Dear Doctor: It seems like if you just wait long enough, everything about nutrition gets contradicted. So now fat is good, carbs are bad, and we don't really need more than 4 servings of vegetables a day? Really? Is everything we've been taught wrong?

Dear Reader: We hear this same frustration from many of our patients, and honestly, we're right there with you. Healthful eating is an important goal, but how are we supposed to achieve it if the rules keep changing?

When it comes to why nutrition advice keeps changing, several factors come into play: science, media and marketing.

Conclusions regarding diet and nutrition are frequently drawn from observational and epidemiological studies. Unlike in a clinical trial, where a single variable, such as a medication or a treatment approach, is scrutinized repeatedly in a randomized and rigidly controlled environment, observational and epidemiological studies draw conclusions by identifying and analyzing trends within large population groups.

Well-designed observational studies can provide valid results. But it's important to understand that the data they use include multiple variables that can't be controlled. Eggs and coffee are two culprit foods that have gone through the "it's bad"/"no, it's good" whiplash. Each of those foods was the focus as researchers combed through piles of data. However, the people in those studies were eating and drinking many more things. Add in variables like lifestyle and environment, and isolating the effect of eating any one specific food becomes challenging.

When researchers publish results that suggest a causal link between a certain food and a particular outcome, they acknowledge that it's just one single data point in an ongoing analysis. It takes time and repetition to reach a reliable conclusion. But scientific rigor pretty much flies out the window when it comes to how the emerging studies get publicized, which leads us to the second factor -- the media.

While scientists are saying, "In this one study, we see a link between eggs and heart disease," in the hands of over-eager newscasters, this is translated as "Eggs will kill you!" Meanwhile, time goes on. Many more studies are conducted. A decade or so later, armed with a wealth of data points collected over the years, the original thesis doesn't hold up. For the scientists, it's the nature of research -- precision and repetition over time. For the rest of us, it's a new -- and contradictory -- headline. "Eggs are good!"

Which brings us to marketing. Diet and nutrition are multibillion-dollar industries in the United States. That means that as food recommendations are crafted, a good bit of lobbying takes place. When new dietary guidelines were released in 2015, a number of nutrition experts, including Dr. David Heber, founding director of the Center for Human Nutrition at UCLA, bemoaned the influence of the food industry in the process.

We need to be savvy consumers of dietary information. And when it comes down to healthful eating, we recommend that our patients stick with the tried-and-true basics. That is, eat fish and lean meats, grains and legumes. Limit or cut out processed foods, and eat a wide variety of fruits and vegetables, especially leafy greens. Limit sugar and alcohol. When you're thirsty, reach for a glass of water.

(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: My eyes are so dry, they often feel as if I have sand in them. I use drops, but they don't always work. Is there anything else I can do? And will this hurt my vision over time?

Dear Reader: No doubt you're frustrated. If it's any comfort, your doctor probably is too. People suffering from dry eyes are frustrated because of the constant irritation; doctors are frustrated because they don't have a miracle cure. The symptoms of dry eyes vary. Like you, many patients complain of a gritty or sandy sensation. Some also report red or watery eyes, a burning feeling, light sensitivity, blurred vision or a combination of these symptoms. Paradoxically, excessive tearing can also be a sign of dry eyes. Rarely, however, does prolonged dry eyes lead to scarring of the cornea or permanently altered vision.

That's fortunate, because 14.5 percent of Americans report having regular dry eye problems, and the likelihood increases with age. The condition is more common in women, especially those undergoing menopause, as well as in people with certain chronic illnesses such as diabetes and Parkinson's disease. People who have had laser eye surgery, wear contacts or use medicated eye drops also have a greater rate of dry eyes, as do people who have allergies.

As for treatment, it's important to look at the possible cause. Some medications, such as antihistamines, antidepressants, niacin, estrogens and the arrhythmia drug amiodarone can cause dry eyes. So can the autoimmune disease Sjogren's syndrome, which leads to inflammation of tear ducts, death of cells in the ducts and decreased formation of tears. Sjogren's, which is accompanied by dry mouth, can be diagnosed with blood tests and other screenings that evaluate production of tears and saliva. Other conditions can affect the tear ducts as well, such as sarcoidosis, lymphoma and diabetes.

