DEAR DR. DONOHUE: I just mastered the subject of good and bad cholesterol. Now they’re talking about omega-3 fats. It’s something discussed in every magazine I pick up. I have asked friends to explain what omega-3 fats are, and they can’t. We would all greatly appreciate it if you would do that for us. – M.M.

ANSWER: Understanding omega-3 fats (or fatty acids) means you have to become conversant with some difficult names. Omega-3 fats are polyunsaturated fats. “Polyunsaturation” means that these fats are missing many hydrogen atoms, and that makes them the kind of fats that don’t clog arteries, don’t raise blood cholesterol and don’t pose a risk for heart attacks. There are three common omega-3 fats. Two are found in fish: EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid). The third omega-3 is alpha-linolenic, ALA, found in flaxseed and other plants.

In the 1970s, studies done on Greenland’s Eskimo population revealed they had few heart attacks in the face of very-high-fat diets. The reason was their fish intake, with its omega-3 fats.

Since then, many authorities encourage people to get two servings of fish a week. Fish with the highest amounts of omega-3s include salmon, sardines, herring, tuna and mackerel. Plants and plant products with lots of the omega-3 acid ALA are flaxseed, walnuts, soybeans and canola oil.

Not every expert subscribes to the omega-3 story. People who should not be encouraged to increase their intake of omega-3s are those who have implantable defibrillators.

Dietary information has a habit of doing an about-face as time goes by. However, the omega-3 story is holding its own for now, even though there have been studies that dispute the elevated status awarded to omega-3s.

DEAR DR. DONOHUE: I am a 30-year-old woman who, as far as I can remember, has had excessively salty sweat. After I sweat for a period of time, there are tiny grains of salt visible on my skin. This sort of thing never concerned me until my mother-in-law mentioned that something must be wrong with me. My question to you is, Is this normal? If it is not, what could possibly cause it? – S.N.

ANSWER: Everyone’s sweat contains salt. When people perspire profusely, their clothes have a salt deposit when they dry. I haven’t seen actual crystals of salt on skin, but skin, after pouring out lots of sweat, tastes salty.

Cystic fibrosis is an illness in which the sweat contains higher-than-normal levels of salt. Perhaps you have heard the advice to kiss your baby to see if its skin tastes salty. If it does, that could be a sign of cystic fibrosis, and it should be reported to the doctor.

Cystic fibrosis is usually diagnosed in the first six months of life. However, it can surface in adulthood, and those adults have no symptoms until then. When symptoms start, they are usually respiratory symptoms with repeated infections and the bringing up of thick secretions. Digestive-tract symptoms are also possible, since cystic fibrosis affects the pancreas’s production of digestive enzymes.

Cystic fibrosis in adults is rare. If you ask your doctor to test you for it, he or she might raise both eyebrows – justifiably. But you can get your sweat tested for excessive chloride, the same test that’s used for infants.

DEAR DR. DONOHUE: My doctor ordered a CT scan of my abdomen and pelvis for right-sided pain. It showed a benign cyst in my liver. The doctor said not to worry. I didn’t think you could tell if a cyst was benign without taking a biopsy. Should I get a second opinion? – S.M.

ANSWER: CT scans can usually tell cancer from benign cysts. Many people have a liver cyst. It’s not a source of concern for most.

Neither your personal doctor nor the doctor who interpreted the scan are going to take a chance by calling a suspicious liver mass a benign cyst if they are not sure that’s what it is. I wouldn’t pursue this any further.

Dr. Donohue regrets that he is unable to answer individual letters, but he will incorporate them in his column whenever possible. Readers may write him or request an order form of available health newsletters at P.O. Box 536475, Orlando, FL 32853-6475. Readers may also order health newsletters from www.rbmamall.com