Dear Doctors: I’ve heard there’s a test that’s more accurate than BMI. Do you know what that is? My daughter is 17 years old, plays tennis, lifts weights and has a sporty build. She’s not overweight, but her BMI of 26 says she is. It upsets her, and it really doesn’t make sense.

Dear Reader: The BMI is a measure known as the body mass index. It is a ratio of a person’s height and weight and is used to estimate body fat percentage. This, in turn, is used to indicate if someone is underweight, overweight or obese.

While fine in theory, in the real world, the BMI falls short. That’s not too surprising, considering the method dates back to 1832, when a Belgian mathematician set out to create a metric to define a “normal man.” Over the decades, the original calculations underwent subsequent tweaks, each time using data based solely on the male body. Those adjustments gradually led to the modern-day BMI and its role as a factor in assessing overall health, well-being and disease risk.

Using only height and weight to estimate body fat percentage leaves out important variables. These include muscle mass, bone density, frame and overall body composition. As has happened in your daughter’s case, someone who is athletic can register higher on the BMI scale. That’s because muscle tissue is denser than fat and weighs more by volume. That’s not a distinction BMI calculations can make. Also excluded are factors such as age, sex and ethnicity, each of which play a role in body fat percentage.

Another important variable regarding body fat is its location. Fat that sits deep in the body, surrounding the organs contained within the abdomen, is known as visceral fat. It is metabolically active in a way that contributes to chronic and systemic inflammation. Visceral fat, which results in a rounded belly and a large waistline, is associated with metabolic disease, insulin resistance, diabetes risk, cardiovascular complications and an increased risk of certain cancers.

The many health risks associated with abdominal fat, as well as the significance of body fat distribution, have led to the update to the BMI that you are asking about. Known as the body roundness index, or BRI, it shifts the focus to visceral fat. Instead of a ratio of someone’s height and weight, the BRI calculation uses their height and their waist circumference. The resulting measurement, which ranges from 1 to 15, reflects how round someone is around their middle.

A recent study analyzed two decades of health data gathered from 33,000 adults. It found the BRI to be more accurate than BMI at assessing whether someone is overweight, obese or underweight. Each of these is associated with an increased risk of disease and death. When BRI scores reached 7 and higher, the risk of developing cardiovascular disease, cancer and metabolic disease increased. People with low scores -- 3.4 and below -- were also at higher risk for illness and death.

Although an improvement over the BMI, the body roundness index is also just an estimate. It should be used alongside blood pressure, blood sugar and blood lipid levels, important metrics of good health.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: I threw out my lower back two years ago while I was helping my cousin move. My back would feel better for a while, but then the pain would always come back. I noticed that since I started a walking program, that cycle stopped. Do you know why that would be?

Dear Reader: You have discovered one of the many -- so many! -- health benefits of walking. As primary care physicians, a large part of our jobs is helping the people we work with to achieve and maintain good health. That makes walking a topic that is near and dear to our hearts.

It is easy to consider walking primarily as a means of locomotion. But it does so much more than get us from point A to point B. The process of walking places physical demands on the body that help us not just physically, but also mentally and emotionally.

Walking is dynamic, rhythmic and can easily become aerobic. Whether a gentle stroll or a speedy workout, it engages muscle groups throughout the body. You’re practicing balance when you walk and maintaining flexibility. Your lungs and heart are more active, and with every step, you are giving a boost to the movement of lymphatic fluid. Walking helps maintain a healthy weight, improves blood pressure, aids in blood sugar control and bolsters the immune system. As an added bonus, regular walking has been shown to be effective at improving sleep, memory and cognition, and also lifting mood and lowering the risk of depression.

Due in part to our upright posture, lower back pain is common. Data show that up to 85% of us will deal with some form of back pain in our lifetimes. As anyone who has experienced it knows, lower back pain can flare unexpectedly, often in response to movement. This can make people leery of doing any kind of exercise, even when recommended by their doctors. The good news is that the vast majority of lower back pain -- the estimate is 90% -- is temporary. The not-so-good news is that having had a lower back injury, such as a sprain or strain or a slipped disc, puts you at risk of a recurrence. And that’s where the walking you are doing enters the picture.

A contributing factor to low back pain is poor muscular support. This includes not just in the back, but also the muscles of the core. As you walk, you are exercising muscles that help stabilize, and thus protect, the spine. A recent study looked at 700 adults with chronic lower back pain. Half were asked to walk 30 minutes per day, five days a week. They also learned about the mechanics of back pain and strategies for recovery. At the end of three years, it turned out that the people in the walking group went twice as long without a recurrence of low back pain as those who did not walk.

We hope that, even with your back pain gone, you will continue your walking program. Your back -- and the rest of your body -- will thank you.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctors: The whitening toothpaste I’ve been using for several years has a pleasant flavor and keeps coffee from staining my teeth. But I just realized it contains xylitol, which I’m hearing can cause heart problems. Do you know if this is true? Do I need to switch?

Dear Reader: Xylitol is an artificial sweetener that falls into a category known as sugar alcohols. Also known as polyols, these compounds contain neither ethanol -- that’s alcohol -- nor sugar. Rather, they are a form of carbohydrate with a chemical structure similar to sugar. In fact, their structure so closely resembles sugar, it triggers receptor proteins on the taste buds that are involved in identifying the flavor of sweetness.

Once activated, those proteins release molecules that stimulate nearby neurons. Signals travel to the region of the brain involved in taste, which interprets sweetness. In an interesting side note, cells from the sweet taste receptor system are scattered throughout the gut. They can detect nutrients and can tell the difference between natural and artificial sweeteners. Some researchers suspect that may play a role in why foods with artificial sweeteners can trigger rather than sate sugar cravings.

Sugar alcohols have from one-half to one-third the calories of natural sugars. Because they are metabolized slowly, they don’t take the same toll on the body’s insulin response as sugar. This has led to their widespread use in diet products, in foods suitable for people living with diabetes and for low-carb diets, such as keto. They are commonly used in diet sodas, sugar-free gum, baked goods and candy and even in some mainstream products.

From the moment these sugar substitutes reached the marketplace, they became a subject of study, debate and also controversy. Polyols occur naturally in small amounts in some fruits and vegetables. But for commercial use, they are lab-made. And as they find their way into a growing array of foods, the increasing intake per person is raising concerns. Investigations into various forms of sugar alcohols have found they can cause gastrointestinal problems such as pain, bloating, flatulence and diarrhea. Two years ago, an observational study linked them to an increased risk of heart attack and stroke.

And that brings us to a new study of xylitol, which pinged your radar. Published in the European Heart Journal, it analyzed blood samples from 3,300 participants who had completed an overnight fast. The researchers analyzed health data three years later, and they found that individuals with the highest blood levels of xylitol had twice the risk of having a heart attack or a stroke as participants with the lowest levels. The risk of death had also doubled.

As always, it is important to note that while an observational study can suggest a connection between certain factors, it can’t prove cause and effect.

In our own practices, we suggest artificial sugars should be used in moderation, and as a temporary aid. Toothpaste is not a significant source of xylitol and should not cause concern. However, if you are worried, you can switch to a natural toothpaste, some of which also have whitening varieties.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10960 Wilshire Blvd., Suite 1955, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)