Eyelid health is also important. Normally, Meibomian glands in the eyelid produce an oily substance that prevents tear evaporation and helps trap tears upon the eyeball. Inflammation of the lid margin, termed blepharitis, causes these glands to malfunction, leading to dry eyes. Treatment of blepharitis includes lid scrubs (using baby shampoo and a warm wet cloth), lid massage, warm compresses and artificial tears.

Speaking of artificial tears, these drops add viscosity to the fluid surrounding the eye and thus maintain a natural tear covering the eye. Adding one drop to each eye three to four times per day will improve symptoms. Note that some people have sensitivity to the preservatives in artificial tears, so using preservative-free tears may be helpful.

For people with inflammatory conditions such as Sjogren's syndrome or blepharitis, the immunosuppressive eye drop cyclosporine, applied twice a day, can significantly improve the symptoms of dry eyes.

A newer eye drop, lifitegrast, decreases inflammation in the eye, with twice-a-day application leading to significant improvement of symptoms. Note, however, that the medication can also cause eye irritation and taste abnormalities.

Not all treatments are drug-related. Omega-3 fatty acids with EPA, DHA and flaxseed oil can enhance tear production, decrease tear evaporation and improve overall symptoms. Similarly, diets deficient in omega-3 fatty acids have been linked to increased rates of dry eyes.

In summary, give artificial tears a try, and increase your intake of omega-3 fatty acids. Also, please see an ophthalmologist. He or she can determine whether an inflammatory disorder, such as blepharitis or Sjogren's, is the cause of your symptoms.

(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'm 79 years old and recently contracted mumps. I've never been so ill, and in fact, I am still suffering the aftereffects. Please help other older adults understand whether they should get the mumps vaccine.

Dear Reader: Although mumps is nowhere near as common in the United States as it once was, outbreaks continue to occur. Last year, mumps cases hit a 10-year high with 6,366 reported to the Centers for Disease Control and Prevention. In 2012, there were just 229 cases. By contrast, prior to 1967, when the mumps vaccination program was launched, about 186,000 cases were reported each year. However, that number is probably too low due to underreporting.

Mumps is caused by a virus that spreads via contact with saliva or mucus from the mouth, nose or throat of an infected person. It is carried via airborne droplets from the upper respiratory tract through coughing, sneezing or even talking. The virus can also be transferred through shared items like plates, cups and cutlery, and on objects or surfaces that have been touched by the unwashed hands of the infected person.

As with many contagious diseases, the infected person begins shedding the virus before symptoms become apparent. He or she will continue to be contagious for up to five days after becoming visibly ill. These symptoms include fever, headache, body aches and pains, exhaustion and lack of appetite. Swollen and tender salivary glands in front of one or both ears give sufferers the puffy cheeks and swollen jaw that is associated with the disease. Some patients experience hearing loss, but it is rarely permanent.

Adults who get mumps are at greater risk for complications than are children. These can include inflammation of: the membrane around the brain or spinal cord (known as meningitis), the testicles, breast tissue, ovaries or pancreas. The most serious complication is encephalitis, which is inflammation of the brain itself. It can lead to permanent disability or even death.

Mumps can be prevented with the MMR vaccine, which also confers immunity to measles and rubella. The CDC recommends two doses of the vaccine for children, the first at 12 to 15 months of age, and the second at 4 to 6 years of age. Teens and adults should also stay current on MMR vaccinations. Pregnant women should not get the vaccine, and women should wait at least four weeks after vaccine administration before becoming pregnant.

Because a number of cases of mumps have occurred in people who have had the two-dose vaccine, there is now a debate about whether immunity confers through adulthood. A recent study in the New England Journal of Medicine analyzed data from a mumps outbreak at the University of Iowa during the 2015-2016 school year. Students who had a mumps booster had a 78.1 percent lower risk of mumps than those with the just the two-dose vaccine. To address waning immunity, researchers suggest that a mumps booster may be called for.

We advocate for vaccination and help our patients stay current. And for anyone with questions or concerns about the vaccine, please do talk to your primary care physician.

(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.)