Dear Doctor: I'm not thin by any means, but I'm not obese either. I lift weights three to four times a week, run about 10 miles a week, play soccer and regularly do half-marathons. Now a new study says that because my BMI is 26, there's no way I can be fit and healthy. Is this true?

Dear Reader: Due in part to the limitations of BMI as a measurement, many athletes and muscular individuals will fall into the overweight category, which is a body mass index in the range of 25 to 30. The body mass index, a measure of body fat based on the ratio between an individual's height and weight, can be a useful tool. However, it doesn't leave room for additional factors like bone density, muscle mass, overall body composition, or the natural variations inherent in the sex, age or ethnicity of an individual.

For example, the BMI of someone who is athletic can skew higher because of the presence of additional muscle, which is denser than fat. Elderly adults tend to have more body fat than younger adults and may have experienced bone loss as well. And on average, women tend to have a higher percentage of body fat than do men.

With athletic individuals, whose habits when it comes to healthful diet and regular exercise are usually quite good, we tend not to worry that much about their actual weight. In these specific cases, we agree that it is possible to fall into the category of being overweight but still be fit.

However, when a patient edges into the upper regions of the BMI category of overweight, or when they register as obese, which is a body mass index of 30 and above, it becomes a different story. At that point we will definitely explain to them the not-insignificant health risks associated with obesity, no matter how physically active the individual may be. These include high blood pressure, heart disease, stroke, kidney disease, Type 2 diabetes, fatty liver disease, metabolic syndrome and certain cancers.

In a large-scale study published last year in the Journal of the American College of Cardiology, researchers from England found that, even when they were otherwise clinically healthy -- that is, their blood pressure, blood sugar and blood lipid levels were all within the normal range -- individuals who were obese were at measurably higher risk of the adverse health outcomes we mentioned above. Even being overweight raised the risk of coronary heart disease up to 30 percent, despite good blood pressure, blood sugar and blood lipid numbers, according to the study results.

The researchers' conclusions came from analysis of data drawn from the electronic medical records of 3.5 million people between 1995 and 2015. However, critics of the study point out that important factors associated with lifestyle, such as exercise habits, diet or stress, each of which can affect or even skew results, were not given equal weight. (Sorry, but we can't pass up an easy pun.)

If the extra pounds that tipped your BMI into the overweight category can be attributed to additional muscle mass because of your athletic endeavors, and if your metabolic markers are all in good order, then in our opinion, you can consider yourself to be fit.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: I want to lose 15 pounds but have had no luck cutting calories or carbs, or even trying that crazy (IMHO) keto diet where you eat mostly fat. What about that new diet where you only eat during certain hours? How does it work?

Dear Reader: We think you're referring to time-restricted eating, which is also called "early time-restricted feeding" or "intermittent fasting." No matter the language, these approaches all boil down to the same basic concept. That is, all of your calorie intake, including meals, snacks and beverages, takes place within a limited period of time. Instead of reducing the calories you take in each day, or limiting the types of food you eat, it's the timeframe in which calories are consumed that is strictly defined.

Before we go any further, we'd like to point out that this isn't a weight-loss regimen per se. Initial studies looked into the potential health benefits of the practice, such as blood sugar control. However, in studies done, as well as in anecdotal evidence from everyday participants, it has emerged that weight loss often takes place.

Restricted eating is based on a growing body of evidence that humans do best when we live in sync with our circadian rhythms, which are guided by the built-in "body clock" that operates within us on a 24-hour cycle. We already know that circadian rhythms influence a number of behavioral and physiological processes. These range from the obvious, such as our sleep/wake cycles, to the unseen, like body temperature, hormone secretion, enzyme function and even the speed at which wounds will heal.

So it's not that surprising to discover that nutritional intake would also have a spot on the list. There's no doubt that we've come a long way from the days of our primitive ancestors, when the rigors of hunting, gathering and preparing food, to say nothing of the challenges of storage, strictly limited mealtimes. These days, however, the average American eats from early morning until well into the night.

One of the thoughts behind restricted eating is that, over time, this type of behavior wreaks havoc on our circadian cycles, which use hormones and enzymes to prep the body in myriad ways for nutritional intake in the morning and afternoon. Then, during the subsequent fast, these processes rest. By front-loading our food consumption, as our ancestors presumably did, we allow our inner clocks to sync up for optimal operation. In a study in which men with pre-diabetes limited caloric intake to a six-hour period for five weeks, researchers saw a drop in participants' blood pressure and lower insulin levels.

There's no single formula for restrictive eating. Some plans suggest an eight-hour window for eating, while others stretch that to 10 hours. The one constant is that during the fasting period, nothing caloric -- and this includes the milk in your morning coffee or that handful of nuts at night -- passes your lips. It also appears that reversing the size of meals -- large breakfast, moderate lunch, light dinner -- helps with hunger management. If you do decide to move forward with a restricted eating plan, please check in with your primary care physician for advice and guidance.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Can white button mushrooms help control my blood glucose? I saw some headlines about this a while back, but it sounds like a correlation to me -- not a clear cause and effect. Maybe it's just that people who eat more mushrooms also eat less junk food?

Dear Reader: Spoken in the true scientific spirit. You are indeed correct that how we view the results of a study depends, in part, on how it is structured. In this case, we're talking about a mouse study in which researchers from Penn State's College of Agricultural Sciences looked at whether a diet that included daily servings of white button mushrooms would affect the gut microbiomes of laboratory mice.

What edges the results of this particular study into the realm of cause and effect is the use of two different groups of mice. One group included typical laboratory mice with a normal gut microbiome. The second group, however, was made up of germ-free mice who did not have a gut microbiome. That allowed the scientists to observe exactly how the microbiota of the "normal" mice changed as compared to the control group.

What they found was that eating a daily serving of white button mushrooms caused subtle changes to the makeup of the gut microbiome in the normal mice. Specifically, adding the mushrooms to their diets resulted in a rise in the overall numbers of a specific bacterium. Known as Prevotella, it produces certain short-chain fatty acids that have a positive effect on genes that improve glucose management.

What's interesting is that the function of the mushrooms in this case was as a prebiotic. That is, they served as food for the community of tens of millions of bacteria that make up the microbiome. For reasons that are not yet fully understood, the mushrooms allowed the Prevotella to flourish, which translated to a measurable effect on blood glucose management in the liver.

The results of the study invite future research into strategies to prevent and manage diabetes, a chronic and progressive disease in which the body cannot properly metabolize glucose. That's important, considering that more than 30 million Americans -- close to 10 percent of the population -- are now living with diabetes, and an estimated 1.5 million new cases are diagnosed each year. Diabetes, which increases the risk of heart attack and stroke, was the seventh-leading cause of death in the United States in 2015.

Another intriguing aspect of this study is the new light it sheds on the effect our diet can have on the populations within the gut microbiome, and how those microbial communities affect the workings of our bodies.

As for whether eating white button mushrooms can help you with your own blood glucose management, it's too soon to say. This study was conducted in mice and for a definitive answer, a human study is needed. According to the researchers in this study, the portion size fed to the mice translates to about 3 ounces of mushrooms per day for us humans. There's no reason not to add a portion of white button mushrooms to your daily diet, barring an allergy, of course. Just don't saute them in a lot of butter.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o UCLA Health Sciences Media Relations, 10880 Wilshire Blvd., Suite 1450, Los Angeles, CA, 90024. Owing to the volume of mail, personal replies cannot be provided.)