Dear Doctor: My cousin says she heard that eating sugar pretty much wipes out the beneficial bacteria in our gut. Please tell me that's not true. I'm an avid baker and love sweets and don't think that I can give them up.

Dear Reader: We can reassure you that the report your cousin is referring to doesn't claim that sugar out-and-out destroys the gut microbiome. But don't celebrate with a home-baked brownie just yet. We're afraid that the new research does contain some bad news for people who have a sweet tooth. According to the study, which was published in the Proceedings of the National Academy of Sciences at the start of this year, high levels of fructose and glucose in the diet wreak havoc on a certain protein that is necessary for beneficial bacteria to colonize the gut.

Why does that matter? The latest research continues to make clear that good health hinges on each of us maintaining a robust and diverse gut microbiome. Made up of bacteria, fungi, viruses and other microorganisms that number in the trillions, the gut microbiome plays a decisive role in digestion, the absorption of vitamins and other nutrients, and the optimal function of the immune system. Whether directly or indirectly, the thousands of species of microbes we host in our bodies affect most of our physiologic functions. At the same time, we have a direct effect on these populations, including through what we eat.

In addition to the word "probiotics," which refers to beneficial gut bacteria, the term "prebiotics" has entered common usage. This refers to the portion of the diet that contains nutrients that are available to those trillions of gut microbes. Dietary fiber, which is made up of long chains of simple sugars bonded together to make a large and complex molecule known as a polysaccharide, sails through the small intestine largely undigested. That fiber reaches the part of the colon known as the distal gut, which houses the lion's share of the gut microbiome. Not only does that dietary fiber provide nutrients to the gut microbiome, but it affects the growth and colonization of the microbial communities.

When it comes to monosaccharides, or simple sugars, like fructose and glucose, which are routinely added to a wide range of prepared and processed foods, it was believed that they were absorbed in the small intestine and never made it to the distal gut. However, it is now known that both fructose and sucrose do reach the distal gut. When they do, they have a negative impact on good bacteria like Bacteroides thetaiotaomicron (B. theta for short), which are associated with a lean and healthy body. Instead of providing food, simple sugars in the distal gut stop the production of a key protein that B. theta needs in order to maintain and expand its presence. Without that protein, B. theta populations become significantly diminished.

It's important to note that this research was done on mice. How or even whether it translates to the human microbiome is not yet known. But considering the many health problems clearly linked to added sugar, including diabetes, obesity and cardiovascular disease, cutting out simple sugars makes sense.

(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: My boyfriend and I were in London over the holidays, and we heard a lot about MG, a new sexually transmitted disease. Is it here in the United States too? Should I be worried?

Dear Reader: You're referring to Mycoplasma genitalium, also known as MG or Mgen. It's a bacterium that is transmitted through sexual contact, and as a result, can infect the reproductive tract. MG was first identified in the United Kingdom more than 30 years ago, so it's not really new. It was found in two men being treated for non-gonococcal urethritis, which is inflammation of the urethra, the tube that carries urine away from the bladder. "Non-gonococcal" means that the infection is not caused by the gonorrhea bacteria.

The Centers for Disease Control and Prevention only officially acknowledged the disease in 2015, so it's a fairly recent addition to the STD conversation. Although reliable statistics are not yet available, infection rates among women in the U.S. are estimated at about 1 percent. That's slightly higher than gonorrhea and lower than the rate of infection with chlamydia, which is 3 percent. However, when looking at high-risk populations, which constitutes young people who have multiple sexual partners and those who don't practice safe sex, some estimates of infection rates are as high as 20 percent.

Mycoplasma genitalium causes inflammation of the urethra in both men and women. Symptoms include an increased urge to urinate, as well as pain and burning, particularly during urination. Some cases are accompanied by an abnormal discharge. It's also possible for MG to produce no symptoms at all. A recent analysis of available data found that close to 60 percent of women who tested positive for MG had no symptoms. However, due to their reproductive anatomy, women face greater risks than men from infection. Among these is developing pelvic inflammatory disease, or PID, an infection of the female reproductive organs that can cause long-term health problems and can lead to infertility. The bacterium has also been linked to adverse birth outcomes, as well as to increased susceptibility to HIV infection.

Mycoplasma genitalium is challenging to diagnose. It lacks a cell wall, so it's not suitable for a gram stain, the most common technique for differentiating and identifying bacteria. It's so slow-growing that a culture takes six months to develop. Instead, nucleic acid amplification testing, or NAAT, which identifies the pathogen via its genetic material, is now being used. The test is typically done on urine in men and via cervical swabs in women. As with all bacterial infections, treatment is with antibiotics. And as with a growing number of bacterial pathogens, MG is developing antibiotic resistance.

As to whether people should be worried, if they have multiple partners and don't practice safe sex, the answer is yes. We're not referring only to MG infections when we say that. The number of new cases of sexually transmitted diseases, including chlamydia, gonorrhea and syphilis, reached an all-time high in 2017. That's part of a steep and sustained increase that has alarmed health care professionals. So, please, always practice safe sex. Anyone with any symptoms of a sexually transmitted disease should seek medical care immediately.

(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: Why is it that vitamin D is so important? I know you can get it from food and supplements, but I'd like to get it the old-fashioned way -- from the sun. Is that too dangerous?

Dear Reader: It can't be overstated: Vitamin D is critical to good health. It's needed for calcium absorption from the intestinal tract, and for the regulation of blood phosphorus levels. Both of these are essential to developing and maintaining strong and healthy bones. Even when you get enough calcium and phosphorus in your diet, you can't absorb it unless you're also getting enough vitamin D.

Without adequate vitamin D, bones become thin and brittle and can easily bend or break. Rickets was once a widespread disorder in which children's bones were so thin and spongy from a lack of the vitamin that their skeletons failed to develop properly. In some cases, their legs became visibly bowed. Since the addition of D to milk, yogurt, breakfast cereals and orange juice, rickets has largely been wiped out.

Vitamin D also plays a vital role in brain development, muscle function, maintaining a healthy respiratory and immune system, and in optimal cardiac function. Recent research has also uncovered a potential link between a deficiency of vitamin D and depression. In fact, we now know that vitamin D receptors are present in virtually every tissue, so we fully expect research to continue to discover even more ways in which the vitamin is essential.

As you point out in your question, our bodies manufacture vitamin D in response to sunlight. That's why it's also known as the "sunshine vitamin." Specifically -- and this is a bit of a deep dive, but we think it's fascinating -- the process of vitamin D synthesis begins when ultraviolet rays strike the skin. This triggers a chemical process known as hydroxylation, which involves the liver, kidneys and certain cellular structures, and ultimately creates the chemical compound that we refer to as vitamin D. This process of synthesis also takes place with the vitamin D that we get from food and supplements.

The challenge is that getting enough D from sun exposure can take vigilance. Variables include air pollution, cloud cover, latitude, time of year, time of day, clothing, the use of sunscreen, and an individual's skin type, age and lifestyle. Each of these can make it more difficult to get enough D.

To trigger vitamin D synthesis in the body, the general recommendation is to spend between 10 to 30 minutes with arms, legs and/or torso exposed to sunlight during peak hours -- without sunscreen -- two to three times per week. Melanin protects the skin from sun damage, so darker skin requires longer exposure. For those who prefer not to deal with the potential risks of sun exposure, good dietary sources of D include the fortified foods we just mentioned, as well as fatty fish such as salmon, tuna and mackerel, and fish liver oils. The vitamin is also present in beef liver and egg yolks, but in small amounts. And if you decide to take supplements, please be sure to follow the dosage guidelines. For all its many benefits, it's possible to get too much vitamin D.

(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.)