed: It used to be referred to as "roughage." But whatever it's being called, fiber is basically the carbohydrates in our food that resist digestion. And though there has been plenty of educated guessing about why we do measurably better over the short and long haul with a regular supply of fiber in our diets, a definitive answer has proven elusive. Now, however, ongoing research into the gut microbiome is offering up some intriguing answers.

First, some of what researchers had already believed about fiber's benefits continue to hold true. Foods that are high in fiber help to slow the absorption of sugars during digestion, which is a boon in regulating blood sugar levels. As a result, fiber in the diet can be a hedge against developing diabetes. Fiber makes you feel full after a meal, helps lower inflammation, and its presence in the intestines aids in elimination. Research shows that a diet high in fiber results in lower rates of heart disease and certain cancers, which may be why eating lots of fiber has been linked to a longer life span in general.

What's really interesting is scientists are now discovering that the fiber we consume feeds the billions of microorganisms each of us hosts in our bodies. Better known as the gut microbiome, it's becoming clear that they play an important role in keeping us healthy. If that number sounds like a lot, you're right. It's now estimated that these tiny hitchhikers make up between 1 and 3 percent of a person's total body weight. That means if your scale says 135 pounds, up to 4 pounds of that weight is beneficial bacteria. And since both quantity and diversity of beneficial bacteria are equally important, making sure they stay fed and happy is key.

Recent studies suggest that the way to cultivate a strong gut microbiome is keeping it supplied with plenty of fiber. When mice were put on a diet loaded with fat and low on fiber, the number and diversity of bacteria in their guts plummeted. At the same time, the mice themselves underwent changes that adversely affected their health. Not only did their intestines shrink, the protective coating of mucus along the intestinal walls became dangerously thin. This woke up the immune system and triggered an inflammation reaction. In time, this chronic inflammation was associated with weight gain and unhealthy blood sugar levels.

Bottom line -- the fiber you consume feeds the beneficial gut bacteria that help to keep us healthy in ways we don't yet fully understand and have yet to discover. So, when you're having that salad of leafy greens, a handful of walnuts, an apple or banana, or a serving of beans, you're not just eating for one -- you're eating for a few billion.

(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 patients with chronic kidney stones reduce their likelihood of problems by adding daily magnesium to their diet?

Dear Reader: There are many hypothetical reasons why supplemental magnesium could decrease the chance of kidney stones.

Kidney stones are quite common, with 10 percent of people in the United States developing them over their lifetime. And the rates are increasing. In fact, the incidence of kidney stones has nearly doubled over the last 30 years. The majority of stones (65 percent) are calcium oxalate; 15 percent are calcium phosphate; and the rest are uric acid or struvite. Thus, the medical community has looked for ways to decrease the amount of calcium in the urine as well as its ability to form crystals there.

Theoretically, magnesium could decrease the possibility of stone formation by binding to oxalate in the intestine, leading to less oxalate in the urine and possibly fewer calcium oxalate stones. It also could bind to oxalate once it's in the urine, enabling the oxalate to be excreted before it binds with calcium. Magnesium preparations could increase urine levels of citric acid, which could bind the calcium and lower the amount of free calcium in the urine. And magnesium could decrease the ability of calcium oxide to form crystals in the urine.

I say "theoretically" for a reason, however. Magnesium also inhibits the reabsorption of calcium by the kidneys, leading to more calcium in the urine and thus a greater chance of kidney stones. (This risk is greater when the urine is overly acidic and less so when the urine is more alkaline.) And while magnesium may increase the amount of calcium in the urine, animal studies have shown that a lower level of magnesium in the urine is actually associated with a greater risk for kidney stones.

In a 2017 study at the University of Texas, researchers analyzed the impact on women of various amounts of dietary magnesium lactate, potassium citrate (which alkalinizes the urine) and ammonium chloride. The goal was to assess how magnesium intake and the pH of urine could affect kidney stone risk. As expected, making the urine more alkaline decreased the calcium excreted in the urine, but did not decrease oxalate in the urine. The magnesium lactate actually increased urine calcium -- even with the addition of potassium citrate -- as well as the risk of calcium oxalate stone formation.

I'm not sure what supplement you've used, but not all supplements would have the same effect as magnesium lactate. Magnesium oxide, for example, provides less magnesium than magnesium lactate, which alkalinizes the urine.

One interesting note from the 2017 study is that magnesium lactate decreased the amount of phosphorus in the urine and thus may decrease the formation of calcium phosphate stones. I'm not sure if your stone was identified as containing calcium phosphate, but that may be interesting to know.

Ultimately, however, there are no good controlled trials assessing whether magnesium has any effect on people with kidney stones, so I can't make a general recommendation. Clearly, more study is needed.

(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 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.)